Kelin Wang Pacific Geoscience Centre, Geological Survey of Canada

1. Observing an Earthquake Cycle Within a Decade Kelin Wang Pacific Geoscience Centre, Geological Survey of Canada (Drawn by Roy Hyndman)

2. Important points • Stress and strain evolve in earthquake cycles. Presently observed interseismic deformation is a snapshot of a changing field. • Earthquake cycle is a common process. There are fundamental similarities between earthquake cycles of different subduction zones. • Study of multiple subduction zones that are presently at different phases of earthquake cycles will help us understand the full cycle. • This will require us to distinguish between common/fundamental processes and site-specific processes.

3. Offshore observations Land observations • Downdip limit of the seismogenic zone • Frictional behavior of deeper part of the fault • Mantle rheology • Updip limit of the seismogenic zone • Frictional behavior of shallow part of the fault

4. Sumatra: A few years after a great earthquake Courtesy Kelly Grijalva and Roland Burgmann

5. Alaska: ~ 40 years after a great earthquake M = 9.2, 1964 Freymueller et al. (2008)

6. M = 9.5, 1960 Chile: ~ 40 years after a great earthquake GPS data: Green: Klotz et al. (2001) Red: Wang et al. (2007)

7. Wells and Simpson (2001) Cascadia: ~ 300 years after a great earthquake

8. Inter-seismic 2 (Cascadia) Inter-seismic 1 (Alaska, Chile) Post-seismic (Sumatra) Co-seismic Coast line Coast line

9. Afterslip Stress relaxation Stress relaxation Locking Rupture ETS Afterslip and transient slow slip: short-lived, fault friction Stress relaxation: long-lived, mantle rheology

10. (c) Viscoelastic stress relaxation model for Chile, viscosity 2.5  1019 Pa s

11. 1995 Antofagasta earthquake, N. Chile (Mw = 8.0) 1993-95 Displacements (dominated by co-seismic) 1996-97 Velocities (2 years after earthquake) Data from Klotz et al. (1999) and Khazaradze and Klotz (2003)

12. Coast line Inter-seismic 2 (Cascadia) ? Inter-seismic 1 (Alaska, Chile) ? Post-seismic (Sumatra) Co-seismic Coast line ? ? ?

13. Coast line Coseismic (contours) and 1-yr postseismic (color) slip of 2005 Nias-Simeulue earthquake GPS stations ? coseismicslip (2 m contours) ? Hsu et al. (2006) Coast line

14. Coast line GPSA off Peru (Gagnon et al., 2005) Updip segment is not slipping. Fully relaxed? Coast line ?

15. Coast line Very-low-frequency earthquakes possibly in Nankai accretionary prism (Ito and Obara, 2006) Coast line ?

16. Coast line Coast line Fluid pressure during a VLF episode ? ? Near-trench boreholes off Mutoto VLF events Davis et al. (2006)

17. Stress increase A few MPa Average stress ~ 15 MPa b  0.04 Stress drop ~ 4 MPa b  -0.01 b > 0

18. Mechanism of the velocity-strengthening behavior: • Soft frontal prism sediment • Presence of stable-sliding minerals • Dilatancy of granular gouge material upon fast shearing • Inability to localize deformation during fast shearing Evidence for a velocity-strengthening shallow segment: • Lack of evidence for massive trench-breaking rupture • Slip patterns from inversion of seismic/tsunami/geodetic data • Inferences based on continental earthquakes • Real-time monitoring at Hokkaido (2003) and Sumatra (2005) Studying the shallow segment is as important as studying the seismogenic zone

19. Importance I: Tsunamigenic seafloor deformation

20. Importance I: Tsunamigenic seafloor deformation Earthquakes of same moment magnitude Less strengthening of the shallow segment leads to trench-breaking rupture. Trench-breaking rupture causes less seafloor uplift.

21. Importance II: Deformation of the frontal prism (Dynamic Coulomb wedge) Inter-seismic: lower basal stress Co-seismic: Basal fault strengthens; greater compression and pore fluid pressure within the prism Cumulative effects of numerous great earthquakes control wedge taper

22. Seismogenic Zone Importance III: Coseismic activation of megasplay (Park et al., 2002)

23. Stress increase; resisting slip Rupture; Stress drop Stress decrease Locked; Stress increase Co-seismic Post-seismic

24. Rate depends on friction properties ? Rate depends on mantle rheology Immediately following an earthquake

25. Slip quickly slows down Fully locked? Stress increases but shortening slowly slows down Longer time after the earthquake

27. Issues to be resolved by observations • Does deformation at differentstages of the earthquake cycle leave different signatures in rock samples? • How far does coseismic rupture propagate updip? How common or rare is trench-breaking rupture? • How do the frontal prism and splay faultsrespond to megathrust motion during, after, and between earthquakes? • How does pore fluid pressure within the frontal prism and along the megathrust evolve in earthquake cycles? • How does the coseismically strengthened shallow segment of megathrust relax after the earthquake? What is the time scale of the relaxation? • How does the oceanic mantle respond to earthquake cycles? What viscosity model and value? Is it similar to the mantle wedge? • … …

28. Important points • Stress and strain evolve in earthquake cycles. Presently observed interseismic deformation is a snapshot of a changing field. • Earthquake cycle is a common process. There are fundamental similarities between earthquake cycles of different subduction zones. • Study of multiple subduction zones that are presently at different phases of earthquake cycles will help us understand the full cycle. • This will require us to distinguish between common/fundamental processes and site-specific processes.

29. Suggestions for SEIZE • Study multiple subduction zones that are presently at different phases of earthquake cycles • Monitor strain, pore fluid pressure, etc., correlate with land-based networks • Transects of shallow boreholes • Monitor locked and creeping segments

30. Earthquake followed by locking

31. Different along-strike rupture lengths and slip magnitudes (surface velocities 35 years after an earthquake; mantle viscosity 2.5 x 1019 Pa s)

32. Coast line Coast line Very-low-frequency earthquakes in Nankai accretionary prism ?