the mechanics of subduction zone tremor and transient slip in japan
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The Mechanics of Subduction Zone Tremor and Transient Slip in Japan. Collaborators: Greg Beroza Stanford University Satoshi Ide University of Tokyo Sho Nakamula ERI, University of Tokyo Takahiko Uchide, University of Tokyo. David R. Shelly UC - Berkeley. Deep Nonvolcanic Tremor.

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the mechanics of subduction zone tremor and transient slip in japan

The Mechanics of Subduction Zone Tremor and Transient Slip in Japan

Collaborators:

Greg Beroza Stanford University

Satoshi Ide University of Tokyo

Sho Nakamula ERI, University of Tokyo

Takahiko Uchide, University of Tokyo

David R. Shelly UC - Berkeley

deep nonvolcanic tremor
Deep Nonvolcanic Tremor
  • Tremor associated with subduction
  • Unclear whether tremor originates from plate boundary or within upper plate

Obara, 2002

initial tremor models in japan
Initial Tremor Models in Japan

Seno and Yamasaki, 2003

Katsumata and Kamaya, 2003

Tremor thought to be generated from fluid flow near the upper plate Moho, either by shear coupling between rock and flowing fluid or by hydraulic fracturing.

episodic tremor and slip

Southwest Japan

Obara et al., 2004

Rogers and Dragert, 2003

Episodic Tremor and Slip

Cascadia

overview
Overview
  • Low-frequency earthquakes (LFEs) beneath Shikoku, Japan are
    • located on the plate interface, coincident with the transient slip
    • generated directly by shear slip in the plate convergence direction
  • Tremor is composed of a nearly continuous sequence of these LFEs
  • Slow slip events do not evolve smoothly, but are composed of many subevents of a range of sizes
western shikoku study region
Western ShikokuStudy Region

Tokyo

Shikoku

Study Region

hi net station distribution
Hi-Net Station Distribution

Hi-net

  • High-sensitivity seismograph network
    • Currently >700 stations
  • 3-component velocity seismometers, natural frequency = 1 Hz
  • Installed at the bottom of boreholes, 100+m deep
tremor and lfes

Low frequency earthquakes (LFEs)

S-wave arrival

30 sec

Tremor and LFEs

1 hour

Shelly et al., Nature, 2006

lfe correlations s wave
LFE Correlations, S-wave

Aligned with catalog S-wave arrival time

Event

Aligned with cross-correlation differential time

Event

Station N.KWBH, north component, filtered 1-8 Hz

Shelly et al., 2006

lfe correlations p wave
LFE Correlations, P-wave

Aligned with catalog (or theoretical) P-wave arrival time

Event

Aligned with cross-correlation differential time

Event

Station N.KWBH, vertical component , filtered 1-8 Hz

Shelly et al., 2006

locations and interpretation

Locked

Transient

Slip

Stable Slip?

Island Arc

Moho

Plate Interface

High Fluid

Pressure

Oceanic Moho

Locations and Interpretation

Shelly et al., Nature, 2006

Suggests LFEs are generated directly by shear slip, possibly enabled by high fluid pressure

low frequency earthquake mechanism

Mechanism of last

large earthquake in

this area, 1946 Nankai

Mechanism of

slow slip events

Low-Frequency Earthquake Mechanism

Empirical moment tensor solution

calculated using LFE waveforms

Ide et al.,

GRL, 2007

But, are LFEs are representative of tremor as a whole?

overview14
Overview
  • Low-frequency earthquakes (LFEs) beneath Shikoku, Japan are
    • located on the plate interface, coincident with the transient slip
    • generated directly by shear slip in the plate convergence direction
  • Tremor is composed of a nearly continuous sequence of these LFEs
  • Slow slip events do not evolve smoothly, but are composed of many subevents of a range of sizes
template lfe events
Template LFE Events
  • Best-recorded LFEs
  • Each with ≥6 stations (≥ 18 channels)
  • 677 LFE template events
template lfe buried in tremor
Template LFE Buried in Tremor?

Continuous Tremor

Template LFE

Is it in here somewhere???

putting detections from all 677 template events together
Putting detections from all 677 template events together….

Strong Detection

Weaker Detection

Each frame =2s

Shelly et al., Nature, 2007

tremor lfes
Tremor=LFEs
  • Tremor can be explained as many LFEs occurring in succession
  • Also provides a method for precisely locating tremor
overview21
Overview
  • Low-frequency earthquakes (LFEs) beneath Shikoku, Japan are
    • located on the plate interface, coincident with the transient slip
    • generated directly by shear slip in the plate convergence direction
  • Tremor is composed of a nearly continuous sequence of these LFEs
  • Slow slip events do not evolve smoothly, but are composed of many subevents of a range of sizes
a weeklong tremor and slip episode
A Weeklong Tremor and Slip Episode
  • April 15-21, 2006
  • Moment Magnitude=6.0 (April 17-20)
  • Average slip = 1.2 cm

Sekine and Obara, 2006

Sekine and Obara, 2006

movie of weeklong episode
Movie of Weeklong Episode

Strong Detection

Weaker Detection

Each frame = 2min

1 day

Shelly et al., G3, 2007

one day
One day…

Shelly et al., G3, 2007

migration examples
Migration Examples

Shelly et al., G3, 2007

tidal triggering of tremor
Tidal Triggering of Tremor

Very strong tidal periodicity for January event, weaker for April

Average Tidal

Period (12.4 hrs)

Shelly et al., G3, 2007

very low frequency vlf events
Very Low Frequency (VLF) events
  • Detected using seismic waves at periods of 20-50s
  • Mw≈3.3
  • Duration ≈10s
  • Mechanism consistent with plate convergence slip

Ito et al., 2007

family of slow shear slip events
Family of slow, shear-slip events

LFEs

VLFEs

SSEs

Megathrust

Ide et al., Nature, 2007

family of slow shear slip events29

Sub-events inferred

from tremor migration??

Family of slow, shear-slip events

Moment scales linearly with duration

Ide et al., Nature, 2007

conclusions section 1
Conclusions - Section 1
  • Precise locations show LFEs locate along a distinct dipping plane, which we interpret as the plate interface
  • High-resolution tomography shows LFEs occur near a region of high Vp/Vs, suggesting high fluid pressure in this region

LFEs are generated directly by shear slip during transient slip events

conclusions section 2

Also provides a means of precisely locating tremor!

Conclusions - Section 2
  • A matched-filter search of tremor using LFE template waveforms reveals that tremor is composed of a “swarm” of LFEs.

2. Therefore, tremor is generated directly by shear slip on the plate interface during slow slip transients

conclusions section 3
Conclusions - Section 3
  • Precise tremor locations indicate transient slip events do not evolve smoothly but rather contain numerous smaller and shorter-duration pulses of slip
  • Most pulses migrate along dip, rather than along strike, possibly reflecting segmentation of the plate boundary
  • Slow events of various sizes occur together in the transition zone.
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