dps thesis etd ffe - ncsu

ABSTRACT

SPRINKLE, DALE P. Analysis of Microearthquake Sequences from the 2015 Eruption of Axial Seamount using Ocean Bottom Seismometer Data. (Under the direction of Dr. Delwayne Bohnenstiehl). In April and May of 2015, an eruption occurred within the summit caldera and along the northern rift

zone of Axial Seamount on the Juan de Fuca Ridge. Seven tri-axial seismometers in the Ocean

Observatories Initiative’s Cabled Array, a sensor suite that provides a continuous stream of near real-time

data from the volcano, recorded more than 107,000 microearthquakes over the period before, during and

after the eruption (Jan. 2015 to Feb. 2017). Using a combined space-time distance metric, a hierarchal

clustering routine identified 99 earthquake sequences comprised of ~20,000 events in total. Hypocenters

within these sequences are distributed entirely within two-outward dipping planes of seismicity associated

with the volcano’s shallow ring fault system. Approaching the eruption, earthquake sequences increase in

frequency and show temporal clustering. The largest sequence, containing more than 4,000 earthquakes,

initiates just after the onset of the 2015 eruption. The frequency-magnitude distributions of earthquakes

within the sequences are similar to those within the larger catalog, and their power-law exponents (b-

values) do not vary systematically leading up to the eruption. Magnitude-time and event-time patterns

suggest that almost 90% of the sequences can be described as swarms, with only a handful of sequences

representing clear mainshock-aftershock or foreshock-mainshock-aftershock sequences. Preceding the

eruption, roughly half of the earthquake sequences exhibit significant hypocenter migration within the

plane of the ring fault at a range of rake angles. Migration rates vary from 0.03-0.785 km/hr over a

median distance of 0.6 ± 0.08 km. In contrast, during the eruption, sequence hypocenters are distributed

along the entire length of the eastern limb of the ring fault, a distance of ~ 6.0 km. In the months

preceding the eruption, sequences initiate and terminate primarily during encouraging ocean tidal stress

regimes (around low tide), but increasingly span larger tidal ranges in the six weeks preceding the

eruption. Although the eruption episode lasted for ~ 1 month, no earthquake sequences were identified

after the first day of eruptive activity, and no sequences were identified during the post-eruptive period.

Analysis of Microearthquake Sequences from the 2015 Eruption of Axial Seamount using Ocean Bottom Seismometer Data

by Dale P Sprinkle

A thesis submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the degree of

Master of Science

Marine, Earth, and Atmospheric Sciences

Raleigh, North Carolina 2018

APPROVED BY:

_______________________________ _______________________________ DelWayne Bohnenstiehl Karl Wegmann Committee Chair _______________________________ Paul Byrne

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DEDICATION

To Irena, You are my guiding light, the air in my lungs and the wind at my back.

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BIOGRAPHY

Dale Parker Sprinkle II was born on 8 April 1983 in Greensboro, North Carolina to Dale and Ann

Sprinkle. After graduating from Northwest Guilford High School in May of 2001, Parker attended North

Carolina State University and took a B.S. in Geology from the College of Physical and Mathematical

Sciences in 2009. Parker went on to work professionally in near-surface applied geophysics from 2009 to

2015. In January of 2015 he returned to graduate school at North Carolina State University to study

Seismology. On 28 August 2018 he orally defended an Earth Science based thesis concentrating in marine

geophysics and seismology. After successfully defending the thesis Parker continued his research career

as a Research Scientist at Pacific Northwest National Laboratory in Richland, Washington.

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TABLE OF CONTENTS

LIST OF FIGURES ...................................................................................................................................... v

Section 1: Introduction ............................................................................................................................ 1

Axial Seamount ............................................................................................................................................ 1

Earthquake Sequences .................................................................................................................................. 3

Section 2: Methods ..................................................................................................................................... 5

Creation of an Earthquake Catalog ................................................................................................. 5

Analysis of Earthquake Sequences ................................................................................................. 6

Defining Sequences ............................................................................................................ 6

Migration Rates .................................................................................................................. 7

Size-Frequency Analysis .................................................................................................... 8

Tidal Analysis .................................................................................................................... 9

Section 3: Results ........................................................................................................................................ 9

Sequence Catalog Statistics ............................................................................................................. 9

Mogi Sequence Types ................................................................................................................... 11

Hypocentral Migration .................................................................................................................. 11

Tidal Cycle Correlations ............................................................................................................... 12

Frequency-Magnitude Distributions ............................................................................................. 13

Spatio-Temporal Extent ................................................................................................................ 13

Section 4: Discussion ................................................................................................................................ 14

Section 5: Conclusions ............................................................................................................................. 18

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LIST OF FIGURES

Figure 1 Bathymetry of Axial Seamount with earthquake locations (grey dots), seismic stations (triangles), hydrothermal fields (stars). 2015 lava flow boundaries outlined in red

(Chadwick et al., 2016), while the magma chamber margins from active seismic imaging are shown by the black, dashed line (Arnulf et al., 2014). Inset map shows the

location of the seamount along the Juan de Fuca Ridge in the eastern Pacific Ocean. ASHES, CASM, and ID 1 (International District 1) denote the location of

high-temperature vent fields along the margins of the caldera ............................................. 25 Figure 2 Number of events versus origin time displaying three types of earthquake sequences (after Mogi, 1963). ................................................................................................................ 26 Figure 3a Locations of sequence events (red dots) superimposed onto the complete seismicity catalog (black dots) showing that sequence earthquakes lie within the same plane identified by Levy et al., 2018. The long-axis of the caldera has a strike orientation of

N28Wº. The location of the seven seismic stations relative to the caldera rim (yellow line) are shown by the green circles. ........................................................................ 27 Figure 3b Sequence event density on the eastern fault plane. The horizontal axis is centered on the centroid location for all sequence events along the fault ................................................ 28 Figure 3c Pre-eruption sequence event depth distribution in (km) binned at 0.1 (km) ........................ 29 Figure 4 a.) Histogram of sequence event count. b) Histogram of sequence durations in hours. c)

Histogram of spatial dimension in km (25-75% quartile for the PC1 scores for each). d) Histogram of median distance link lengths for each sequence. e) Histogram of median

time link lengths for each sequence. f) Histogram of median space-time link lengths for each sequence. These plots exclude the largest sequence, which occurred over a ~25 hr period co-incident with the eruption and spanned a range of ~ 6 km of the fault zone ........ 30

Figure 5 a) Stem plot of median sequence time verses number of events in the sequence with

cumulative moment (overlain blue line). b) Stem plot of median sequence time verses spatial dimension (defined as the 25-75% quartile range of the PC1-scores). c) Stem plot of median sequence time verses the duration of each sequence in hours ...................... 31 Figure 6 Mainshock-Aftershock sequence from Axial Seamount. Cumulative seismic moment vs event origin time (Top). Number of events vs event origin time (blue line) with moment magnitude (grey stems) and largest event (red dot) in the sequence (Middle). Scatter plot of seismicity along the eastern fault plane with events proportional to rupture radius and colored by time (circles) ......................................................................... 32 Figure 7 Foreshock-Mainshock-Aftershock sequence from Axial Seamount. Cumulative seismic moment vs event origin time (Top). Number of events vs event origin time (blue line) with moment magnitude (grey stems) and largest event (red dot) in the sequence (Middle). Scatter plot of seismicity along the eastern fault plane with events proportional to rupture radius and colored by time (circles) ................................................ 33

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Figure 8 Swarm sequence from Axial Seamount. Cumulative seismic moment vs event origin time (Top). Number of events vs event origin time (blue line) with moment magnitude (grey stems) and largest event (red dot) in the sequence (Middle). Scatter plot of seismicity along the eastern fault plane with events proportional to rupture radius and

colored by time (circles) ....................................................................................................... 34 Figure 9 Migrating sequences from Axial Seamount. Scatter plot of seismicity along the eastern fault plane with proportional rupture radii colored by time (circles) .................................... 35 Figure 10 Histograms of sequence migration rates (a) and distance migrated (b) ................................ 36 Figure 11 Migration direction vectors (arrows) for sequences occuring on the eastern limb of the ring fault. Along strike (red arrows) vectors and up/down and oblique (blue arrows) vectors are scaled by migration distance. Vectors begin at sequence centroid location ....... 37 Figure 12 Sequence event origin times versus tidal phase (small colored dots) with slack tidal phase brackets (large vertical black dots) and largest event in each sequence (black asterisks) .................................................................................................................... 38 Figure 13 Bar chart displaying percent excess numbers of earthquakes during encouraging stress for different subsets of events before (a) and after (b) March 5 2015. First, biggest and last categories refer to individual events within each sequence. Significance is determined by evaluating the percent excess outcomes against a binomial model .............. 39 Figure 14 Histogram showing the distribution of the largest event in each pre-eruption sequence with tidal phase (a). Tidal phase vs largest seismic moment (b) from each sequence (black dots) showing that most events occur between slack (red dots) and low-tide (0º) .... 40 Figure 15 Temporal evolution of b-value (black dots) for sequence events (a) and main catalog events (b) with linear best-fits (cyan) and uncertainty (red error bars) ................................ 41 Figure 16 Cross-section along east fault plane showing spatial variation in b-value for sequence

catalog (a) and the full microearthquake catalog (b). Grid nodes are masked for areas of high uncertainty and/or low hypocentral density ............................................................. 42 Figure 17 Estimated rupture radius vs equivalent sequence magnitude. Equivalent magnitudes are

estimated from the cumulative sum of seismic moment in each swarm. Burst radius is defined as the mean distance from all events in a sequence to its centroid. Constant

lines of stress drop for 100 Pa (green line) and 1000 Pa (red line) ....................................... 43 Figure 18 Largest magnitude event in each sequence vs the number of events in each sequence (dots) colored by sequence duration. Sequence with a relatively large event but few total number of events (red circle). Sequence where largest event is relatively small but has many events (green circle) ........................................................................................ 44 Figure 19 Spatiotemporal expansion of rupture areas vs magnitude difference from initiating event and the largest remaining event for each sequence. Red dashed line denotes unity ratio where there is no expansion of the rupture area .................................................. 45

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1.0. Introduction

1.1. Axial Seamount

Submarine volcanism is responsible for creating almost 70% of the Earth’s surface and largely occurs

along the ~ 65,000 km of mid ocean ridge (MOR) that circumscribe the planet (Crisp, 1984). The

inaccessible nature of the MOR system has limited our ability to characterize seafloor-spreading events

and deep-sea eruptions (Kelley, et al., 2014). In an effort to better characterize physical, chemical and

biological aspects of these processes, the Ocean Observatories Initiative (OOI) installed in 2012 and 2013

a cabled sensor array at the summit of Axial Seamount in the northeast Pacific Ocean.

Axial Seamount is a basaltic shield volcano located at ~ 45º 57’ N and 130º 01’W. It lies at the

intersection of the Cobb-Eickelberg hotspot and the intermediate spreading rate Juan de Fuca (JdF) Ridge.

The summit of Axial rises ~ 700 m above the ridge and is capped by a shallow (~ 160 m deep), elliptical

(~ 3 km wide by 8 km long) caldera breached by lavas along its southern rim (Figure 1) (Clague et al.,

2013). Active-source seismic imaging reveals the presence of a 3 km wide by 14 km long by ~ 1 km thick

magma chamber whose roof lies at depths between 1.1-2.3 km beneath the caldera floor (Arnulf et al.,

2014; Arnulf et al., 2018; West et al., 2001).

Three eruptions at Axial Seamount, in 1998, 2011 and 2015, have been documented using a combination

of seismic and acoustic sensors, bottom-pressure recorders (BPRs), and seabed and water column

mapping (Chadwick et al., 2012; Dziak et al., 2012; Dziak & Fox, 1999; Nooner & Chadwick, 2016;

Wilcock et al., 2016). Each eruption was characterized by a rapid deflation of the volcanic edifice in

response to the withdrawal of magma from the crustal reservoir during lateral diking. This series of

eruptions followed a volume-predictable model; whereby, after a sustained period of re-inflation, diking

initiated after the uplifting caldera floor reached a threshold elevation (Chadwick et al., 2012; Nooner &

Chadwick, 2016).

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The 2015 eruption of Axial Seamount initiated just three months after the activation of the seismic assets

of the OOI Axial Seamount Cable Array. This local network of seven, cabled ocean-bottom seismometers

(OBSs) detected hundreds of thousands of microearthquakes in the months before, during (24 April – 20

May) and after the eruption—providing an unprecedented view of a MOR volcanic system in the deep

ocean (Wilcock et al., 2016). Seismicity revealed two steeply dipping outward-facing fault planes

underlying the southern caldera rim. These ‘ring fault’ structures were activated during magma chamber

inflation and deflation and accommodated a component of the associated uplift and subsidence of the

caldera summit block (Levy et al., 2018).

These fault systems additionally may act as fluid conduits enabling the development and sustainment of

hydrothermal systems (Embley, Murphy, & Fox, 1990; Wilcock & Fisher, 2004). Three high temperature

vent fields exist along the margins of the caldera, CASM in the north and ASHES and International

District in the south on either side of the caldera’s breached walls (Figure 1), as do numerous areas of

diffuse low-temperature discharge (Embley et al., 1990). Along the eastern margin of the caldera, where

the percentage of melt is highest, the ring fault system extends near the margins of the chamber and the

depth to the chamber roof can be seen to increase beneath the areas of active venting (Arnulf et al., 2018).

This highlights the strong coupling between magmatic, tectonic and hydrothermal processes within the

shallow crust.

There is significant evidence from studies of microearthquakes on the Juan de Fuca Ridge and the East

Pacific Rise that solid Earth and ocean tidal loading stresses can trigger earthquakes (e.g., Stroup et al.,

2007; Tolstoy et al.,, 2002; Wilcock, 2001). Moreover, the rate of stressing from these cyclic loads is

typically several times the rates of tectonic stress accumulation (Emter, 1997) Melchior, 1983). In the

northeastern Pacific, tidal stresses are dominated by the ocean tidal loading effect (Tolstoy et al., 2002;

Wilcock, 2001). Earthquake initiation is favored during low tide when the seafloor is unloaded (𝜎! =

30-40 kPa) by the removal of 3-4 m of water (Tolstoy et al., 2002; Wilcock, 2001; Wilcock et al., 2016).

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This work examines the spatial-temporal distribution of microseismicity over a two-year period (Jan 2015

through Feb 2017) spanning the eruption of Axial Seamount in April and May of 2015. After filtering the

dataset of hypocenters for completeness level and depth, we analyze a set of ~ 81,000 earthquakes with

magnitudes between 0.4 and 3.2 Mw. A hierarchal single-link clustering (SLC) algorithm (e.g., Frohlich

and Davis, 1991) is applied to identify earthquake sequences that exhibit clustering in both space and

time. This approach is ideal for sequence selection because it avoids decision making for temporal

duration and spatial extent of the sequences and does not require the number of clusters to be

predetermined. The resulting subset of microearthquakes is systematically explored for spatial and

temporal patterns considering hypocenter migration and distribution, correlations with tidal phase and

sensitivity to tidal triggering, as well as seismic moment release and the frequency-magnitude parameter,

b.

1.2. Earthquake Sequences

Volcanic activity is often associated with earthquake sequences that are clustered spatially and

temporally. Mogi (1963) classified earthquake sequences into three principal types (Figure 2) as follows:

type 1, a large main shock followed by aftershocks; type 2, a foreshock-mainshock-aftershock sequence;

and type 3, a swarm of temporally and spatially clustered events without a distinct main shock. It is

important to note that this classification scheme represents a continuous spectrum with type 1 and 3 being

end members and that most sequences are some combination of the three. The physical mechanisms that

generate swarm events are not well constrained, although, in volcanic or geothermally active regions,

most models invoke stress changes driven by the movement of fluids (water or magma) within a highly

fractured and structurally heterogeneous crust (Hill, 1977; K Mogi, 1963; Sykes, 1970).

Earthquake sequences exhibit statistical properties (Hill, 1977; Roland & McGuire, 2009) that can aid in

determining their causative mechanisms and potentially add predictive value in eruption forecasts. For

example, b-values associated with volcanic swarm events can be in excess of 2.5, whereas b-values from

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swarm events associated with continental rifts or subduction zones are close to the global average ~ 0.8-

1.0 (Ruhl et al., 2010). Moment release during swarms also differs from typical mainshock-aftershock

sequences in that the largest events in a swarm tend to occur near the end of the sequence. Aftershocks

relax stress concentration resulting from a larger magnitude earthquake (i.e., a step in the stressing

function), and their frequency per unit time has been observed to decay in a systemic fashion following a

modified Omori law (Omori, 1894; Utsu, Ogata, S, & Matsu’ura, 1995). By comparison, when changes in

effective stress are driven by other types of loadings, such as magma chamber inflation or pore pressure

changes, their overall event-time histories are typically not well described by an Omori decay law

(Roland & McGuire, 2009). On short time scales, however, each earthquake will trigger additional

earthquakes (i.e., aftershocks), and so Omori-like behaviors can sometimes be observed embedded within

longer duration swarm sequences (Hainzl, 2004).

Migration (or lack thereof) of earthquake hypocenters has been used to understand the mechanisms

generating earthquake sequences and can also be used to distinguish swarms from type 1 and 2 sequences.

While the spatial pattern of aftershocks is typically stationary in time (Scholz, 1990; Bohnenstiehl et al.,

2002), the migration of hypocenters during some swarms supports the interpretation that mobilized fluids

(water or magma) are dynamically perturbing the stress field and initiating earthquakes. In volcanic

systems, hypocenter migrations have been proposed to track the lateral propagation of dikes over distance

of 10’s km (e.g., Battaglia et al., 2005; Robert P. Dziak & Fox, 1999; Hayashi & Morita, 2003; Martens

& White, 2013). Migrating swarms also have been attributed to pore pressure increases associated with

magmatic, hydrothermal and meteoric waters that move laterally or radially within permeable fault zones

(e.g., Hensch et al., 2008; Hill, 1977; Vidale & Shearer, 2006; Yukutake et al., 2011). Weakening of

stressed fault zones by hydrothermal alteration may make these areas particularly susceptible to swarm

activity (Heinicke et al., 2009).

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2.0. Methods

2.1. Creation of an Earthquake Catalog

Primary (P) and Secondary (S) phase arrivals were picked with an automated triggering algorithm using

short-term amplitude to long-term amplitude (STA / LTA) ratios and signal kurtosis. This algorithm

operated on 4-Hz high-pass filtered waveforms obtained from the five short-period and two broad-band

OBSs in the OOI cabled array (Wilcock et al., 2016). The signals associated with water-column multiples,

explosions and whale calls were removed from the detection set, and the remaining detections were then

associated to form a catalog of ~ 107,000 microearthquakes for the time period 22 January 2015 through

16 February 2017 (Wilcock et al., 2016). Hypocenters were determined using the NonLinLoc package

(Lomax et al. 2000) using the 3D velocity model developed by Arnulf et al. (2018). Average location

errors are 50 m laterally and ~100 m vertically for events inside the array.

P- and S-wave arrivals were used to estimate the attenuation corrected seismic moments (Mo). Time

series velocity waveforms for each arrival were integrated to displacement and then used as inputs to a

multi-taper spectral analysis routine (Prieto et al., 2009). The displacement spectra �! were then fit

according to a Boatwright model by solving simultaneously for the low-frequency spectral amplitude

(�!), corner frequency (𝑓!), and attenuation (Q).

Boatwright ∶ ! =�!!

!(!"! )

[!! !!!

!]!/!

(1)

The low frequency spectral amplitude was then used to estimate the seismic moment.

𝑀! =!��!!!�!

!" (2)

where 𝜌 is rock density, x is source receiver distance, c is the P or S wave velocity, and K and R are

constants referencing surface corrections and radiation pattern effects, respectively. Parameter values

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appropriate for mid-ocean ridge settings were: 2.7 g/cm3 for 𝜌, 5.0 km/s for P-wave velocities and 2.78

km/s for S-wave velocities, 1.5 for Kp and 1.7 for Ks, and 0.42 for Rp and 0.59 for Rs (Weekly et al.,

2013).

The moment magnitudes (𝑀!) were calculated (for Mo in dyne-cm) for each arrival:

𝑀! = !!log!"(𝑀!) 10.7

(3)

Using the parameters outlined above (Eq. 2), Mw estimates obtained from P arrivals were found to be

systematically larger than Mw estimates obtained from S arrivals, with this offset being independent of

radiation pattern effects. Linear regression analysis was therefore used to correct each P-wave magnitude

to its equivalent S-wave magnitude. Using a subset of 19,102 earthquakes with S-wave arrivals recorded

at each of the seven stations within the array, additional station correction terms were derived based on

the average offset of each station magnitude from the network mean, with the constraint that the sum of

the station correction terms must equal zero (Appendix XX).

2.2. Analysis of Earthquake Sequences

2.2.1. Defining Sequences

The initial catalog of ~107,000 earthquakes was first filtered to remove earthquakes with magnitudes less

than the approximate completeness level of the catalog (Mw ~ 0.395), reducing the total number of

earthquakes to ~ 81,000. A hierarchal clustering algorithm was used to define earthquake sequences using

a combined spatio-temporal metric (dST):

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𝑑!" = (𝑑! + 𝐶!𝑇!)

(6)

where d is the separation between two earthquake hypocenters, T is their time difference in days, and C is

a scaling parameter relating time to distance (Frohlich and Davis, 1991). Values of C between 0.25 and

0.9 km/hr were evaluated and found to produce the same number of sequences with a nearly identical set

of events belonging to each cluster. Analysis presented here uses C = 0.6 km/hr, meaning that two

earthquakes separated by 0.6 km but occurring at the same time would have the same separation as two

earthquakes occurring at the same location 1 hour apart. A nearest distance algorithm was then used to

link events into a hierarchical tree and clusters are formed using a cut-off length (dco) of 0.350 km-hr,

which corresponds to 95% of the median link length within the catalog (e.g., Bohnenstiehl et al., 2002;

Frohlich and Davis, 1991). In this study, only clusters containing ≥ 50 earthquakes were retained for

analysis.

2.2.2. Migration Rates

Principal component analysis (PCA) was applied to the hypocenters within each sequence. The first two

principal component (PC) vectors represent orthogonal coordinate axes that lie within the plane of the

seismicity, and the third PC vector represents a normal to that plane. Since the PC-1 axis represents the

direction that explains the largest amount of variation within the data, the PC-1 scores (position along that

axis) and earthquake origin times were used in a linear regression to constrain the rate of migration for

each sequence. To measure the significance of linear dependence between PC-1 scores and origin times,

p-values were computed and rates determined significant at p < 0.05. The migration direction was

determined using the PC-1 axis direction.

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2.2.3. Size-Frequency Analysis

The power law relationship that describes earthquake frequency-magnitude distributions is referred to as

the Gutenberg-Richter Law and is described by

𝑙𝑜𝑔!" 𝑁 = 𝑎 − 𝑏𝑀

(6)

where N is the cumulative number of earthquakes with size greater than or equal to magnitude M and a

and b are empirical constants. The a-value represents an earthquake productivity parameter while the b-

value is a measure of the relative amounts of small to large earthquakes in the volume of space from

which sample population is drawn. Since magnitude is a logarithmic measure of earthquake size, the

slope of the linear relationship on a log10(N) vs. M plot is the b-value.

Following Aki (1965) a maximum likelihood approach was used to estimate the b-value:

𝑏 = !"#!"!!!!!

(7)

where Mc is the magnitude of completeness, the magnitude above which the catalog is considered

complete, and 𝑀 is the mean magnitude of earthquakes with M ≥ Mc. The iterative procedure proposed

by Wiemer and Wyss (2000) was used to solve for Mc and b simultaneously. Uncertainty in the b-value

estimate was calculated following Shi and Bolt (1982):

𝑏 = 2.3𝑏! (!!!!)!!!!(!!!)

(8)

where n is sample size.

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2.2.4. Tidal Analysis

Volumetric stress changes associated with ocean loading were estimated based on OOI bottom-pressure

recorder (BPR) data that were averaged over 1-minuate intervals. Semi-diurnal water level variations

above Axial Seamount result in periods of relative volumetric extension at times of low-water level, and

relative volumetric compression at times of high water level. A tidal phase is interpolated for and

assigned to each earthquake event. Times of maximum crustal extension (low-tide) are represented with a

phase of 0º, whereas times of maximum compression (high-tide) are assigned a tidal phase of ±180º.

Following this convention, an earthquake that occurs at slack tide would be assigned a tidal phase of ±90º

where the sign is determined by falling or rising water levels. Regression analysis was used to explore

correlations between sequence times and tidal phase. A binomial model was employed to evaluate

whether there was a statistically significant preference for earthquakes to occur at times of low water level

(encouraging stress conditions) (e.g., Cochran et al., 2004; Stroup et al., 2007). The percentage of excess

events, Nex, during periods of encouraging stress is assessed by:

𝑁!" = 100 ∗ !!"#!(

!!"!! )

!!"!

(9)

where Nenc is the number of events that occur during encouraging stress and Ntot is the total number of

events.

3.0. Results

3.1. Sequence catalog statistics

Using a hierarchal single link distance approach, 99 spatially and temporally clustered earthquake

sequences (with ≥ 50 events) were identified. These sequences represent ~1/4 (20,564 of 81,318) of the

cataloged seismicity. Most (92) of the sequences occurred along the eastern side of the caldera, where

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seismicity rates are much higher overall (Wilcock, 2016; Levy et al., 2018), with only seven sequences

occurring along the western side of the caldera. The sequence events exclusively lie along the two

outward dipping fault planes identified by Wilcock and Levy and further constrain the dimensions of the

plane and corresponding damaged zone (Figure 3).

A total of 95 sequences were identified in the three months preceding the eruption and contain 16,278 of

the 20,564 events. Four sequences were identified on the day of the eruption during which the remaining

4,286 sequence events occurred. The smallest pre-eruption sequence had 51 events and the largest 770

(Figure 4a). Three of the four eruption-day sequences occurred on the western fault and contained 221

events. One eruption-day sequence occurred on the eastern fault and contained 4065 events, more than

55% of the located events that occurred that day. The sequences along the western fault contained

significantly fewer events than the eastern fault sequences, with median counts of 80 and 112

earthquakes, respectively.

Excluding the largest sequence, which occurred over a 25 hour period on the first day of the eruption, the

remaining eruption sequences exhibited a median duration of 2.6 hours (Figure 4b) and a median spatial

dimension of 0.53 km (Figure 4c). The median spatial and temporal separation (i.e., spatial and temporal

link-lengths) of their hypocenters was 0.42 ± 0.26 km and 0.54 ± 0.5 hr, respectively (Figure 4d and 4e).

The observed sequences are temporally clustered (coefficient of variation = 1.2). As shown in Figure 5,

they increase in frequency and size in early March, coincident with an overall increase in the rate of

seismicity reported for the entire catalog (Wilcock et al., 2016; Levy et al., 2018). The frequency, size

and duration of sequences, however, then decreases during a two-week period immediately prior to the

eruption, during which time the overall rate of seismicity remains high or increases slightly (Wilcock et

al., 2016; Levy et al., 2018). No sequences were identified after 25 April 2015, reflecting the much lower

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rates of seismic activity throughout much of the eruptive episode, which continue through ~19 May

(Wilcock et al., 2016; Nooner and Chadwick, 2016), and during the first two years of re-inflation.

3.2. Mogi Sequence Types

Event and moment-release rates were analyzed to distinguish type 1 and 2 sequences (i.e., aftershock

sequences) from type 3 (swarm) sequences. Only around 10% of the sequences appear to be dominated by

a single large magnitude event followed by the production of a simple aftershock sequence. Figure 6, for

example, shows a seismic sequence led by an earthquake with magnitude Mw 2.4. This event is followed

by a series of 109 smaller magnitude aftershocks that occur in the next 2.5 hours. The rate of activity

delays with an Omori p-coefficient of 0.9886. Figure 7 displays a sequence with several dozen small

foreshocks over a period of ~ 45 minutes, followed by a Mw 3.1 mainshock and Omori-style aftershock

pattern (p=0.7282). By contrast, most other sequences showed a more temporally distributed moment

release and a series of similar size earthquakes throughout their duration (e.g., Figure 8).

The pre-eruption sequences constitute 25% of the located events in the main catalog, and they account for

~ 25% of the cumulative moment release. This is consistent with their representing swarm activity. Had

aftershock activity dominated these sequences, the presence of larger magnitude events would have

resulted in a disproportionate release of seismic moment.

3.3. Hypocentral Migration

Sequences throughout the pre-eruption time frame display non-migrating (Figure 8) and migrating (Figure

9) behaviors. A total of 47 sequences showed significant (p < 0.05) migration. Their rates varied from

0.03 ± 0.05 km/hr up to 0.785 ± 0.05 km/hr with a median propagation distance of ~ 0.6 ± 0.08 km

(Figure 10a, b). These migration rates are consistent with values reported previously for either diking or

aqueous fluid driven swarms (Koyanagi et al., 1988; Dziak and Fox, 1999; Hayashi and Morita, 2002;

Battaglia et al., 2005; Yukutake et al., 2011).

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As seismicity occurs in the plane of the ring fault, the orientation of the PC-1 axis can be used to identify

the direction of propagation. During the pre-eruption period, there was a tendency for migration to occur

along strike, but there were also a significant number of sequences that migrate up-down dip, as well as

several that migrate with oblique rake directions (Figure 14). Moreover, all but three of these sequences

have centroid locations that cluster within the brittle crust just above the top of the mapped magma

chamber (Arnulf et al., 2014; 2018).

3.4. Tidal Cycle Correlations

During the pre-eruption period, the earthquakes belonging to the sequences occurred primarily at times of

crustal extension induced by the tides (|∅| ≤ 90 º) (Figure 12). All but three of the 23 sequences that

occurred before March 5th were limited temporally to times of encouraging stress, with earthquakes in

these sequences displaying Nex = 37% (Figure 13). Typically, these sequences initiate during the fall from

slack water to low tide (∅ = -90º to 0º), with only five sequences initiated after the time of the lowest

water level (peak extension), but before slack tide (∅ = 0º to 90º). After March 5th sequence earthquakes

still occur primarily during times of encouraging stress conditions (Nex = 33.5%); however, a greater

percentage of the sequences (20 of 72) were initiated during times of discouraging stress (90 ≤ |∅| ≤ 180

º). Earthquakes within the sequences display percent excess that is somewhat greater than the catalog as a

whole (Figure 13).

Even for sequences that initiated during times of discouraging stress, they were more likely to initiate as

the water levels fell and the relative volumetric extension of the crust began to increase (∅ ≤ 90 º). The

timing of the largest magnitude event was similarly biased to times of falling water level (Figure 14a);

however, the maximum size of an earthquake in a sequence was not correlated with the earthquake’s

phase (Figure 14b).

13

3.5. Frequency-Magnitude Distribution

b-values are estimated for both main catalog and sequence catalog events and examined both temporally

and spatially. In general, b-values at Axial are well above the global average (~ 1.0) associated with

conventional tectonic event catalogs, and more representative of values from within regions of active

volcanism and high thermal gradients (Warren & Latham, 1970). Although sequences increase in

frequency and size approaching the eruption, no systematic change in their frequency-magnitude

distribution is observed (Figure 15 top). Moreover, the temporal trend and spatial patterns of b for

sequence events and main catalog events bear striking similarities (Figure 15). Temporally, both catalogs

show a marginally increasing trend in b approaching the eruption, each with a slope of (0.03 +/- 0.001

units/per day). Unexpectedly however, b-values associated with the main catalog are slightly higher than

the sequence catalog (Figure 15).

Figure 16 shows the spatial pattern of b-value mapped onto the eastern fault plane for both the sequence

events and full catalog, and again, both catalogs are similar. Elevated b-values extend over a large central

swath of the fault zone, but decrease rapidly below ~ 1.2-1.5 km depth along the south and central regions

of the fault. For both the sequence and full catalogs, elevated b-values extend ~ 500 to 750 m deeper

along the northern region of the fault plane. Although a slight gradation exists going from areas of high to

low b, variation in b changes dramatically over relatively short distances (2.4 to 1.2 over ~ 200 to 400 m).

3.6. Spatiotemporal Extent

Figure 17 shows the burst radius of each sequence plotted against its equivalent magnitude—where the

burst radius is calculated as the mean distance from the sequence centroid to all events and the equivalent

magnitude is estimated from the total seismic moment released by the sequence. For an individual

earthquake, the rupture area and moment release are related through the stress drop on that portion of the

fault (Madariaga, 1976). Here, the same concept is applied to the sequences as a whole (e.g., Vidale and

Shearer, 2006 ). The results indicate that the value of the stress drop averaged over the burst radius is

14

somewhere between 10-3 to 10-4 MPa, quite small when compared to the 2-3 MPa values reported for

individual earthquakes (Moyer et al., 2018). This indicates that the sequences do not effectively relieve

the stress over the entire sequence area, and suggests a heterogeneous concentration of stresses on the ring

faults.

Moreover, the number of events in sequences appears to be uncorrelated with the size of the largest event

in the sequence (Figure 18) demonstrating again that these sequences are not controlled by their largest

event. For instance, the number of events in the sequence marked with the green circle is ~ 8 times the

number of events in the sequence marked with a red circle despite its largest event being an order of

magnitude (32 times less energetic) smaller.

Spatially and temporally, many sequences diffuse away from their point of initiation (Figure 19). The

expansion is measured as the ratio of the median distance from the first event in each sequence to each

event in the second and first halves of the sequence. In this way, the second half of the time evolution of

each sequence is compared to the first to probe spatio-temporal expansion of the sequence hypocenters.

Ratios over one denote expansion, close to one suggests little expansion and less than one suggests

contraction. A large group of sequences have initiating events ~ 1 – 2 orders of magnitude smaller than

the largest remaining event and the paucity of mainshock sequences in general is apparent (i.e. there is

only one sequence with a magnitude difference greater than one). The relationship between magnitude

difference and distance ratio demonstrates that for many sequences the initial event is not driving a

cascade of events that make up the rest of the sequence (i.e. not a mainshock) and that unlike mainshock

sequences, hypocentral expansion is occurring.

4.0. Discussion

Most of the identified sequences appear to be swarms (type 3), consisting of a series of similar size

earthquakes that lack a discernable mainshock event. Earthquake swarms are common in many volcanic

15

and hydrothermal areas (Mogi, 1963; Sykes, 1970, Hill 1977; Benoit, Benoit, & McNutt, 1996; Dziak and

Fox, 1999; Hainzl and Fischer, 2002; Brauer et al., 2003; Hainzl, 2004; Ake et al., 2005; Hainzl & Ogata,

2005). A smaller number of the sequences were classified as type 1 (mainshock-aftershock) or type 2

(foreshock-mainshock-aftershock) activity. Previous work examining the aftershocks of larger magnitude

events (> M 5.0 mainshocks) along the global oceanic ridge-transform system (e.g., Bohnenstiehl et al.,

2002) has reported p-values that are elevated relative to the global median of ~ 1.1 (Utsu et al., 1995).

This has been interpreted to reflect the faster relaxation of the crust in high heat-flow areas (Kisslinger &

Jones, 1991; Kiyoo Mogi, 1967; Rabinowitz & Steinberg, 1998; Creamer, 1994). At Axial, the rate of

aftershock production can be described well assuming a p-value ~ 0.72 - 0.98, implying that aftershock

rates decay more slowly than in other environments. However, at Axial, most sequences appear to

terminate with semi-diurnal tidal stress changes, and therefore aftershock durations typically only extend

over a time period of a few hours. Notably, p-value estimates are known to be sensitive to the time

segment considered—typically showing the lowest values when calculated for time windows immediately

after the mainshock (e.g., Helmstetter & Shaw, 2009).

Sequence event locations are exclusively concentrated within the two steeply dipping outward-facing

fault zones identified by Levy et al. (2018) and Wilcock et al. (2016) (Figure 2). The spatial pattern of

sequence events, which represent ~20% of the pre-eruption catalog, mirrors the distribution of the larger

catalog—being concentrated predominantly within the plane of the eastern portion of the ring fault

between depths of 0.1 and 2.5 km below the seafloor (Figure 3c). Sequence earthquakes also display

similar size-frequency distributions, in both space and time, when compared to the larger catalog. High

b-values are observed at shallow depths, consistent with lower stress levels and/or the heterogeneous

nature of the fracture network within the upper (extrusive) sections of the ocean crust. There is a general

decrease in b-value as stresses increase with depth, with a localized zone (near 45.9535° N, 129.9841° W)

of high b-value seismicity extending to the depth of the magma chamber (Figure 16). This pattern may

indicate elevated pore fluid pressures (low effective stress) within this zone. In map view, it is positioned

16

beneath the southern limit of the lava flows (i.e., near the site of likely dike initiation) observed during the

2015 eruption (Wilcock et al., 2016; Chadwick et al., 2016), but to the north of the International District

high-temperature vent field.

On semi-diurnal time scales, seismicity rates at Axial Seamount are controlled strongly by the tidally

induced stress changes, which are driven primarily by the ocean loading (Wilcock, 2001; Tolstoy et al.,

2002; Wilcock et al., 2016). These stress changes are too small (~30 kPa) to fracture intact basaltic rock,

which has a tensile strength of 2-5 MPa, but they are sufficient to modulate the timing of events along the

critically stressed fault system beneath Axial Seamount (Wilcock et al., 2016). While seismicity rates rise

and fall during each semi-diurnal tidal cycle, prior to the eruption, less than half of the tidal cycles contain

sequences with spatio-temporally clustered events (as defined using the criteria in Section 2.2.1). The

onset, termination and moment release within these sequences may be particularly sensitive to tidal

stressing, with significantly higher percent excess values reported for the sequences than the catalog as a

whole (Figure 13).

Nearly 48 percent of the observed sequences display a significant migration of hypocenters, with rates of

up to ~ 800 m/hr. This movement of hypocenters through time is consistent with triggering of seismicity

by fluid migration, and these observed rates are similar to those previously ascribed to either magmatic

diking or pore pressure fluctuations associated with aqueous fluids (e.g., Ake et al., 2005; Raleigh et al.,

1976; Prejean et al., 2002 ). The relatively short duration (~ 3.2 hr median) and distance (~ 600 m

median) over which the swarms migrate argues against magmatic diking as a mechanism for generating

these sequences. Many sequences have centroid locations near the roof of the magma chamber, with some

sequences migrating upward or away from the chamber, and others migrating downward toward the

chamber, within the plane of the ring fault.

17

Brittle faults at the scale of the ring structures beneath Axial Seamount represent a damage zone that may

extend over a width of several hundred meters away from the central plane of the fault (e.g., Caine,

Evans, & Forster, 1996; Mitchell & Faulkner, 2009; Tadokoro, 2000; Vermilye & Scholz, 1998). The

precipitation of minerals within the sub-surface hydrothermal system can form seals (Lowell et al., 1995;

Rona, 1976) within this more permeable zone, leading to the development of areas of elevated pore-fluid

pressure. The characteristics of the observed swarms beneath the summit of Axial Seamount suggest that

they may be triggered when high pore pressure aqueous fluids are released along the preexisting ring fault

system, thereby prompting the release of accumulated stress.

Given the burst radius and moment release of each swarm, the stress drop averaged over the area

activated by each sequence would be only 10-3 to 10-4 MPa. This may suggest the development of a

heterogeneous stress distribution within the activated area, with higher stresses between small areas of

earthquake rupture that have undergone greater (2-3 MPa) stress drops. Such heterogeneity, however,

appears not to be reflected in the sequence b-values, which are not systematically elevated relative to the

catalog as a whole. Conceivably, aseismic slip could produce a more homogeneous slip distribution

across the activated area. Creep has been suggested as a mechanism for generating swarms in other

settings, where these types of aseismic motions can be independently identified using precision GPS

measurements (e.g., Linde et al., 1996; Smith et al., 2004). Unfortunately, such observations are not

readily available in submarine volcanic environments.

Statistical seismology can be a useful tool in eruption forecasting. With regard to the earthquake

sequences observed here, we find that as stress increases approaching the 2015 eruption, swarm

sequences occur for longer durations and increasingly proceed into and initiate at times of discouraging

stress. Moreover, the number of events and the frequency with which they occur also increase. No

sequences were identified beyond the first ~ 25 hours of the eruption, or over the following two years as

the volcanic system continued to re-inflate. As seismicity rates continue to increase in the coming years,

18

the size, timing and clustering of sequences leading up to the next eruption may provide additional insight

into their potential value as a forecasting tool.

5.0. Conclusions

A hierarchal clustering approach is used to identify spatio-temporal clusters of microseismicity over a

two-year period (Jan 2015 through Feb 2017) spanning the eruption of Axial Seamount in April and May

of 2015. Our analysis reveals the following:

• Approximately one fifth of the earthquakes occurring during the pre-eruptive period are

associated with spatially and temporally clustered sequences having at least 50 earthquakes.

• Sequences are located exclusively along the shallow ring fault system. The spatial distributions,

as well as size-frequency scaling, mirror that observed for the larger catalog of seismicity beneath

the volcanic summit (e.g., Wilcock et al., 2016; Levy et al., 2018).

• More that 90% of the sequences observed at Axial Seamount are classified as swarms.

Mainshock-aftershock or foreshock-mainshock-aftershock sequence are relatively rare and

exhibit Omori p-value exponents < 1 over their relative short (hours) durations.

• Thetimingofearthquakesequencesbeneaththesummitisstronglymodulatedbyocean

tidalloadingstresses.Mostsequencesinitiatingduringtimesoffallingwaterlevel

approachinglowtide,whenthecrustundergoesrelativevolumetricextensionthat

encouragesslip,andendbeforethesucceedingslacktide,whenrisingwaterlevelscreate

compressivestressconditionsthatarediscouragingtoslip.

19

• Hypocentermigrationoccurredinalmosthalftheidentifiedsequences.Thedistances,

directionsandvelocitiesofpropagationareconsistentwiththeiroccurrenceduetothe

releaseofhighporepressureaqueousfluidsalongthepreexistingringfaultsystem.

• Intheweeks-to-monthsbeforethe2015eruption,earthquakesequencesincreasein

frequency,size(i.e.,numberofevents)andduration.Additionally,theybegantospan

longerportionsofthetidalcycles,withanincreasedproportioninitiatingduring,or

proceedinginto,timesofdiscouragingstress.Trackingthetiminganddurationof

earthquakesequencesmayaidinforecastingsubsequenteruptionsatAxialSeamount.

• Although the eruption persisted for ~26 days, earthquake sequences are not identified beyond ~1

day after the onset of the eruption. Earthquakes within these eruption sequences are also

positioned entirely along ring fault structures, consistent with the downward movement of the

caldera floor relative to the rim (Levy et al., 2018) as the volcano rapidly deflates (Chadwick et

al., 2016).

20

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Figure 1 Bathymetry of Axial Seamount with earthquake locations (grey dots), seismic stations (triangles), hydrothermal fields (stars). 2015 lava flow boundaries outlined in red (Chadwick et al., 2016), while the magma chamber margins from active seismic imaging are shown by the black, dashed line (Arnulf et al., 2014). Inset map shows the location of the seamount along the Juan de Fuca Ridge in the eastern Pacific Ocean. ASHES, CASM, and ID 1 (International District 1) denote the location of high-temperature vent fields along the margins of the caldera.

46˚0

0’N

45˚5

5’N

130˚05’W

Depth (m)2050 1850 1500

45˚N

120˚W130˚W

CASM

ASHES

ID 1

26

Figure 2 Number of events versus origin time displaying three types of earthquake sequences (after Mogi, 1963).

Time

Type1:M/Asequence

HomogeneousMaterial&UniformExternalStress

Time

Type2:Foreshocksequence

LessHomogeneous&Non-UniformStress

Time

Type3:Swarm

HeterogeneousMaterial&ConcentratedStress

27

Figure 3a Locations of sequence events (red dots) superimposed onto the complete seismicity catalog (black dots) showing that sequence earthquakes lie within the same plane identified by Levy et al., 2018. The long-axis of the caldera has a strike orientation of N28Wº. The location of the seven seismic stations relative to the caldera rim (yellow line) are shown by the green circles.

28

Figure 3b Sequence event density on the eastern fault plane. The horizontal axis is centered on the centroid location for all sequence events along the fault.

-2000 -1500 -1000 -500 0 500 1000 1500 2000 2500Distance along East fault

from center of seismicity (m)

-2000

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0D

epth

(m)

Sequence Event Density for East Fault

-6

-5.5

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-3

29

Figure 3c Pre-eruption sequence event depth distribution in (km) binned at 0.1 (km).

30

Figure 4 (a-f) a.) Histogram of sequence event count. b) Histogram of sequence durations in hours. c) Histogram of spatial dimension in km (25-75% quartile for the PC1 scores for each). d) Histogram of median distance link lengths for each sequence. e) Histogram of median time link lengths for each sequence. f) Histogram of median space-time link lengths for each sequence. These plots exclude the largest sequence, which occurred over a ~25 hr period co-incident with the eruption and spanned a range of ~ 6 km of the fault zone.

31

Figure 5 a) Stem plot of median sequence time verses number of events in the sequence with cumulative moment (overlain blue line). b) Stem plot of median sequence time verses spatial dimension (defined as the 25-75% quartile range of the PC1-scores). c) Stem plot of median sequence time verses the duration of each sequence in hours.

0

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800N

umbe

r of

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nts

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ulat

ive

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ent

01/25 02/01 02/08 02/15 02/22 03/01 03/08 03/15 03/22 03/29 04/05 04/12 04/19 04/26

Median Sequence Time (days)

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ial D

imen

sion

25%

- 75

% r

ange

01/25 02/01 02/08 02/15 02/22 03/01 03/08 03/15 03/22 03/29 04/05 04/12 04/19 04/26


0

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Sequ

ence

Dur

atio

n (h

r)

01/25 02/01 02/08 02/15 02/22 03/01 03/08 03/15 03/22 03/29 04/05 04/12 04/19 04/26


32

Figure 6 Mainshock-Aftershock sequence from Axial Seamount. Cumulative seismic moment vs event origin time (Top). Number of events vs event origin time (blue line) with moment magnitude (grey stems) and largest event (red dot) in the sequence (Middle). Scatter plot of seismicity along the eastern fault plane with events proportional to rupture radius and colored by time (circles).

23:30 23:40 23:50 00:00 00:10 00:20 00:30 00:40 00:50 01:00 01:10 01:20 01:30 01:40 01:50 02:00 02:10Origin Time

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e M

omen

t

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ive

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ber

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s

Axial Seamount Microearthquake Sequence Date 04/19 - 04/20

23:30 23:40 23:50 00:00 00:10 00:20 00:30 00:40 00:50 01:00 01:10 01:20 01:30 01:40 01:50 02:00Origin Time

0

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ber

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s

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t Mag

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de (M

w)

Mogi Sequence Plot

-1000 -500 0 500 1000

Along Strike from center of seismicty (m)

-1400

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-600

Dep

th (m

)

Rupture Radius Proportional / Events Colored by Time

23:45 00:00 00:14 00:28 00:43 00:57 01:12 01:26 01:40 01:55

K= 37p =0.9886c = 0.004MOL=

33

Figure 7 Foreshock-Mainshock-Aftershock sequence from Axial Seamount. Cumulative seismic moment vs event origin time (Top). Number of events vs event origin time (blue line) with moment magnitude (grey stems) and largest event (red dot) in the sequence (Middle). Scatter plot of seismicity along the eastern fault plane with events proportional to rupture radius and colored by time (circles).

23:15 23:30 23:45 00:00 00:15 00:30 00:45 01:00 01:15 01:30 01:45 02:00 02:15 02:30 02:45 03:00 03:15Origin Time

0

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s

Axial Seamount Microearthquake Sequence Date 01/30 - 01/31

23:15 23:30 23:45 00:00 00:15 00:30 00:45 01:00 01:15 01:30 01:45 02:00 02:15 02:30 02:45 03:00Origin Time

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ber

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s

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(Mw

)

Mogi Sequence Plot

-3000 -2000 -1000 0 1000 2000 3000

Along Strike from center of seismicty (m)

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0

Dep

th (m

)


23:31 00:00 00:28 00:57 01:26 01:55 02:24 02:52

K= 91p =0.7282c = 0.004MOL=

34

Figure 8 Swarm sequence from Axial Seamount. Cumulative seismic moment vs event origin time (Top). Number of events vs event origin time (blue line) with moment magnitude (grey stems) and largest event (red dot) in the sequence (Middle). Scatter plot of seismicity along the eastern fault plane with events proportional to rupture radius and colored by time (circles).

01:00 01:10 01:20 01:30 01:40 01:50 02:00 02:10 02:20 02:30 02:40 02:50 03:00 03:10 03:20 03:30 03:40 03:50Origin Time

0

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1C

umul

ativ

e M

omen

tAxial Seamount Microearthquake Sequence Date 03/18

01:00 01:15 01:30 01:45 02:00 02:15 02:30 02:45 03:00 03:15 03:30 03:45Origin Time

0

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35

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s

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(Mw

)

Mogi Sequence Plot

-2000 -1500 -1000 -500 0 500 1000 1500Along Strike from center of seismicty (m)

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Dep

th (m

)


01:12 01:26 01:40 01:55 02:09 02:24 02:38 02:52 03:07 03:21 03:36

35

Figure 9 Migrating sequences from Axial Seamount. Scatter plot of seismicity along the eastern fault plane with proportional rupture radii colored by time (circles).

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000Along Strike from center of seismicty (m)

-2000

-1500

-1000

-500

0D

epth

(m)

Vertical Migration Sequence Date 02 March - 03 March 2015

23:02 23:31 00:00 00:28 00:57

-3000 -2000 -1000 0 1000 2000 3000Along Strike from center of seismicty (m)

-2000

-1500

-1000

Dep

th (m

)

Horizontal Migration Sequence Date 22 Feb 2015

14:24 15:36 16:48

-2000 -1000 0 1000 2000 3000Along Strike from center of seismicty (m)

-1800-1600-1400-1200-1000

-800

Dep

th (m

)

Oblique Migration Sequence Date 22 Feb 2015

10:48 12:00 13:12

MigrationRate = 413 m/hr



36

Figure 10 Histograms of sequence migration rates (a) and distance migrated (b).

37

Figure 11 Migration direction vectors (arrows) for sequences occuring on the eastern limb of the ring fault. Along strike (red arrows) vectors and up/down and oblique (blue arrows) vectors are scaled by migration distance. Vectors begin at sequence centroid location.

0

Migration Vectors from Sequence Centroids

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Ver

tical

pos

ition

from

ce

nter

of s

eism

icity

(m)

500

1000

-1000 0

Along Strike from Center of Seismiciy (m)1000 2000 3000

38

Figure 12 Sequence event origin times versus tidal phase (small colored dots) with slack tidal phase brackets (large vertical black dots) and largest event in each sequence (black asterisks).

-200 -100 0 100 200Tidal Phase Angle

01/25

02/01

02/08

02/15

02/22

03/01

03/08

03/15

03/22

03/29

04/05

04/12

04/19

04/26

Ori

gin

Tim

eTidal Phase vs Origin Time

39

Figure 13 Bar chart displaying percent excess numbers of earthquakes during encouraging stress for different subsets of events before (a) and after (b) March 5 2015. First, biggest and last categories refer to individual events within each sequence. Significance is determined by evaluating the percent excess outcomes against a binomial model.

Pre March 5 Percent Excess Events

Sequences Biggest Event Last Event Full Catalog First Event0

10

20

30

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50

Post March 5 Percent Excess Events

Sequences Biggest Event Last Event Full Catalog First Event0

5

10

15

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30

35

P << 0.001 forall classes

P << 0.001 forall classes

40

Figure 14 Histogram showing the distribution of the largest event in each pre-eruption sequence with tidal phase (a). Tidal phase vs largest seismic moment (b) from each sequence (black dots) showing that most events occur between slack (red dots) and low-tide (0º).

41

Figure 15 Temporal evolution of b-value (black dots) for sequence events (a) and main catalog events (b) with linear best-fits (cyan) and uncertainty (red error bars).

Feb Mar AprDate

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4Se

quen

ce b

-Val

ues

Sequence Event Pre-Eruption b-values

errorbvaluey=0.003 ±0.001 + 1.49

Feb Mar Apr

Date

1

1.5

2

2.5

3

3.5

Cat

alog

b-V

alue

s

Larger Catalog Pre-Eruption b-values

errorbvaluey=0.003 ±0.0007 + 1.69

42

Figure 16 Cross-section along east fault plane showing spatial variation in b-value for sequence catalog (a) and the full microearthquake catalog (b). Grid nodes are masked for areas of high uncertainty and/or low hypocentral density.

-2000 -1500 -1000 -500 0 500 1000 1500 2000 2500

Along strike distance from center of seismicity (m)

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Dep

th (m

)Sequence b-values along East Fault

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Dep

th (m

)

Full Catalog b-values along East Fault

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43

Figure 17 Estimated rupture radius vs equivalent sequence magnitude. Equivalent magnitudes are estimated from the cumulative sum of seismic moment in each swarm. Burst radius is defined as the mean distance from all events in a sequence to its centroid. Constant lines of stress drop for 100 Pa (green line) and 1000 Pa (red line).

1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2Equivalent Magnitude (Mw)

200

300

400

500

600

700

800

900

Burs

t Rad

ius

(m)

Estimated Sequence Rupture Radius vs Equivalent Magnitude

Sequence

100 Pa

1000 Pa

44

Figure 18 Largest magnitude event in each sequence vs the number of events in each sequence (dots) colored by sequence duration. Sequence with a relatively large event but few total number of events (red circle). Sequence where largest event is relatively small but has many events (green circle).

0.5 1 1.5 2 2.5 3 3.5

Largest Event Magnitude

0

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Largest Event vs Number of Events

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ence

Dur

atio

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r)

45

Figure 19 Spatiotemporal expansion of rupture areas vs magnitude difference from initiating event and the largest remaining event for each sequence. Red dashed line denotes unity ratio where there is no expansion of the rupture area.

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5Magnitude Difference

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Dis

tanc

e R

atio

Expansion of Sequence vs Relative Magnitude of 1st Event

dps thesis etd ffe - ncsu

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