repeating coupled earthquakes at shishaldin volcano,...

www.elsevier.com/locate/jvolgeores

Journal of Volcanology and Geotherm

Repeating coupled earthquakes at Shishaldin Volcano, Alaska

Jacqueline Caplan-AuerbachT, Tanja Petersen

Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Dr.,

Fairbanks, AK 99775-7320, United States

Received 13 November 2003; accepted 20 January 2005

Abstract

Since it last erupted in 1999, Shishaldin Volcano, Aleutian Islands, Alaska, has produced hundreds to thousands of long-

period (1–2 Hz; LP) earthquakes every day with no other sign of volcanic unrest. In 2002, the earthquakes also exhibited a

short-period (4–7 Hz; SP) signal occurring between 3 and 15 s before the LP phase. Although the SP phase contains higher

frequencies than the LP phase, its spectral content is still well below that expected of brittle failure events. The SP phase was

never observed without the LP phase, although LP events continued to occur in the absence of the precursory signal. The two-

phased events are termed bcoupled eventsQ, reflecting a triggered relationship between two discrete event types. Both phases are

highly repetitive in time series, suggestive of stable, non-destructive sources. Waveform cross-correlation and spectral

coherence are used to extract waveforms from the continuous record and determine precise P-wave arrivals for the SP phase.

Although depths are poorly constrained, the SP phase is believed to lie at shallow (b4 km) depths just west of Shishaldin’s

summit. The variable timing between the SP and LP arrivals indicates that the trigger mechanism between the phases itself

moves at variable speeds. A model is proposed in which the SP phase results from fluid moving within the conduit, possibly

around an obstruction and the LP phase results from the coalescence of a shallow gas bubble. The variable timing is attributed to

changes in gas content within the conduit. The destruction of the conduit obstacle on November 21, 2002 resulted in the abrupt

disappearance of the SP phase.

Published by Elsevier B.V.

Keywords: Shishaldin Volcano; volcano seismology; long-period earthquake

1. Introduction

Since late 1999, shortly after it produced a

Subplinian eruption, Shishaldin Volcano, Unimak

0377-0273/$ - see front matter. Published by Elsevier B.V.

doi:10.1016/j.jvolgeores.2005.01.011

T Corresponding author. Now at the Alaska Volcano Observatory,

U.S. Geological Survey, 4200 University Dr., Anchorage, AK

99508, United States. Tel.: +1 907 786 7460.

E-mail address: [email protected] (J. Caplan-Auerbach).

Island, Alaska (Fig. 1), has exhibited up to thousands

of daily small long-period (LP) earthquakes. This

level of seismicity is in stark contrast to the other 24

volcanoes monitored by the Alaska Volcano Observ-

atory (AVO), which generally experience a few

earthquakes per week (Dixon et al., 2003). While

swarms of earthquakes are typical on volcanoes, the

continuous nature of Shishaldin seismicity makes it

unique among Alaskan volcanoes and unusual among

al Research 145 (2005) 151–172

-165o 00'

-165 00'

-164o 40'

-164 40'

-164o 20' -164o 00' -163o 40' -163o 20' -163o 00' -162o 40' -162o 20'

54o 20'

54o 40'

55o 00'

55 20'

-165 00' -164 40'

55 20'

54o 20'

54o 40'

55o 00'

54o 20'

-164o 20' -164o 00' -163o 40' -163o 20' -163o 00' -162 o40' -162o 20'

SSLN

SSLS

SSLW

BRPK

ISNN

ISTK

False Pass

Cold Bay

Shishaldin

Isanotski

Fisher

50˚

180

180

-170

-170

-160

-160

-150

-150

50 50

60 60

70 70

0 5 10

km

Westdahl

Fig. 1. Map of Unimak Island, Aleutian Arc, and the Shishaldin seismic network. Shishaldin Volcano is marked with an arrow, as are Westdahl,

Fisher and Isanotski volcanoes. The solid circles represent the locations of short-period seismic stations operated by the Alaska Volcano

Observatory. The nearest population centers, the villages of False Pass and Cold Bay, are marked by triangles.

J. Caplan-Auerbach, T. Petersen / Journal of Volcanology and Geothermal Research 145 (2005) 151–172152

monitored volcanoes in general. Shishaldin’s behavior

is also perplexing in that the elevated seismicity is

apparently unassociated with other evidence of

volcanic unrest such as thermal anomalies. Finally,

this activity is notable for the extreme similarity

evidenced among events over long time scales

(Petersen et al., 2002).

Volcanic seismicity commonly heralds eruptive

activity. While there is a great deal of variety in the

type and duration of pre-eruptive seismicity, it is

largely accepted that a high degree of seismicity on a

volcano reflects the activation of internal eruption

processes such as the rise of magma from depth, dike

propagation, or pressure increase related to changes in

gas flux (e.g. Klein et al., 1987; Lahr et al., 1994;

McNutt, 2000; Chouet, 2003). LP earthquakes in

particular are commonly seen as indicators of volcanic

unrest, as they are believed to relate to the presence of

fluids such as magma (Fischer et al., 1994; Lahr et al.,

1994; Chouet, 1996). Thus, a continuously high level

of LP activity is troubling, especially for an organ-

ization such as AVO which is responsible for eruption

forecasting and hazard assessment.

In early 2002, the appearance of Shishaldin seis-

micity changed, with the addition of a precursory short-

period (SP) phase to the LP events. We refer to this pair

of earthquake phases as a bcoupledQ event and show

that it is unlike other multi-phase earthquakes com-

monly presented in the volcano seismology literature.

In this article we first give an overview of Shishaldin

seismicity, and then explain how the coupled earth-

quakes differ from more common earthquake types

such as LP or hybrid events. We describe the coupled

events in detail, discussing event similarity, earthquake

J. Caplan-Auerbach, T. Petersen / Journal of Volcanology and Geothermal Research 145 (2005) 151–172 153

location and the relationship between the signal’s two

phases. Finally we propose a mechanism by which

coupled events may be generated and discuss the

implications for hazards monitoring at Shishaldin.

2. Geologic setting

Shishaldin Volcano is among the most active

volcanoes in Alaska, with 29 confirmed eruptions in

historic time (Miller et al., 1998; Nye et al., 2002). Its

eruptions are composed primarily of basalt and

basaltic andesite, although rare lavas with up to 64

wt.% silica have been identified (Fournelle and

Marsh, 1991; Miller et al., 1998). The tallest volcano

in the Aleutians, Shishaldin’s summit reaches 2857 m

above sea level and glaciers and snowfields blanket its

surface year-round. A puffing gas plume nearly

always emanates from the volcano’s summit.

Although the composition of the plume is unknown,

observations have been made of sulfur deposits within

the mouth of the conduit and a strong sulfur smell has

been associated with gas puffs (P. Stelling, personal

communication, 2003). This suggests that some

portion of the plume results from magmatic gases.

Although most of Shishaldin’s eruptions have

consisted of small ash and steam plumes, at least 11

Subplinian deposits have been identified in Holocene

stratigraphy (Beget et al., 1998). The most recent

eruption, in April–May 1999, consisted of both

Strombolian bubble and ash bursts and a 16-km

Subplinian eruption column. Thus, while the majority

of its eruptive activity is small and local to the region,

the fact that Shishaldin is capable of generating high

and voluminous ash plumes confirms that it must be

recognized as a hazardous volcano.

Unimak Island, the easternmost island in the

Aleutian chain (Fig. 1) comprises 5 volcanoes,

including Shishaldin. The two nearest population

centers are False Pass, located ~60 km from Shish-

aldin on the island’s eastern coast, and Cold Bay,

located on the Alaska Peninsula ~92 km from

Shishaldin. Unfortunately, Shishaldin cannot be seen

from False Pass because the nearer volcanoes Round

Top and Isanotski block the view. AVO is therefore

dependent on local pilots or observers in Cold Bay for

visual descriptions of the volcano and its activity.

Overall, however, visual observations are minimal and

AVO relies heavily on telemetered seismic data and

satellite imagery for volcanic monitoring. This lack of

visual observation makes it difficult to examine

possible correlations between Shishaldin seismicity

and factors such as precipitation, or the strength of the

ubiquitous gas plume at the volcano’s summit.

3. Seismic network and history

In 1997, AVO installed a 6-station seismic network

near Shishaldin, and in 1998 a network was installed

on Westdahl volcano west of Shishaldin (Fig. 1). Five

of the Shishaldin stations have short period Mark

Products L-4 vertical seismometers with 1 Hz natural

frequency. Station SSLS has a 3-component Mark

Products L-22 seismometer with 2 Hz natural

frequency. A Setra 239 pressure sensor was co-located

with station SSLN on the volcano’s north flank, but

stopped functioning in late 1999, and was replaced in

2003 by a Chaparral Model 2 microphone. Data from

all instruments are telemetered to the nearby towns of

Cold Bay and King Cove and then to AVO over

analog telephone lines. Once in Fairbanks the data are

digitized at 100 Hz with 12-bit resolution.

While the 1999 eruption proved that the network is

effective for volcanic monitoring (Moran et al., 2002;

Nye et al., 2002; Thompson et al., 2002; Caplan-

Auerbach and McNutt, 2003), the large distance

between the outer stations and Shishaldin’s summit

(14–19 km; Fig. 1) makes location of small events

extremely difficult. Furthermore, station ISNN suffers

from a poor telemetry link, resulting in large data gaps

from that station. Finally, LP and coupled earthquakes

do not exhibit clear S-waves. Thus for earthquake

locating we can typically use a maximum of 5 P-

waves, with 4 phases the more common situation.

Consequently, conventional locations of Shishaldin

earthquakes are not always possible, although pre-

liminary locations place them beneath the summit at

shallow (0–3 km) depths (Dixon et al., 2003).

The seismicity commonly observed in volcanic

regions can be described using an event classification

scheme such as those developed by Minakami (1960)

and Lahr et al. (1994) based on the present under-

standing of source mechanisms. The volcanic event

types most commonly observed include volcano-

tectonic (VT) earthquakes, LP events and hybrids.


Volcano tectonic (VT) events are high-frequency

earthquakes, occurring within or beneath the volcanic

edifice. They represent the brittle failure of rocks due

to stress changes within the volcano. Although the

source of the stress may be volcanic in nature, such as

magmatic activity or transport of volatiles to the

surface (Lahr et al., 1994; Moran, 2003), VT earth-

quakes are otherwise indistinguishable from tectonic

earthquakes. VT events show characteristics similar to

common double-couple tectonic earthquakes, e.g.

they have clear P- and S-phases with significant

energy at frequencies N5 Hz and a variety of first-

motions (Lahr et al., 1994). In March 1999, a shallow

ML 5.2 earthquake occurred 16 km to the west of

Shishaldin, followed by a several-month long after-

shock sequence of VT events (Moran et al., 2002).

Since then, only a few VT earthquakes have been

located at Shishaldin, almost exclusively restricted to

the area 10–15 km west of the summit.

LP events are low-frequency (1–2 Hz) earthquakes

commonly associated with fluids moving in volcanic

conduits or cracks (Chouet, 1988, 1996; Julian, 1994;

Neuberg et al., 1998). They are characterized by

emergent onsets due to a gradual increase of ground

shaking caused by fluid-driven oscillation (Lahr et al.,

1994). The S-waves of LP events are not distinct and

their codas are extended. At Shishaldin, the vast

majority of earthquakes are LP events. The omnipre-

sent steam plume and steam puffs suggest that these

events might be related to degassing processes,

however, the underlying source mechanisms are still

poorly understood.

A third type of earthquake, termed the hybrid, is

related to both VT and LP events. Hybrid earthquakes

typically begin with an impulsive, short-period onset

followed by an LP-like coda. Initial descriptions of

hybrid events noted that, like VT events they

commonly exhibit a variety of first motions and clear

S-phases (Lahr et al., 1994; White et al., 1998).

However, other researchers have discarded this

requirement and note that hybrids are part of a

continuum of events that include LPs (Neuberg et

al., 2000). Lahr et al. (1994) suggest that hybrids

result from brittle failure on a plane intersecting a

fluid body, thereby combining the source mechanism

for both VT and LP events. Earthquakes of this sort

have been observed at a number of volcanoes,

including Deception Island (Ibanez et al., 2003),

Galeras (Gil Cruz and Chouet, 1997), Montserrat

(Neuberg et al., 1998; White et al., 1998), and

Redoubt (Lahr et al., 1994).

Prior to March 2002, Shishaldin activity was

composed almost exclusively of LP events. On March

13, however, a new type of earthquake was identified,

and became the predominant event type until Novem-

ber. These earthquakes begin with a short-period (SP)

phase, followed by what appears to be a normal LP

event. Subsequent investigation of earlier records

indicated that these events were present prior to March

13 but in small numbers and at weak amplitudes. We

refer to these events as coupled because, like hybrid

earthquakes, they involve two discrete phases. How-

ever, for a number of reasons, we believe that the source

process commonly invoked for hybrid events, brittle

failure near a fluid filled crack, is unlikely for

Shishaldin events. First of all, the SP phase of

Shishaldin coupled earthquakes is strongly bandlimited

between 4 and 7 Hz, which is quite low for local brittle

failure. There are also few identifiable S-phases for

either the SP or LP phases. As we will show, the SP

portion of Shishaldin events is highly repetitive for

prolonged time periods (days to weeks). It is unlikely

that repeating brittle failure event could retain such

similarity in waveform over such a long time, since

with time the source process would alter conditions at

the source region. Because the SP phases are generally

small, and somewhat emergent, first motions are

difficult to constrain. Unlike traditional hybrid events,

the timing between SP and LP phases in Shishaldin

coupled earthquakes is highly variable, as will be

described later in this article. Finally, the LP phase

bears significant similarity to bnormalQ LP events,

suggesting that it is triggered by the SP phase and is not

simply a later phase of the same earthquake. Con-

sequently we call these events bcoupled earthquakesQand propose a different mechanism for their formation.

4. Coupled events observed elsewhere

Because some Shishaldin coupled events bear

similarity to hybrid earthquakes recorded elsewhere,

it is non-trivial to determine how common such events

are at other volcanoes. However, events that appear

similar in time series to Shishaldin coupled events

have been noted on a handful of other volcanoes


(Galeras, Gil Cruz and Chouet, 1997; Stromboli, J.

Lees, personal communication, 2002; Deception

Island, Ibanez et al., 2003; Koryakski, Gordeev and

Senyukov, 2003; Popocatepetl, R. White, personal

communication, 2003). Signals called bsecondaryeventsQ were identified as precursory to LP events at

Redoubt volcano, but were only visible on a single

station and thus could not be thoroughly examined

(Stephens and Chouet, 2001). Precursory signals

identified at Koryakski volcano, Kamchatka were

interpreted by Gordeev and Senyukov (2003) to be

related to magma injection into cracks. Gil Cruz and

Chouet (1997) note their occurrence on Galeras

volcano in 1991 and suggest a source mechanism

involving two discrete cracks and a rising gas bubble.

Ibanez et al. (2003) were able to determine that the

short and long-period phases of these earthquakes have

different locations and attribute their source to brittle

failure on faults lubricated by fluids. Thus, although

they are not common features, events similar to

Shishaldin coupled events have been identified at a

number of volcanoes. However, Shishaldin events

appear to be unique in the variable timing observed

between the SP and LP phases, and the spectral content

of the SP phase is generally lower than that observed

50 100 150 200 250 3TIM

180 185 190 195 200 205TIM

-400

-200

0

200

400

-200

0

200

400

Fig. 2. Cross-correlation technique used to extract repeating events from th

correlated with segments of the continuous record and aligned at the point o

the frequency band of the reference event (4–7 Hz for SP events) and eve

continuous data. Note that the first event shown in the top panel was not s

analysis.

elsewhere. Consequently, we propose a slightly

modified mechanism for the events described here.

5. Identification and extraction of repeating

earthquakes

A preliminary examination of Shishaldin seismic-

ity shows that many of its earthquakes, both LP and

coupled, appear similar in time series (Petersen et al.,

2002). Because these events are small, only a low

percentage trigger the data acquisition system using

the EARTHWORM triggering algorithms Carlstatrig

and Carsubtrig (Dixon et al., 2003). Consequently,

most events are not automatically segmented for

further analysis. To examine a larger subset of coupled

events we took advantage of the fact that the earth-

quakes are repetitive and developed a method by

which similar events could be extracted from the

continuous record (Fig. 2). In this algorithm, a

breference eventQ is selected against which other

earthquakes will be compared. The reference event

is selected somewhat arbitrarily, with the only criteria

being a good signal-to-noise ratio and no overlap with

other events. The continuous record is divided into

00 350 400 450 500 550E (s)

210 215 220 225 230E (s)

e continuous record. An SP reference event, shown in red, is cross-

f maximum correlation. The spectral coherence is then calculated in

nts with coherence estimates exceeding 0.9 are segmented from the

ufficiently similar to the reference event to be considered for further

Table 1

Time frames and number of events for the eight time periods

selected in this study

Event group Start date End date Number of

events extracted

by SP phase

1 March 3 April 27 1442

2 April 27 June 19 1258

3 June 14 July 25 1773

4 July 14 July 21 426

5 July 22 August 19 380

6 August 24 September 16 1151

7 September 7 October 9 202

8 September 28 November 21 943

Each period reflects a time in which a single reference event was

dominant. Time periods were considered to end if two days passed

in which no events similar to the reference event were found. Note

that there is some overlap in the time periods, reflecting times in

which some variation in event appearance was observed.


segments of length equal to that of the reference

event, and each segment is cross-correlated with the

reference event. The maximum cross-correlation value

yields the lag position at which the two signals

correlate most strongly, and a data segment beginning

at that time is selected from the continuous record for

coherence testing. The spectral coherence between

this data segment and the reference event is calculated

and the mean coherence in a specified frequency band

is examined. All data segments with mean coherence

exceeding a threshold value (typically 0.9 for the SP

phase of coupled events) are selected as part of an

event group. The initial selection of events was made

using station SSLN, the station on which Shishaldin

events typically exhibit the highest signal-to-noise

ratio, although we were successful in performing the

same task using other stations. Once the repeating

events are selected, a 30-s window around each event

is extracted for all stations in the network, and the data

are thereby segmented. This method allows us to

select a sequence of repeating events from the

continuous record that are too small to trigger the

data acquisition system. This method is similar to the

one used by Stephens and Chouet (2001) in their

analysis of LP seismicity at Redoubt volcano. As with

all other methods used to count events at Shishaldin

(Petersen et al., 2002), this method only selects a

small subset of earthquakes. However, this subset was

sufficient to examine the coupled events, their

locations and their temporal behavior. Petersen et al.

(2002) performed a similar analysis for the LP events.

Each reference event was used for a time period

ranging from days to weeks, until no events were

found that fit the coherence criteria for selection. When

the number of daily extracted events dropped to zero

for more than 2 days we selected another reference

event and tested an overlapping time period. In total,

we were able to extract N7000 coupled events between

March 13 and November 21, 2002, using the SP phase

and eight reference events (Table 1). We note that

although we have used eight reference events, this

does not reflect eight distinct event families, but rather

a continuously evolving source process.

It is important to note that the extraction algorithm

could be simplified by selecting events with high

correlation coefficients rather than spectral coheren-

cies, since coherence requires conversion to the

frequency domain. We chose to use the spectral

coherence for two reasons. First of all, the SP phase

of the coupled events typically has much smaller

amplitude than the LP phase. Consequently, we often

find the highest correlation coefficients when earth-

quakes are aligned with their LP rather than SP

portions in phase. Secondly, because the signals are

fairly monochromatic, they may be misaligned by a

unit number of periods, yet still have a high

correlation coefficient.

6. How similar are coupled events?

The coupled events discussed in this work were

selected because their SP phases were determined to

be similar to a reference event. Thus, each SP event is

known to have a coherence estimate of 0.9 relative to

one specific SP earthquake. This does not provide

information about how similar all SP phases in the

cluster are to one another, and how they compare to

events extracted using other reference events. Nor

does it provide information on the repeatability of the

LP phases of coupled events. Thus, we sought a

means by which the entire data set, or a generalized

subset thereof, could be examined.

As previously noted, Shishaldin coupled events do

not typically trigger the data acquisition system. Thus,

in order to investigate all of the coupled events at

Shishaldin, not just those that meet specific similarity

criteria, we needed to segment the data in a manner


that would identify all earthquakes rather than those

that meet similarity criteria. To this end we first

filtered the data between 3 and 8 Hz, to enhance the

SP phase of coupled events. A new short-term-

average/long-term-average (STA/LTA) ratio (STA

period=0.2 s, LTA period=5 s, ratio=4) was used on

the filtered data to select all events in that frequency

band occurring on a given day. With this algorithm we

were able to identify hundreds of SP events per day,

regardless of their similarity. Because the sheer

quantity of events made it impossible to perform this

function on all of the data, we selected a subset of

days for which signal-to-noise was strong and

identified all events for those days. We chose 1 day

for each time period covered by a reference event,

with the exception of the first time period, for which

we examined 2 days, and period 4 which overlaps

both periods 3 and 5 (Table 2). This gave us a subset

of data with which we could examine the similarity

between events from different time periods as well as

events from within the same time period.

Because spurious signals may be selected using

the STA/LTA algorithm, we examined all of the

segmented events and manually deleted any events

that were obviously not SP phases of coupled events.

Events that were identified by the algorithm included

calibration pulses, electronic spikes due to telemetry

problems and regional earthquakes, all of which

were removed from the data subset. Additionally, we

removed earthquakes that overlapped the coda of

previous events. Of 2847 events of all types initially

extracted, 1400 coupled events were selected for

further analysis.

Table 2

Number of events extracted on selected days using eight different referen

Date Time

period

Number of coupled

events (STA/LTA)

Ref

event 1

Ref

even

March 26 1 234 59 10

March 30 1 166 55 5

May 11 2 142 0 21

July 10 3 265 0 0

July 25 5 169 0 0

August 29 6 311 0 0

September 23 7 134 0 0

October 24 8 244 0 0

In general, events were extracted using only the reference event associated

given day were found to be similar to multiple reference events. Thi

simultaneously, or that events were similar to several reference events, re

One of the goals of this analysis was to evaluate our

decision to use separate SP reference events for each of

eight time periods (Table 1). To that end we calculated

the spectral coherence between each reference event

and all 1400 events in the data subset. In general, only

days in the time period originally assigned to a

reference event contained earthquakes selected by that

event. In other words, events on May 11, a day initially

assigned to time period 2, were selected only when

reference event 2 was used. No events occurring on

May 11 were selected using any other reference event.

Events in time periods 6 and 7 were found to be mostly

interchangeable, suggesting that the two time periods

comprise similar event types. The only other interesting

exception is that reference event 2 was found to be

highly similar (coherenceN0.9) to 15 events that

occurred during first time period. In contrast, no

earthquakes on the day selected from time period 2

were extracted using reference event 1. This suggests

that there may have been two distinct SP sources in the

initial time period, but that the second type became

dominant by late April. Overall, these results (Table 2)

confirm that the decision to use different reference

events in each time period was robust.

Next we examined all earthquakes in the data

subset to investigate their similarity. To that end we

calculated the coherency between the SP phases of all

1400 events in the data subset. To ensure that we are

looking at the similarity of the SP phase only, we

calculated the mean coherence in the frequency range

of the SP phase, between 4 and 7 Hz. The total results

and mean coherence values for each event are shown

in Fig. 3a. The matrix diagonal (running from the

ce events

t 2

Ref

event 3

Ref

event 4

Ref

event 5

Ref

event 6

Ref

event 7

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

56 11 0 0 0

0 17 0 0 0

0 0 75 93 0

0 0 8 18 2

0 0 0 0 14

with that day’s event group. In some instances, however, events on a

s suggests either that there were multiple event types occurring

flecting a gradually evolving source.

Fig. 3. Estimated spectral coherencies for (a) the SP phase and (b) the LP phase of 1400 coupled earthquakes occurring over the span of the time period of this study (March–November

2002). For the SP phase, the coherence was calculated for each event pair in the frequency band between 4 and 7 Hz for the 5 s around the P-wave. A frequency band of 0.5–3 Hz was

used to estimate the spectral coherence for the first 5 s of the LP phase. The scale for spectral coherence is shown to the right. Values lying on the diagonals represent the auto-coherence

between an event and itself and are everywhere equal to 1. Points close to the diagonal represent events that occur closely spaced in time while those farther from the diagonal are

separated by longer time periods. The days from which data were extracted are shown on the top of the figure. While each event exhibits a range of coherence values, the overall values

are high, typically exceeding 0.7 for SP phases and 0.6 for LP phases, indicating a high degree of similarity over 9 months. Also evident is the fact that for the SP phase, certain time

periods exhibit long-term similarity while others change more rapidly. Events occurring between March and May are highly similar to one another, as are events occurring between

August and October. This suggests a minor but long-term change in the SP source process. In contrast, the LP phase shows little temporal evolution.

J.Caplan-Auerb

ach,T.Petersen

/JournalofVolca

nologyandGeotherm

alResea

rch145(2005)151–172

158


lower left to upper right in Fig. 3a) shows the auto-

coherence for an event (the spectral coherence with

that event and itself) and is equal to 1. Values close to

the diagonal represent events that are closely spaced

in time, while values plotted farther from the diagonal

represent the coherence between events spanning

longer time scales. In all cases, coherence values are

relatively high (0.7–0.9; Fig. 3a), with highest values

clustering near the diagonal. This indicates that while

all SP events are fairly similar, events are more likely

to be similar if they occur over a short time scale when

source conditions have not changed significantly.

Furthermore, events that occurred between March

and May have high coherence values relative to one

another, as do events occurring between August and

October. There is a strong decrease in coherence when

events in the former group are compared with events

in the latter (Fig. 3a). In contrast, events extracted in

July bear strong similarity only to each other. This

indicates that there was a slight but significant change

in the SP phase between May and August 2002.

A similar test was performed for the LP phase of

the 1400 coupled events included in the SP analysis.

As with the SP phase, the LP portions of coupled

events were cross-correlated with one another and the

coherencies calculated for a 5-s window (Fig. 3b).

This analysis shows that overall, LP phases have

slightly lower correlation coefficients than the SP

phases for the same events, averaging 0.6–0.8.

Furthermore, LP phases do not show the same

temporal trends evident in the SP phase analysis.

This suggests that LP phases do not change signifi-

cantly over long time scales and may have a more

stable source mechanism.

7. Coupled event delay times

A benefit of identifying groups of events by

extracting similar events is that the events are

segmented such that the earthquakes are aligned in

time series. This method of viewing the activity

promotes identification and timing of different phases.

Examples of such a plot for a single day of coupled

earthquakes are shown in Fig. 4. This perspective

clearly shows that when the SP phases are aligned in

time series, the arrival times of the LP phases occur

anywhere from 3 to 15 s after the SP phase (Fig. 4). In

this article, the time period between the SP and LP

phases will be called the bSP–LP delay timeQ, or

simply the bdelay timeQ. At times the delay time is

found to vary dramatically over short time scales (Fig.

4a and b) while at other times the delay time is

relatively consistent (Fig. 4c and d). As previously

noted, when delay times change rapidly, there does

not appear to be a systematic timing change. In some

instances a change in delay time is followed by a long

sequence of events while in other cases, only a few

events occur before delay times change yet again. The

observed changes in delay time are visible at all

stations, suggesting that the delay results from a

source effect rather than one associated with site or

path. Finally, more distant stations exhibit slightly

longer delay times (Fig. 4). This timing difference

indicates that the two phases have different source

locations or different phase velocities.

To estimate delay times for all events we need to

identify the start of both phases. The SP phase onset is

easily determined as it is identified during extraction of

similar events. To determine the beginning of the LP

phase, we filtered the data between 0.5 and 4 Hz and

used an STA/LTA algorithm. Because the LP phase

often begins in the coda of the precursory SP phase, and

because the onset of LP activity is emergent, we

estimate a 2-s uncertainty in the actual delay time.

Although the actual delay times are approximate, the

general trend, that of wide variability at some times and

consistent delay times at others, is robust.

When delay time is plotted for the time period of the

study (March–November 2002; Fig. 5) we see that

there are time periods (e.g. late March) with highly

variable delay times, while at other times (e.g. late July,

November) the two phases occur at predictable

intervals. Furthermore, there are general trends in the

data: delay times are short in April and May but longer

between June and September. This suggests that the

mechanism acting between the two phases not only

changes on short time scales but occasionally stabilizes

at the maximum or minimum observed values.

To further investigate SP–LP delay times, we

plotted delay time against several parameters for

Shishaldin coupled events (Fig. 6). We found no

correlation between delay time and interevent time

(Fig. 6a). There is only a weak correlation between the

maximum amplitude of the SP phase and the delay

time (Fig. 6b); high amplitude SP phases correlate

0 5 10 15 20 25 30TIME (s)

ALL EVENTS BELOW, STACKED

ISTK:SHZ, 3/29/02

0 5 10 15 20 25 30TIME (s)


SSLN:SHZ, 3/29/02

0 5 10 15 20 25 30TIME (s)


ISTK:SHZ, 5/1/02

0 5 10 15 20 25 30TIME (s)


SSLN:SHZ, 5/1/02

a) b)

c) d)


04/01 05/01 06/01 07/01 08/01 09/01 10/01 11/010

5

10

15

20

SP

-LP

DE

LAY

(s)

DATE, 2002

Fig. 5. SP–LP delay time plotted against date for station SSLN. Some time periods, such as mid-March and mid-July exhibit wide variation in

delay time, while other time periods, such as early August and mid-November, show consistent delay times. In most cases the variability in

delay time is no more than 5–7 s.


with short delay times. However, at small delay times

the apparent maximum amplitude for SP events may

be influenced by overlapping signal from the larger

LP phase resulting in larger apparent amplitudes.

Although there appears to be a slight decrease in event

count with the time of day (Fig. 6c), this may be due

to an increase in noise resulting from afternoon winds

(occurring toward the end of the UTC day). Spectral

analysis of event occurrence times shows no obvious

peaks, thereby ruling out tidal or diurnal effects.

Coupled events occurred throughout the time period

from March to November 2002, suggesting that there

is no correlation between seismic activity and

precipitation. However, since there are no meteoro-

logical stations on Unimak Island, this lack of

correlation cannot be positively confirmed.

8. Relationship between LP phases and LP events

The LP phase of the coupled event bears strong

similarity to the LP events that occurred prior to March

13 and after November 21. The possibility therefore

Fig. 4. (a) Shishaldin coupled events occurring on March 29, 2002, record

from the continuous record using a common 5-s reference event taken from

extracted events, plotted according to their time of occurrence during the d

panels show stacks of all of the events plotted below. Although the SP phas

~15 seconds after the SP phase. (b) Shishaldin coupled events occurring on

variable timing of SP and LP phases is evident on both stations. (c) Shishal

represents a day in which the time between the SP and LP phases is relativ

Again, the timing of the two phases is consistent at all stations.

exists that the LP phase visible within coupled events

is the same as other LP events, with the only difference

being the SP trigger. Thus, to better understand the

coupled events, we investigate the relationship

between Shishaldin LPs and the LP phase of coupled

earthquakes. We can examine the LP signals by

comparing the time series of a btypicalQ LP event (that

is to say, unassociated with an SP precursor) and the

LP phase of a coupled event (Fig. 7a). In this format it

is evident that the two events are highly similar from

the onset time at 20 s until approximately 37 s into the

file. This similarity is visible at all stations, again

suggesting that the similarity is due to source rather

than path or site effects. When the data are bandpass-

filtered to enhance SP energy (Fig. 7b) it is clear that

only one of the LP events has an associated SP

precursor. These data suggest that the LP phase of

Shishaldin coupled events shares a common source

mechanism and location with other Shishaldin LPs; the

only difference between the two events is the presence

of the precursory SP phase for the coupled events.

As a final check as to the relationship between LP

and coupled events, we scanned through several days

ed at station SSLN, 6.5 km from the vent. All events were extracted

the SP phase of a coupled earthquake. The lower panels show the

ay (morning events plotted toward the top of the panel). The upper

es are aligned in time series, the LP phases occur anywhere from 3 to

March 29 2002, recorded at station ISTK, 17 km from the vent. The

din coupled events on April 30 2002, recorded at station SSLN. This

ely constant, at ~3 s. (d) The same events as (c), recorded at ISTK.

0

5

10

15

20

25

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

INTEREVENT TIME (s)

SP

-LP

DE

LAY

TIM

E (

s)

0 5 10 15 200

50

100

150

200

250

300

350

HOUR OF DAY

NU

MB

ER

OF

EV

EN

TS

SP

-LP

DE

LAY

TIM

E (

s)

SP PHASE MAX AMPLITUDE (cts)0 50 100 150 200 250 300

0

5

10

15

20

25

a)

b)

c)

Fig. 6. Representations of the SP–LP delay against various parameters associated with the coupled events. (a) Interevent time for repeating

coupled events plotted against delay time. (b) Maximum amplitude of the SP phase versus delay time. (c) Number of events that occurred during

each hour of the day. No strong correlation exists between delay time and interevent time. There are slightly fewer events towards the end of the

UTC day, corresponding to late afternoon Alaska time. This probably reflects an increase in background noise due to wind rather than an actual

decrease in event numbers. There is a very weak correlation between the amplitude of the SP phase and the delay time; high amplitude SP

phases occur for short delay times only.


0 5 10 15 20 25 30 35TIME (s)

0 5 10 15 20 25 30 35TIME (s)

-200

0

200

a)

b)

-50

0

50

Fig. 7. (a) Waveforms for an LP event at 10:31 UTC with no apparent SP precursor (blue), and the LP phase of a coupled event at 11:36 UTC

(red), both on November 21. The strong similarity between events suggests that they share a common source. b) The same waveforms shown

above, bandpass filtered between 4 and 8 Hz to enhance any SP precursory signal. The blue seismogram shows no noticeable increase in SP

energy prior to the LP phase, while the red event shows a clear SP signal between 5 and 10 s.


of data to estimate the ratio of bnormalQ LP events to

coupled events. Identifying LP events that are not

associated with SP precursors is difficult because SP

phases are small and may easily be masked by noise,

giving the event the appearance of a standard LP.

However, our estimated counts suggest that during the

time frame of this study (March–November 2002),

there were roughly twice as many coupled events as

LPs. In contrast, on January 3 2002, 183 LP events

were counted, compared to only 38 coupled events.

This activity occurred before the coupled events were

identified as a distinct event type. Intriguingly, we

note that when coupled events were dominant, the LP

phase of the coupled events were significantly larger

than plain LPs. Similarly, on January 3, when LP

events were among the largest noted at Shishaldin, the

amplitude of the SP phase of coupled events was so

small that they were not identified except in retro-

spect. Thus there appears to be a trade-off in energy

content; the bulk of the energy released by Shishal-

din’s seismicity resides in one event type at a time.

9. Earthquake locations

Shishaldin earthquakes are extremely difficult to

locate due to their small magnitude and the station

geometry described above. Initial locations of the

largest coupled and LP events place their hypocenters

at 0–3 km directly beneath Shishaldin’s summit

(Dixon et al., 2003), but hypocenters for the majority

of events cannot be calculated by traditional means.

We made use of the similarity of Shishaldin activity to

estimate locations of the smaller earthquakes.

Locating the SP phase of the coupled events first

required a means to enhance the first arrivals. To

that end, we began by stacking all of the events

selected with a single reference event. In some cases

this consisted of events spanning several weeks

while for other reference events we found similar

events occurring for only a few days (Table 1).

Following the removal of noisy traces, events for

each group were aligned by cross-correlation and

stacked at each station. P-wave arrival times were

picked for the stacks, and the events were located

using HYPOELLIPSE (Lahr, 1999). The velocity

model used for Shishaldin is composed of seven

constant-velocity layers and was originally derived

for Pavlof volcano (McNutt and Jacob, 1986). We

were unable to identify P-wave arrivals at station

BRPK (Fig. 1), even after stacking several hundred

earthquakes, so that station was not used in the

locations. As previously mentioned, station ISNN

suffers from a poor telemetry link, so for event

group 5 (Table 1) we found no usable traces at that

station.


Hypocentral locations for the stacked events are

shown in Fig. 8. Although the events are clearest at

station SSLN they occur just west of the summit,

midway between stations SSLN and SSLS. Depths are

very poorly constrained due to (a) the lack of reliable

S-phases and (b) the low number of stations recording

the activity, but the best fit locations are at shallow

(b4 km below the summit) depths. Thus, although we

do not know exact hypocentral depths, we take 4 km

to be the maximum likely depth for the source of the

SP phase. Note that the apparent bairquakesQ visible inFig. 8 (earthquakes whose depths are above regional

topography) are a consequence of the location

algorithm’s assumption of flat topography. These

events are obviously poorly located but indicative of

shallow depths.

Although the small amplitude of Shishaldin events

makes it impossible to pick reliable P-waves for

individual events, we were able to estimate arrival

times by cross-correlating each waveform with the

stacked events. The number of lags at maximum

correlation yields the number of samples by which

each event is offset relative to the stack and allows us

to pick an arrival time to within 0.01 s. Picks were

weighted according to the correlation coefficient, such

164o 10'

164o 10'

164o 00'

164o 00'

163o 50

163o 50

54o 40'

54o 50'

SSLS

SSLW

SSLN

a)

Fig. 8. (a) Approximate hypocentral locations for stacks of events extracte

hypocenters for 1370 coupled events, with P-wave arrivals selected by cros

with white triangles. (b) North–south cross section for the hypocenters sho

shallow portion of the edifice. Hypocenters that plot above ground are

topographic relief.

that events with correlations exceeding 0.8 were given

full weight, and those with correlations of b0.4 were

not used in the location. Of the ~7000 earthquakes

selected for this study, 1400 were located with

HYPOELLIPSE quality factors of A or B, indicating

68% confidence levels for horizontal and vertical

locations of b2.67 km (Lahr, 1999). Not surprisingly,

those locations cluster tightly in the same region as the

stacked events, on the west flank of Shishaldin, near

the summit. As with the stacked events, depths are

poorly constrained between 0 and 4 km. In all cases,

however, the events appear to be shallow and within

the Shishaldin edifice. While these hypocenters may

be considered suspect due to the low number of

stations, we note that in the vast majority of cases, the

best correlations for all stations were found with

offsets of less than 2 samples (0.02 s), requiring that

all of the events have their source within ~100 m of

each other. No temporal trends in earthquake locations

are evident.

We attempted the same stacking analysis on the LP

phase of the coupled earthquakes but were unsuc-

cessful in determining reliable hypocenters. Because

the LP phase occurs after the SP phase, first arrivals in

LP data were often difficult to identify. Furthermore,

'

'

ISNN

-3

-2

-1

0

1

2

3

54.7o 54.8o

54.7o 54.8o

-3

-2

-1

0

1

2

3

ELE

VA

TIO

N (km

)

b)

d with a common reference event (black stars) and the most reliable

s-correlation (gray circles). Short period seismic stations are marked

wn in panel (a). Depths are very poorly constrained but cluster in the

due to the fact that the location algorithm does not account for


even in stacked data, the onset of the event was too

emergent to accurately time first arrivals. Because of

the low-frequency nature of the event, a half-wave-

length uncertainty in first arrivals corresponds to a

long time interval (~0.5 s) and consequently a large

degree of location uncertainty. However, there is

reason to believe that, like the SP phase, the LP phase

of the coupled event has its source within Shishaldin’s

edifice. Firstly, delay times are slightly offset between

stations; an event with a 5-s delay time at station

SSLN (6.3 km from the vent) exhibits just over 5–6 s

of delay at SSLW (10 km from the vent) and 7–8-s of

delay at ISTK (17.1 km from the vent). This allows us

to conclude that the SP and LP sources have discrete

locations, since delay times would be consistent

across the network for co-located phases. The

relatively small move-out further suggests that the

two phases have sources near one another, or at least

within the immediate vicinity of Shishaldin. If one

source region lay significantly farther we would

expect to see larger move-out at the more distant

stations. Finally, conventional location of pure LP

events places them at shallow depths beneath the

summit (Dixon et al., 2003) and stacking analysis of

pure LP events yield similar hypocenters (Petersen et

al., 2004). If LP phases share a common source with

LP events, then they would also be located in the

shallow edifice.

0 5 10 15 20

TIME (

3/26/02, 03:07 UTC

3/26/02, 14:30 UTC

Fig. 9. Shishaldin coupled earthquakes recorded on March 26, 2002. Bot

correlation, although the SP–LP delay time varies by ~3 s. The SP phase

coherence between the LP phases at 0.5–3 Hz is 0.76.

10. A proposed model for Shishaldin coupled

earthquakes

Several parameters must be considered when

proposing a model for Shishaldin coupled earth-

quakes. The first is the coupled nature of the event;

each SP phase is followed by an LP phase, and no SP

events have been identified without a succeeding LP

phase. In contrast, LP events occur without the SP

precursor, and waveforms for some standard LPs are

virtually identical to the LP phase of Shishaldin

coupled events. Secondly, the repetitive nature of both

the SP and LP phases suggests that each represents a

stationary, non-destructive source process, although

there is more variability in the appearance of the LP

phases (Fig. 3b; Petersen et al., 2002). Nonetheless,

events with variable SP–LP delay times have been

identified for which both the SP and LP phases are

highly similar (Fig. 9). Thus, if both source regions

are fixed, then the variable timing between the two

phases must be caused by variations in the mechanism

that relates the two, i.e. the trigger mechanism. Next,

the prolonged nature of the SP–LP delays, between 3

and 15 s, requires a trigger mechanism that can

change speed by a factor of 5. And finally, since both

source regions are believed to lie within the Shishal-

din edifice, the trigger mechanism cannot travel more

than a few kilometers over the delay time, and

25 30 35 40

s)

h the SP and LP phases of these events exhibit a strong degree of

s have a spectral coherence between 4 and 7 Hz of 0.79 while the


therefore must move at slow speeds (b1 km/s). In fact,

if we take the maximum depth of the SP phase to be 4

km below the summit (the maximum depth calculated

for hypocentral locations), and assume that the LP

phase is shallower, we can place constraints on the

velocity of the triggering mechanism. The approx-

imate range in delay time of 3–15 s for 4 km spacing

yields a velocity range of approximately 270–1333 m/

s. If the SP phase has a shallower source, or the LP

source is located at greater depth, then these values

would be considerably lower.

We believe that both the SP and LP phases of

Shishaldin coupled events result from the presence of

fluids within the volcano. This is consistent with the

low frequency of these signals as well as the

repeatability of the source. Furthermore, the plume

at Shishaldin’s summit confirms that gas is constantly

escaping the volcano, although the relative contribu-

tions of magmatic and meteoric gas are unknown.

While no correlation has been confirmed between

seismicity and the puffing steam plume, the observed

timing between successive gas puffs (1–3 min) is

similar to the timing between LP events recorded by

the seismic network. Furthermore, the fact that the

plume comes in bpuffsQ indicates that gas is releasedin discrete bursts. Finally, the installation of a more

sensitive pressure sensor in 2003 allowed us to

correlate LP events at Shishaldin with infrasonic

signals (Petersen et al., 2004). Although a detailed

investigation of this relationship is beyond the scope

of this article, this correlation encourages us to assume

a shallow, gas-related source for Shishaldin LPs and

the LP phase of coupled events. Consequently, all the

models we consider are based upon a volcano with

two fluid-based sources.

There are many means by which fluids, and gas in

particular, may generate seismic signals. One involves

the sudden coalescence of foam into a single gas slug.

This mechanism has been observed in laboratory

experiments (Jaupart and Vergniolle, 1988, 1989) and

has been invoked as a source of LP seismicity (Ripepe

and Gordeev, 1999). When the slug reaches the top of

the fluid it oscillates prior to bursting, inducing

seismic and infrasonic signals (Vergniolle and Bran-

deis, 1994, 1996; Gil Cruz and Chouet, 1997;

Vergniolle et al., 2004). Alternatively, LP events

could be due to explosions resulting from magma–

water, or fuel–coolant, interactions (Wohletz, 1986,

2002; Buttner and Zimanowski, 1998). An additional

means by which fluid may generate seismic signals is

by promoting oscillation of a crack or reservoir

(Chouet, 1985, 1988; Julian, 1994; Garces, 1997).

Conduit vibration may also be induced by choked

flow at obstructions (Julian, 1994; Morrissey and

Chouet, 1997) or by turbulent flow (Hellweg, 2000).

Finally, fluid transfer through volcanic conduits has

been invoked to explain LP events and tremor at a

number of volcanoes, including Kilauea (Aki et al.,

1977; Almendros et al., 2001), Lascar (Hellweg,

2000) and Sakurajima (Honda and Yomogida, 1993).

We first considered a model in which gas bubbles

coalesce into a slug at a deep boundary and rise under

their own buoyancy through magma or hydrothermal

fluid. The SP phase of the coupled event is produced

when the bubbles coalesce at depth. At some point as

it rises through the conduit, the slug enters a second

fluid body, perhaps a magma reservoir or crack,

initiating the LP signal. This signal may result from

the vibration of the reservoir walls or from the

oscillation of the bubble itself. Thus the trigger event

between the two phases of the coupled earthquake is a

mechanical one, the rising of the slug itself. In this

model we hypothesize that variations in SP–LP delay

time result from changes in the rise speed of slugs at

depth due to slug radius or fluid density and viscosity.

There are several reasons to immediately discount

such a model. If the SP phase is both the trigger and

the source for the LP phase, then the LP events cannot

occur without the precursor. Yet the LP signals are

observed to occur before, coincident with and after the

time period in which coupled events were dominant.

Thus it is more likely that the two phases have

independent sources, but that the LP phase can be

triggered into activity by the SP phase. Secondly, even

for large slugs in dense fluid, the rise velocity is only a

few m/s. For coupled events in which the LP phase

follows immediately after the SP phase, this would

require the SP and LP reservoirs to be within a few

tens of meters of each other. It seems to be highly

unlikely that two discrete reservoirs could coexist at

such proximity for months without merging. Sec-

ondly, bubbles coalescing in a reservoir tend to form a

slug which fills the conduit through which it rises;

variable radii are not observed in laboratory models

(Jaupart and Vergniolle, 1989; Vergniolle et al., 1996).

Finally, we have shown that velocities for the

SP SIGNAL

LP SIGNAL

Fig. 10. A proposed model for Shishaldin coupled earthquakes

Fluid moving within the Shishaldin conduit encounters an obstacle

on the conduit wall. The restricted flow promotes oscillations of the

conduit walls, producing the SP phase. Seismic energy from the SP

signal propagates through the fluid at a rate dependent on the void

fraction in the fluid. The wave encounters a region with high void

fraction near the top of the conduit and induces bubble coalescence

and the LP seismic signal.


triggering mechanism are likely to be as high as

several hundred m/s, a speed that is unattainable by a

rising gas slug in magma.

In our preferred model, the trigger mechanism is

seismic energy generated by the precursory SP event,

rather than mechanical transport. In this model, the

velocity of the trigger mechanism depends on the

acoustic velocity of the material through which the

wave propagates; in this case, the conduit fluid. We

will show that small changes in gas content are

sufficient to explain the observed variability in delay

times.

We propose that coupled events initiate when a

pressure transient within the conduit fluid creates the

SP signal. Julian (1994) has shown that a small

perturbation to steady-state flow within a conduit can

result in transient seismic events. While many of the

source mechanisms described above can also explain

a signal such as the SP phase, a key parameter

constraining the SP source is the fact that the

similarity evident in SP waveforms is independent

of the observed delay time. This rules out any source

mechanism associated with conduit resonance, as

crack wave velocities are dependent on the acoustic

wave speed of the fluid (Chouet, 1988, 1996; Gil Cruz

and Chouet, 1997). Because many fundamental

parameters of the SP phase, such as source depth

and regional material properties, remain unknown, an

investigation of the SP power spectrum is beyond the

scope of this study.

The same seismic energy recorded by the seis-

mometers as the SP phase also travels within the

volcano itself to a shallow portion of the conduit or

reservoir where it triggers the LP source. There are

two mechanisms that we considered for the LP source:

magma–water interactions (Wohletz, 1986; Buttner

and Zimanowski, 1998) and bubble coalescence

(Jaupart and Vergniolle, 1988, 1989). In the former

case, seismic waves impinging on a magma–water

boundary collapse an insulating film layer at the

boundary, causing superheated water to flash to steam.

In the latter model, SP phase energy passes through a

shallow layer of bubbles or foam (Fig. 10), causing

the bubbles to decompress and rapidly coalesce into a

bubble slug. The slug coalescence generates an LP

signal and releases a pulse of gas. Both models can

generate LP events with or without a triggering event:

Superheated water can flash to steam without a trigger

.

if it continues to heat past a critical temperature

(Wohletz, 2002) and a foam layer that grows beyond a

critical thickness will likewise collapse on its own

(Jaupart and Vergniolle, 1988, 1989).

While both models are reasonable for Shishaldin,

we prefer the bubble coalescence model. Magma–

water interactions can generate instabilities at the

magma surface, promoting magma break-up and an

ensuing explosive eruption (Wohletz, 1986, 2002),

which was not observed at Shishaldin in 2002. The

ability of the volcano to continually supply water that

is rapidly superheated is also questionable. Finally,

there is no evidence of shallow magma in the

Shishaldin conduit. In the case of the bubble

coalescence model, there is a large supply of gas

available in the form of volcanic and hydrothermal

gases, and shallow magma is not required. In the

models presented here, we assume the presence of

magma but discuss the consequences for the case of

other fluids.

To explain the variable SP–LP delay timing in this

model, we seek a means by which the velocity of


seismic energy can be dramatically changed over

short time periods. We invoke changes in fluid void

fraction as a means of accelerating or decelerating the

trigger mechanism. After Gil Cruz and Chouet (1997)

we calculate am, the velocity of a pressure wave in a

fluid according to

1

am¼ 1

alþ

ffiffiffiffiffiffiffiffiffiffiqmegcmP

rð1Þ

(Hsieh and Plesset, 1961; Gil Cruz and Chouet, 1997).

The fluid velocity is given by al, P represents the

pressure in the reservoir, and eg is the void fraction of

the fluid. qm, the density of the gas–liquid mixture,

depends on the void fraction eg and liquid and gas

densities ql and qg according to,

qm ¼ egqg þ 1� eg� �

ql ð2Þ

Because these values have not been measured

precisely at Shishaldin, we use commonly accepted

values for gas and basalt and take ql to be 2700 kg/m3

(Williams and McBirney, 1979) and qg to be 1 kg/m3.

The seismic velocity of the liquid magma is estimated

0 0.1 0.20

500

1000

1500

VOID FR

P-W

AV

E V

ELO

CIT

Y (

m/s

)

εg= 0

εg= 0.0004

εg= 0.037

εg= 0.042

INC

REA

SIN

G P

RES

SUR

E

0

Fig. 11. Velocity of seismic waves through gas-rich basaltic magma. Veloc

pressure within the fluid. Thin solid curves represent values calculated at

solid line represents a pressure of 10 MPa for a magma-filled conduit. Th

water to a depth of 4 km, resulting in static pressure of ~4 MPa. White ci

maximum and minimum velocities estimated for the coupled event trigge

as 2500 m/s. The isentropic coefficient of the mixture

cm depends on the specific heats of the gas and liquid

as well as the mass ratio of gas to liquid. Because

these values are not known specifically for Shishaldin

we follow Gil Cruz and Chouet (1997) and take the

value of cm to be 1.0066.

Seismic velocity in low-viscosity fluid is strongly

dependent on void fraction qg, with only a small

amount of gas dropping the velocity far below that of

either pure liquid or pure gas. In Fig. 11 we show

seismic velocity for 2-phase fluids with a range of

void fractions and reservoir pressures. Because the

composition of the fluid within the Shishaldin conduit

is unknown, we cannot precisely determine the

pressure within the conduit. We can, however,

estimate the regional pressure for the endmember

cases of a conduit filled with magma (q=2700 kg/m3)

and one filled with water (q=1000 kg/m3). We

estimate the pressure as P=qgh where q is fluid

density and is related to void fraction according to Eq.

(2) above, g is the acceleration due to gravity and h is

the depth within the conduit, between 0 and 4 km

below the summit, according to the hypocentral

0.4 0.5 0.6

ACTION εg

P = 1 MPa

P = 100 MPa

.3

ity is dependent on the fluid void fraction qg as well as the ambient

pressures between 1 and 100 MPa, at 10 MPa intervals. The heavy

e heavy dashed line represents the same conduit filled with gas-rich

rcles mark the gas velocities of 270 and 1300 m/s, the approximate

r mechanism.


depths of The SP phase. These values give us an

approximate pressure range of 5–100 MPa. All of

these values decrease, of course, if we consider the

presence of gas within the fluid. While we cannot

determine the precise pressure within the conduit,

these values allow us to investigate the effects of on

seismic velocity at pressures that are reasonable for

the conditions (Fig. 11).

The values shown in Fig. 11 confirm that only a

small change in qg is required to explain the

observed variability in delay time. For example, a

magma at 10 MPa (heavy solid line in Fig. 11)

experiences a 5-fold decrease in seismic velocity for

a change in eg of 0.0004–0.042. More importantly,

the predicted seismic velocities for this range of egare between 270 and 1300 m/s, the precise range of

velocities required to explain the observed delay

times for an SP–LP source separation of 4 km. The

results are similar for an identical water filled

conduit, with slightly lower values for eg. At 4 km

depth, pressure in a water-filled conduit is ~0.39

MPa (heavy dashed line in Fig. 11) and velocity is

found to range between 270 and 1300 m/s for egbetween 0 and 0.037 (Fig. 11).

The relationship described above, in which

increased gas content results in increased SP–LP

delay time, causes us to wonder if there might be a

relationship between the interval between successive

seismic events and the delay time. In the bubble

coalescence model, long delay times indicate that

more gas is available to generate bubble layers or

foam, which would presumably result in more LP

events. No correlation between interevent time and

delay time was initially visible in our data (Fig. 8a).

However, the interevent times presented in Fig. 8a are

for SP phases only. To compare delay times to the

total number of events, we selected 2 days and

counted all earthquakes regardless of event type. On

May 11, a day when delay times were low (3–5 s),

indicating low gas content, we counted a total of 368

events. In contrast, we counted 957 earthquakes on

July 25, when high delay times (10–12 s) suggest high

gas content. This fact supports the bubble coalescence

model for the LP phase.

As previously noted, when coupled earthquakes

are the predominant event type, pure LP events appear

to be of low amplitude. This model may indicate that

the SP phase forces overpressurized bubbles to

coalesce. Ripepe and Gordeev (1999) have shown

that bubbles layers that coalesce freely do so when the

hydrostatic pressure has diminished beyond a critical

value. Bubbles that are forced to coalesce, however,

exhibit larger overpressure and generate stronger

seismic waves.

11. Demise of the coupled events

The last day on which the coupled earthquakes

were dominant at Shishaldin was November 21,

2002. We find coupled events occurring until

approximately 09:15 UTC, after which the events

appear to be normal LP earthquakes. No other

unusual features, such as VT earthquakes or thermal

anomalies, were observed at the time, and the overall

occurrence rate of earthquakes appears unchanged.

This suggests that the source of the SP phase was

destroyed rapidly, and that the LP phase was

unaffected by the change. The most likely explan-

ation is that the conduit obstruction that caused the

SP phase was destroyed on November 21, although

no associated seismic signal was observed. LP

activity continued without trigger activity after the

coupled events ended. The total number of events at

Shishaldin remained similar following the cessation

of the SP phase.

12. Discussion

The model proposed here indicates variations in

SP–LP delay time are likely due to changes in gas

content within the conduit. Events such as these can

therefore provide insight into the amount of gas within

the system and hence the state of unrest of the

volcano. Because Shishaldin is not visible from any

populated area, and because little is known about gas

flux at the volcano, events such as the coupled

earthquakes could provide a rare window into changes

in gas content. At present, the conduit system at

Shishaldin appears to be open, allowing constant

degassing, so increases in gas flux may not indicate a

hazardous change in the volcano’s plumbing system.

In 1931, however, Hubbard (1935) reported that

Shishaldin’s summit crater was filled with a solidified

lava lake, indicating that it has the capacity of shutting


and blocking gas release. At a volcano with a closed

system, events such as the coupled earthquakes could

be used as an indicator of gas content for use in hazard

evaluations.

As previously noted, there are times when SP–LP

delay times vary significantly over short time scales

(Figs. 4 and 5). More commonly, however, the LP

phase seems to follow the SP phase within a few

seconds of a predictable time (Figs. 4 and 5). This

suggests that gas flux and gas content at Shishaldin

usually exhibit relatively steady state behaviour, but

occasionally fluctuate rapidly. There appear to be

times (e.g. near June 1; Fig. 5), in which delay

times suddenly increase markedly. This reflects the

addition of new gas into the system. The sharp

change from short delay times in April–June to

consistently long delay times evident between June

and October may indicate that a new fracture

opened through which gas could enter the system.

Increased gas content may be expected to correlate

with increased gas flux at the vent, but no

observations exist to evaluate this relationship.

Periodic sealing of the crack could regulate delay

times over short periods.

The presence of the SP phase is the key feature

that defines a coupled earthquake. If this phase does

indeed result from an obstruction in the conduit or

other such feature, then it may be a simple case of

good fortune that these signals occurred such that

they could be identified and studied. However,

earthquakes similar to Shishaldin coupled events

have been noted on a handful of other volcanoes

(Galeras, Gil Cruz and Chouet, 1997; Popocatepetl,

R. White, personal communication, 2003; Stromboli,

J. Lees, personal communication, 2002; Deception

Island, Ibanez et al., 2003; Koryaksi, Gordeev and

Senyukov, 2003), so they may be more common than

has been previously noted. In other instances, the

association between the two phases may not be

noted, possibly due to the small amplitude of the SP

phase. The increased popularity of cross-correlation

techniques in the study of volcanic seismicity (e.g.

Gillard et al., 1996; Lees, 1998; Aster and Rowe,

2000; Wolfe et al., 2003; Rowe et al., 2003; Battaglia

et al., 2004) may promote the identification of

similar types of events. We are therefore optimistic

that this type of locally triggered seismicity may be

found in other seismically active regions. Events

such as these could provide a telling view on the

internal structure of active volcanoes and a new

means by which gas content can be remotely and

safely monitored.

Acknowledgments

We are grateful to our colleagues at the Alaska

Volcano Observatory, notably S.R. McNutt, for

helpful discussions regarding Shishaldin seismicity.

The work was greatly improved following helpful

reviews by Timothy Horscroft, Eisuke Fujita and an

anonymous reviewer. The manuscript further bene-

fited from discussion with R. White and S. Prejean

of the U.S.G.S. Several figures were produced

using the Generic Mapping Tools software by

Wessel and Smith (1998).

References

Aki, K., Fehler, M., Das, S., 1977. Source mechanism of

volcanic tremor: fluid-driven crack models and their applica-

tion to the 1963 Kilauea eruption. J. Volcanol. Geotherm.

Res. 86, 7095–7110.

Almendros, J., Chouet, B.A., Dawson, P.B., 2001. Spatial extent of

a hydrothermal system at Kilauea Volcano, Hawaii, determined

from array analyses of shallow long-period seismicity: 2.

Results. J. Geophys. Res. 106, 13581–13597.

Aster, R., Rowe, C., 2000. Automatic phase pick refinement and

similar event association in large seismic datasets. In:

Thurber, C.H., Rabinowitz, N. (Eds.), Advances in Seismic

Event Location. Kluwer Academic Publishers, the Nether-

lands, pp. 231–263.

Battaglia, J., Thurber, C.H., Got, J.-L., Rowe, C.A., White, R.A.,

2004. Precise relocation of earthquakes following the 15 June

1991 eruption of Mount Pinatubo (Philippines). J. Geophys.

Res. 109, B07302.

Beget, J., Nye, C., Stelling, P., 1998. Postglacial collapse and

regrowth of Shishaldin Volcano, Alaska, requires high eruption

rates. EOS Trans. 70, S359.

Buttner, R., Zimanowski, B., 1998. Physics of thermohydraulic

explosions. Phys. Rev., E 57, 5726–5729.

Caplan-Auerbach, J., McNutt, S.R., 2003. New insights into the

1999 eruption of Shishaldin Volcano, Alaska, based on acoustic

data. Bull. Volcanol. 65 (6), 405–417.

Chouet, B.A., 1985. Excitation of a buried magmatic pipe: a

seismic source model for volcanic tremor. J. Geophys. Res.

90, 1881–1893.

Chouet, B.A., 1988. Resonance of a fluid-driven crack: radiation

properties and implications for the source of long-period events

and harmonic tremor. J. Geophys. Res. 93, 4373–4400.


Chouet, B.A., 1996. Long-period volcano seismicity: its source and

use in eruption forecasting. Nature 380, 309–316.

Chouet, B.A., 2003. Volcano seismology. Pure Appl. Geophys. 160,

739–788.

Dixon, J.P., Stihler, S.D., Power, J.A., Tytgat, G., Estes, S., Moran,

S.C., Paskievitch, J., McNutt, S.R., 2003. Catalog of earthquake

hypocenters at Alaskan volcanoes: January 1, 2002 through

December 31, 2002: U.S. Geological Survey Open-File Report

OF 03_267, 58 pp.

Fischer, T.P., Morrisey, M.M., Calvache, M.L., Gomez, D., Torres,

R., Stix, J., Williams, S.N., 1994. Correlations between SO2 flux

and long-period seismicity at Galeras volcano. Nature 368,

135–137.

Fournelle, J., Marsh, B., 1991. Shishaldin Volcano; Aleutian high-

alumina basalts and the question of plagioclase accumulation.

Geology 19, 234–237.

Garces, M., 1997. On the volcanic waveguide. J. Geophys. Res. 102

(10), 22547–22564.

Gil Cruz, F., Chouet, B.A., 1997. Long-period events, the most

characteristic seismicity accompanying the emplacement and

extrusion of a lava dome in Galeras Volcano, Colombia, in

1991. J. Volcanol. Geotherm. Res. 77, 121–158.

Gillard, D., Rubin, A.M., Okubo, P., 1996. Highly concentrated

seismicity caused by deformation of Kilauea’s deep magma

system. Nature 384, 343–346.

Gordeev, E.I., Senyukov, S.L., 2003. Seismic activity at Koryaksi

volcano in 1994: hybrid seismic events and their implications

for forecasting volcanic activity. J. Volcanol. Geotherm. Res.

128, 225–232.

Hellweg, M., 2000. Physical models for the source of Lascar’s

harmonic tremor. J. Volcanol. Geotherm. Res. 101, 183–198.

Honda, S., Yomogida, K., 1993. Periodic magma movement in the

conduit with a barrier: a model for the volcano tremor. Geophys.

Res. Lett. 20 (2), 229–232.

Hsieh, D.Y., Plesset, M.S., 1961. Theory of rectified diffusion of

mass into gas bubbles. J. Acoust. Soc. Am. 33, 206–215.

Hubbard, B.R., 1935. Cradle of the Storms. Dodd, Mead, New

York. 285 pp.

Ibanez, J.M., Carmona, E., Almendros, J., Saccorotti, G., Del Pezzo,

E., Abril, M., Ortiz, R., 2003. The 1998–1999 seismic series at

Deception Island volcano, Antarctica. J. Volcanol. Geotherm.

Res. 128, 65–88.

Jaupart, C., Vergniolle, S., 1988. Laboratory models of Hawaiian

and Strombolian eruptions. Nature 331, 58–60.

Jaupart, C., Vergniolle, S., 1989. The generation and collapse of a

foam layer at the roof of a basaltic magma chamber. J. Fluid

Mech. 203, 347–380.

Julian, B., 1994. Volcanic tremor: nonlinear excitation by fluid flow.

J. Geophys. Res. 99 (6), 11859–11877.

Klein, F.W., Koyanagi, R.Y., Nakata, J.S., Tanigawa, W.R., 1987.

The seismicity of Kilauea’s magma system. In: Decker, R.W.,

Wright, T.L., Stauffer, P.H. (Eds.), Volcanism in Hawaii, U. S.

Geol. Surv. Profess. Pap., vol. 1350, pp. 1019–1185.

Lahr, J.C., 1999. HYPOELLIPSE: a computer program for

determining local earthquake hypocentral parameters, magni-

tude, and first-motion pattern (Y2K compliant version). U.S.

Geological Survey Open-File 99-23.

Lahr, J.C., Chouet, B.A., Stephens, C.D., Power, J.A., Page, R.A.,

1994. Earthquake classification, location, and error analysis in a

volcanic environment: implications for the magmatic system of

the 1989–1990 eruptions at Redoubt Volcano, Alaska. J.

Volcanol. Geotherm. Res. 62, 137–151.

Lees, J., 1998. Multiplet analysis at Coso Geothermal. Bull.

Seismol. Soc. Am. 88 (5), 1127–1143.

McNutt, S.R., 2000. In: Sigurdsson, H., Houghton, B., McNutt,

S.R., Rymer, H., Stix, J. (Eds.), Volcanic Seismicity, Chapter

63 of Encyclopedia of Volcanoes. Academic Press, San Diego,

pp. 1015–1033.

McNutt, S.R., Jacob, K.H., 1986. Determination of large-scale

velocity structure of the crust and upper mantle in the vicinity of

Pavlof volcano, Alaska. J. Geophys. Res. 91, 5013–5022.

Miller, T.P., McGimsey, R.G., Richter, D.H., Riehle, J.R., Nye, C.J.,

Yount, M.E., Dumoulin, J.A., 1998. Catalog of the historically

active volcanoes of Alaska. U.S. Geological Survey Open-File

Report 98-482, 104 pp.

Minakami, T., 1960. Fundamental research for predicting volcanic

eruptions (I)-earthquakes and crustal deformations originating

from volcanic activities. Bull. Earthq. Res. Inst. Univ. Tokyo 38,

497–544.

Moran, S., 2003. Multiple seismogenic processes for high-

frequency earthquakes at Katmai National Park, Alaska:

evidence from stress tensor inversions of fault-plane solutions.

Bull. Seismol. Soc. Am. 93 (1), 94–108.

Moran, S.C., Stihler, S.D., Power, J.A., 2002. A tectonic earthquake

sequence preceding the April–May 1999 eruption of Shishaldin

Volcano, Alaska. Bull. Volcanol. 64, 520–524.

Morrissey, M.M., Chouet, B.A., 1997. A numerical investigation of

choked flow dynamics and its application to the triggering

mechanism of long-period events at Redoubt Volcano, Alaska. J.

Geophys. Res. 102, 7965–7983.

Neuberg, J., Baptie, B., Luckett, R., Stewart, R., 1998. Results from

the broadband seismic network on Montserrat. Geophys. Res.

Lett. 25 (19), 3661–3664.

Neuberg, J., Luckett, R., Baptie, B., Olsen, K., 2000. Models of

tremor and low-frequency earthquake swarms on Montserrat. J.

Volcanol. Geotherm. Res. 101, 83–104.

Nye, C.J., Keith, T., Eichelberger, J.C., Miller, T.P., McNutt, S.R.,

Moran, S.C., Schneider, D.J., Dehn, J., Schaefer, J.R., 2002. The

1999 eruption of Shishaldin Volcano, Alaska: monitoring a

distant eruption. Bull. Volcanol. 64, 507–519.

Petersen, T., Caplan-Auerbach, J., McNutt, S.R., 2002. Temporal

distribution and rates of repetitive low-frequency earthquakes at

Shishaldin Volcano, Alaska. Eos Trans.-Am. Geophys. Union

83 (47) (Fall Meet. Suppl., Abstract V21A-1173).

Petersen, T., Caplan-Auerbach, J., McNutt, S.R., 2004. The

continuously high level of long-period seismicity at Shishaldin

Volcano, Unimak Island, Alaska, 1999–2004, abstract, IAVCEI

General Assembly, November, 2004.

Ripepe, M., Gordeev, E., 1999. Gas bubble dynamics model for

shallow volcanic tremor at Stromboli. J. Geophys. Res. 104 (5),

10639–10654.

Rowe, C.A., White, R.A., Thurber, C.H., White, R.A., 2003.

Precise, correlation-based seismic event locations at Sourfriere

Hills volcano: insights into magma extrusion behavior through


detailed mapping of seismic energy release. Seismol. Res. Lett.

74 (2), 214.

Stephens, C.D., Chouet, B.A., 2001. Evolution of the December 14,

1898 precursory long-period event swarm at Redoubt Volcano,

Alaska. J. Volcanol. Geotherm. Res. 109, 133–148.

Thompson, G., McNutt, S.R., Tytgat, G., 2002. Three distinct

regimes of volcanic tremor associated with the eruption of

Shishaldin Volcano, Alaska 1999. Bull. Volcanol. 64, 535–547.

Vergniolle, S., Brandeis, G., 1994. Origin of the sound generated

by Strombolian explosions. Geophys. Res. Lett. 21 (18),

1959–1962.

Vergniolle, S., Brandeis, G., 1996. Strombolian explosions: 1. A

large bubble breaking at the surface of a lava column as a source

of sound. J. Geophys. Res. 101 (9), 20433–20448.

Vergniolle, S., Brandeis, G., Mareschal, J.-C., 1996. Stromblian

explosions: 2. Eruption dynamics determined from acoustic

measurements. J. Geophys. Res. 101, 20449–20466.

Vergniolle, S., Boichu, M., Caplan-Auerbach, J., 2004. Acoustic

measurements of the 1999 basaltic eruption of Shishaldin

Volcano, Alaska: 1. Origin of Strombolian activity. J. Volcanol.

Geotherm. Res. 137 (1–3), 109–134.

Wessel, P., Smith, W.H.F., 1998. New, improved version of the

Generic Mapping Tools released. EOS Trans.-Am. Geophys.

Union 79, 579.

White, R.A., Miller, A.D., Lynch, L., Power, J., 1998. Observations

of hybrid seismic events at Soufriere Hills volcano, Montserrat:

July 1995 to September 1996. Geophys. Res. Lett. 25 (19),

3657–3660.

Williams, H., McBirney, A.R., 1979. Volcanology. Freeman,

Cooper. 397 pp.

Wohletz, K., 1986. Explosive magma–water interactions: thermo-

dynamics, explosion mechanisms, and field studies. Bull.

Volcanol. 48, 245–264.

Wohletz, K., 2002. Water/magma interaction: some theory and

experiments on peperite formation. J. Volcanol. Geotherm. Res.

114, 19–35.

Wolfe, C.J., Okubo, P.G., Shearer, P.M., 2003. Mantle fault zone

beneath Kilauea volcano, Hawaii. Science 300, 478–480.

repeating coupled earthquakes at shishaldin volcano,...

Documents