Krohne 1

Nathan KrohneProfessor HamburgerG454 Plate TectonicsMay 1, 2017

The Driving Forces of Slow Slip Events and Seismic Tremor at Subduction Zones

Abstract:

Episodic tremor and slip (ETS) is a new geological process that has been observed in

various subduction zones around the world. Through continuous GPS monitoring

and seismic data, it is apparent that ETS events are a system of shear slip failure

along the plate interface that is driven by unknown forces. The analysis of thermal

and pore fluid models have provided insight to the new phenomena which has led

people to the belief that fluids have a controlling factor in both tremor and slip

activity. The discovery of ETS has enlarged our definition of slow earthquakes and

reformed our understanding of how faults adapt to plate motions.

Introduction

Episodic tremor and slip (ETS) was discovered in the early 2000 and first noticed in

the southwest Japan subduction zone (Obara 2002). Soon after, evidence of ETS was

found in subduction zones across the world and it marked the beginning of daily

instrument measurements for ETS. This phenomena isn’t home to just subduction

zones, it can be found in transform faults were frictional properties are similar

(Peng & Gomberg 2010). However, ETS is mainly found in subduction zones due to

interplate friction. When a plate is subducting, there are zones of locking and

slipping along the plate boundary. The locked zone is near the trench where the

subducting lithosphere is still cold and brittle. The lower end of the subducting

Krohne 2

lithosphere is associated with the slip zone due to the warmer and more ductile

environment. This leaves a transition zone in between where the slow slip events

and seismic tremor take place.

The importance of ETS is a

growing concern, considering

these events have an impact on

lithospheric plate motions and

play a role for mega thrust

earthquakes.

Slow Earthquakes

The discovery of ETS opened

up an entire spectrum of

seismic frequencies and

expanded the term slow

earthquakes. These

earthquakes can occur in faults

all over the world including

divergent, convergent and

transform boundaries. Low

frequency earthquakes (LFEs)

are defined as seismic events

that radiate waveforms far

longer than an ordinary

Figure 1. Slow slip seismic signals a) seismic signal from a tremor b) VLF seismic signal from Japan c) LFE seismic signal from Japan d) Earthquake with a magnitude of 1.9 in Washington (Peng & Gomberg 2010)

Krohne 3

earthquake. They are associated with magnitudes lower than 3 and normally

fluctuate between 1 and 3 Hz. The depths at which LFEs can occur are around the

transitional zone of a subducting lithospheric plate, characteristically about 20-45

kilometers. LFE events are vey sensitive and can be triggered by distant

earthquakes or even tidal influences, as observed in the southwest Japan subduction

zone (Nakata et al 2008). Very low frequency earthquakes (VLFs) are considered a

subgroup of LFEs and differ by duration and magnitude, which often occurs in

bursts. They have even lower frequencies, ranging from .01 to .03 Hz, and have a

slightly larger magnitude range of 3 to 3.5 M. They are harder to detect, largely due

to their frequency and short burst duration (Ghosh et al 2015). Figure 1 illustrates

the different waveforms LFEs and VLFs can produce. Notice the difference between

the two types of slow earthquakes. Often times, VLFs are found buried deep within

LFEs implying that LFE’s can occasionally arrange themself to radiate at lower

frequencies (Peng & Gomberg 2010).

Cascadia subduction zone

The Cascadia subduction zone is one of most active ETS locations. This subduction

zone extends over 1000 kilometers from the British Columbia to California

(Chapman & Melbourne 2009). ETS is observed throughout the entire subduction

zone by GPS observations and broadband seismometers, but is segmented into three

regions roughly 300-500 kilometers apart (Brudzinski & Allen 2007). Figure 2

illustrates the three regions and their locations along the Cascadia subduction zone.

The regions differ by recurrence cycles and duration. The recurrence cycle is where

Krohne 4

the “E” originates from in episodic tremor and slip (ETS). Slow slip and tremor are

not a continuous event but rather a cycle that reoccurs periodically. These cycles

can last a few days or extend into months of ETS activity. Considering ETS is taking

place along the entire subduction zone, we can exclude localized events particular to

a specific region that is

excluding ETS. In other words, If

ETS is observed along the entire

subduction zone, then local

geological processes will not have a

limiting factor on ETS. The northern

region of Cascadia is associated with

a recurrence interval of 14 ± 2

months. About 400 km south in the

central region, ETS is associated

with a recurrence interval of 19±4

month. This is almost double the recurrence interval for the southern region as they

occur in only 10±2 month cycles (Brudzinski & Allen 2007). Consistent with

Schwartz and Rokosky (2007), continuous GPS data from the same networks

Figure 2. Map illustrating the regions in which ETS occurs. Squares represent GPS stations. Triangles represent boradband seismometers. (Brudzinski & Allen 2007).

Krohne 5

revealed that there have been 8 slow-slip events since 1992, each lasting 2 to 4

weeks, with an average recurrence of 13-16 months. Rodgers and Dragert (2003)

went a step further and correlated tremor with the same slow slip events, from

1997, by cross correlating GPS and seismic data from five specific sites. Five of the

six slow slip events correlated with tremor migrating along the Vancouver Island

and past GPS measurements. Slow slip events have been detected at depths of 20-40

km, consistent with the seismogenic zone found on the subducted lithosphere.

Clusters of tremor have also been detected by seismometers in the same region with

frequencies between 1 and 5 Hz (Schwartz & Rokosky 2007). Figure 3 displays the

correlation between slow slip events and tremor activity from 1997 to 2003. Rogers

& Dragert (2003) used a GPS

station located on the

Vancouver Island to record

the east component and a GPS station on the North American plate to record the

west component. Then, seismic data were used to correlate the tremors with the slip

events. To test this correspondence, continuous seismic data were analyzed from

1999 to 2003 to find any tremor activity outside of the slow slip events (Rogers &

Dragert 2003). This correlation implies that slow slip and tremor occur under the

same processes and are not independent of each other.

Observations

Figure 3. Correlation between slow slip events and tremor recorded by a GPS station in Victoria British Columbia. Blue circles show the changes in the Victoria GPS site with respect to the west GPS site, which is located on the North American plate. (Rogers & Dragert 2003)

Krohne 6

Slow slip events (SSE) are characterized by shear slip occurring over an extended

time. These events are considered silent earthquakes because they lack the ability

to propagate seismic waves (Schwartz & Rokosky 2007). In subduction zones, slow

slip occurs in the transitional zone between the upper locked phase and the lower

slipping phase of the subducting plate. SSEs can occur during the inter-seismic stage

or during the post-seismic stage of the earthquake cycle. Under the inter-seismic

stage, strain is accumulating and slow slip occurs as a way to relieve some of the

strain. The post-seismic stage is associated with afterslip following earthquakes

(Schwartz & Rokosky 2007). A network of global positioning systems (GPS) is used

in order to observe slow slip events. A large network of GPS stations is usually

required in order to obtain accurate measurements; however, according to

Brudzinski & Allen (2007), ETS information generated by an automated

identification of ETS events at an individual GPS and seismic station is sufficient.

Due to the slow nature in which slow slip events happens, continuous recording GPS

data is the most accurate way to observe slip. Today, almost all research on slow slip

events is observed through multiple GPS sites recording continuous data. These GPS

sites continuously record data over years, like Jiang et al. (2012), who observed 5

slow slip events over a 9-year continuous recording period along the Middle

American Trench in Costa Rica.

Krohne 7

including Schwartz and Rokoskey (2007), that there could be three possibilities to

explain this apparent relationship: 1. Slow slip events and tremor are unrelated and

arise from different processes. 2. Slow slip and tremor are a result of a specific

region with specific characteristics. 3. Slow slip and tremor are related and originate

from the same process. Tremor generally occurs in clusters and contains low

frequencies with uncertain P and S wave arrival times. It also contains a weak signal

to noise ratio, making it even more difficult to identify and distinguish as tremor can

continue on for days or weeks (Satoshi et al. 2007). Seismometers are the best

instrument in order to observe tremor; however, it can’t be done with just one

station. It is only capable to recognize tremors when multiple seismographs are

view at the same time. Tremors are strongest when they are being recoded

horizontal on seismographs (Rogers & Dragert 2003). Since the 1995 M 6.9

earthquake in Japan, the Japanese government established a policy to supply many

universities and institutions with a nationwide, high sensitivity borehole seismic

network (Obara et al. (2005) as cited in Beroza & Ide (2011)). This hi network of

borehole seismometers has greatly increases our ability to detect tremors. Figure 4

Krohne 8

displays the number of earthquakes overlapping with the number of LFE events in

the Japan subduction zone. It also illustrates the importance of the Hi-net

seismometer network and open data access that was sparked by the 1995 Kobe

earthquake. In order to identify the tremor, Japan Meteorological Agency (JMA)

created an earthquake catalog capable of identifying low frequency tremors (Beroza

& Ide 2011).

Models

A thermal-petrologic model is used to illustrate thermal conditions for which ETS

occurs. The Juan de Fuca plate is tectonically young at less than 10 million years old

and has a modest convergence rate of 40mm/yr. This is consistent with Peacocks

(2009) thermal model,

which suggest that Cascadia

has warmer fore arc

temperatures compared to

other subduction zones. In

this model, Peacock (2009)

used the Wadati-Benioff

zone and seismic

reflections to determine the geometry of the subducting lithosphere. Marine

magnetic anomalies were used to determine the age of the lithosphere. A

continental geothermal gradient was used for the arc side boundary as well as

thermal conductivity of 2.5 W/(m K) for the crust and 3.1 W/(m K) for the mantle.

Figure 5. A) Surface Heat flux B) Thermal Structure. Inferred location of ETS is shaded grey (Kao et al., Cited in Peacock 2009)

Krohne 9

Radioactive elements lie within the upper (0-15km) and lower crust (15-30km),

producing 1.3 and .27 µW/m2. Temperatures at depths greater than 30km have

some uncertainties and are primarily measured by the thermal structure and

convergence rate of the subducting lithosphere. Figure 5 illustrates the surface heat

flux and thermal structure for Vancouver Island in the Cascadia subduction zone.

This model shows ETS events

occurring around 30km in depth at

temperatures of about 500°C. It also

considers a 1.2 km sediment layer that

acts as insulation for the subducting

lithosphere, which could perhaps

justify some of the high temperatures

in the Cascadia subduction zone. This

is important to consider because not

many models take this into account,

such as Yoskioka et al (2008), who

developed a thermal model that was

90°C cooler compared to peacock’s

(2009) model of the southwest Japan subduction

zone.

Not much evidence is available to correlate high

temperatures with ETS events, suggesting that

they do not coincide with a specific temperature range (Peacock 2009). However,

Figure 5. Illustrates one hour of tremor. a) S wave arrivals. b) Expanded view displaying one LFE event (Shelly et al. 2006).

Krohne 10

there is evidence that suggests the dehydration of minerals can act as a lubricant for

shear slip along the plate interface. Satoshi et al (2006), was able to formulate a

tomographic model to illustrate high Vp/Vs ratios between LFE events, consistent

with the occurrence of pore fluids. Though, In order to correlate the casual

relationship between slow slip, tremor and fluid flow, a model needs to demonstrate

that fluid flow is a response to a change in stress and strain generated by shear slip

(Shelly et al. 2006). Figure 5 illustrates a lengthy phase of tremor that includes four

LFE events. Shelly et al. (2006) was able to identify these events by cross correlating

a combination of waveforms and also using double difference tomography to

relocate the LFE events. Once relocated, these LFE events coincide with a zone of

high Vp/Vs ratios, found in figure 6, which is consistent with a source of

fluid movement, associated with dehydrated minerals within the

subducting lithosphere. The Moho is identified based on average

crustal thickness of 8 kilometers. Likewise; the calculated Moho depth

also coincides with P and S wave velocities that are consistent with

seismic data. This model doesn’t directly prove that fluid flows are a

response to strain change; however, it does imply that fluid flow could

promote shear slip failure.

Interpretations

The thermal model of the Cascadia subduction zone showed no correlation between

temperatures and ETS occurrence. This suggests that the age of the subducting

lithosphere does not play an important role in the occurrence of ETS. However,

other young and warm subduction zones such as the southwest Japan subduction

Krohne 11

zone or the Mexican subduction zone, all experience higher rates of ETS compared

to older and colder subduction zones. The three regions in the Cascadia subduction

zone that differ by recurrence cycle could be related to the age of the overriding

continental plate instead of the subducting plate. The north and south region of ETS

in Cascadia had recurrence cycles of 14 ± 2 and 10 ± 2 months. These two regions

correspond to older tertiary crust while the middle region, associated with 19 ± 4

month recurrence cycles, corresponds with thick siletzia crust. This represents an

age gap of 10-20 Ma and could justify the difference in the recurrence cycles (Trehu

et al. (1994); Jones et al. (1997); Harden (1998) as cited in Brudzinski & Allen

(2007)). Pore fluid models have shown to be more convincing than some thermal

modeling, as illustrated by Shelly et al. (2006). The pore fluid model identifies high

Vp/Vs ratios that correspond well with mantle velocities. The pore fluid

found in the subducting lithosphere is at consistent depths at which

fluid is normally expelled from the dehydration of minerals. Tremor

sources in Cascadia and southwest Japan also correlate with depths at

which fluid is expelled from minerals, suggesting that the dehydration

of minerals is a controlling influence on tremor. Clusters of tremors were also

located in areas of high Vp/Vs ratios, reinforcing the connection between

fluids and tremor. (Schwartz & Rokosky 2007). Further evidence from Seno &

Yamaski (2003), confirms that areas of no tremor activity in the southwest Japan

subduction zone are associated with island arc crust with low water content.

Conclusions

Krohne 12

ETS is a relative new geological process that still needs some further explanation.

Through various models and instruments, people have tired to observe tremor and

slip through a network of continuous GPS data and Hi-net borehole seismometers.

Due to the low frequency, duration, and lack of clear body wave arrivals, this

phenomenon is difficult to capture. As time progresses, more GPS locations and Hi-

net borehole seismometers will be put in place to improve our understanding of

slow slip events and seismic tremor. The future monitoring of slow earthquakes has

become and increasing concern, not only for ETS data and information, but also for

the role they play in mega thrust earthquakes. The Cascadia subduction zone has a

history of 8.0 M mega thrust earthquakes occurring every 300-500 years, with the

last one reputing over just over 300 years ago (Schwartz & Rokosky 2007). This

means that the Cascadia subduction zone is scheduled for a mega thrust earthquake

in the years to come. By monitoring these LFEs, new information could arise that

may help with the prediction of mega thrust earthquake eruptions. As stated by

Rogers and Dragert (2003), a slow slip event would increase the stress on the

locked segment of the subducting lithosphere, increasing the likelihood of a mega

thrust earthquake. From the evidence we have available today, it seems clear that

ETS events are a function of shear slip failure along the plate interface and the

dehydration of minerals could very well act to promote this failure. Based off my

research, I have interpreted that the overriding lithosphere is just as important as

the subducted lithosphere. As observed in Cascadia, Mexico, and Japan, the rheology

of the overriding plate plays a role on ETS recurrence cycles.

References

Krohne 13

1. Beroza, Gregory C., and Satoshi Ide. "Slow Earthquakes and Nonvolcanic Tremor." Annual Review of Earth and Planetary Sciences 39.1 (2011): 271-96.

2. Brudzinski, Michael R., and Richard M. Allen. "Segmentation in episodic tremor and slip all along Cascadia." Geology 35.10 (2007): 907.

3. Chapman, James S., and Timothy I. Melbourne. "Future Cascadia megathrust rupture delineated by episodic tremor and slip." Geophysical Research Letters 36.22 (2009).

4. Ghosh, Abhijit, Eduardo Huesca-Pérez, Emily Brodsky, and Yoshihiro Ito. "Very low frequency earthquakes in Cascadia migrate with tremor." Geophysical Research Letters 42.9 (2015): 3228-232.

5. Ide, Satoshi, David R. Shelly, and Gregory C. Beroza. "Mechanism of deep low frequency earthquakes: Further evidence that deep non-volcanic tremor is generated by shear slip on the plate interface." Geophysical Research Letters 34.3 (2007).

6. Ito, Yoshihiro, Kazushige Obara, Katsuhiko Shiomi, Shutaro Sekine, and Hitoshi Hirose. "Slow Earthquakes Coincident with Episodic Tremors and Slow Slip Events." Science 315.5811 (2007): 503-06.

7. Jiang, Yan, Shimon Wdowinski, Timothy H. Dixon, Matthias Hackl, Marino Protti, and Victor Gonzalez. "Slow slip events in Costa Rica detected by continuous GPS observations, 2002-2011." Geochemistry, Geophysics, Geosystems 13.4 (2012).

8. Nakata, Ryoko, Naoki Suda, and Hiroshi Tsuruoka. "Non-volcanic tremor resulting from the combined effect of Earth tides and slow slip events." Nature Geoscience 1.10 (2008): 676-78.

9. Obara, K. "Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan." Science 296.5573 (2002): 1679-681.

10. Peng, Zhigang, and Joan Gomberg. "An integrated perspective of the continuum between earthquakes and slow-slip phenomena." Nature Geoscience 3.9 (2010): 599-607.

11. Rogers, G., and Herb Dragert. "Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip." Science 300.5627 (2003): 1942-943.

12. Schwartz, Susan Y., and Juliana M. Rokosky. "Slow slip events and seismic tremor at circum-Pacific subduction zones." Reviews of Geophysics 45.3 (2007).

13. Seno, Tetsuzo, and Tadashi Yamasaki. "Low-frequency tremors, intraslab and interplate earthquakes in Southwest Japan-from a viewpoint of slab dehydration." Geophysical Research Letters 30.22 (2003).

14. Shelly, David R., Gregory C. Beroza, Satoshi Ide, and Sho Nakamula. "Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip." Nature 442.7099 (2006): 188-91.

15. Yoshioka, Shoichi, Mamiko Toda, and Junichi Nakajima. "Regionality of deep low-frequency earthquakes associated with subduction of the Philippine Sea plate along the Nankai Trough, southwest Japan." Earth and Planetary Science Letters 272.1-2 (2008): 189-98.