The Mechanics of Subduction Zone Tremor and Transient Slip in Japan

1. 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

2. Deep Nonvolcanic Tremor • Tremor associated with subduction • Unclear whether tremor originates from plate boundary or within upper plate Obara, 2002

3. 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.

4. Southwest Japan Obara et al., 2004 Rogers and Dragert, 2003 Episodic Tremor and Slip Cascadia

5. Earthquake warning

6. 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

7. Western ShikokuStudy Region Tokyo Shikoku Study Region

8. 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

9. Low frequency earthquakes (LFEs) S-wave arrival 30 sec Tremor and LFEs 1 hour Shelly et al., Nature, 2006

10. 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

11. 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

12. 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

13. 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?

14. 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

15. Template LFE Events • Best-recorded LFEs • Each with ≥6 stations (≥ 18 channels) • 677 LFE template events

16. Template Event Waveforms

17. Template LFE Buried in Tremor? Continuous Tremor Template LFE Is it in here somewhere???

18. Shelly et al., 2007

19. Putting detections from all 677 template events together…. Strong Detection Weaker Detection Each frame =2s Shelly et al., Nature, 2007

20. Tremor=LFEs • Tremor can be explained as many LFEs occurring in succession • Also provides a method for precisely locating tremor

21. 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

22. 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

23. Movie of Weeklong Episode Strong Detection Weaker Detection Each frame = 2min 1 day Shelly et al., G3, 2007

24. One day… Shelly et al., G3, 2007

25. Migration Examples Shelly et al., G3, 2007

26. 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

27. 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

28. Family of slow, shear-slip events LFEs VLFEs SSEs Megathrust Ide et al., Nature, 2007

29. Sub-events inferred from tremor migration?? Family of slow, shear-slip events Moment scales linearly with duration Ide et al., Nature, 2007

30. 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

31. 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

32. 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.