Slow Slip: An Ubiquitous yet Poorly Understood Mode of Strain Release

1. Slow Slip: An Ubiquitous yet Poorly Understood Mode of Strain Release Susan Y. Schwartz Department of Earth and Planetary Sciences UC Santa Cruz

2. OBJECTIVES Global Characterization of Slow Slip Events and Associated Seismic Tremor • Ubiquitous Phenomena- Almost all subduction zones with instrumentation capable of recording slow slip events have done so • Require broadening of existing frictional framework for the seismogenic zone and phenomena has promise to improve our understanding of mechanical behavior • Variations in the depth range of slow slip and in the nature of associated tremor between subduction zones may be important to our understanding • Large scale experiment focused on understanding slow slip phenomena might be considered by future SEISMOGENIC ZONE EXPERIMENT

3. SLOW SLIP

4. Modes of Strain Release Velocity weakening Velocity strengthening VLFE LFE Discovery of plate boundary “slow slip” requires modification of existing frictional framework

6. Global Distribution of Slow Slip ETS

8. Cascadia- Earthscope/PBO Southwest Japan- Hinet and GEONET Modified from Dragert and Rogers [2004] Modified from Obara and Hirose [2005]

9. Shelly et al., 2006

10. E-layer- high reflectivity and electrical conductivity and low shear velocity- FLUIDS Kao et al., 2008, preprint

11. OBSERVATIONS: Slow slip is frequent and regular Slow slip is always accompanied by tremor and occurs in regions inferred to have fluids Slow slip occurs at 30-40 km depth (the downdip edge of the seismogenic zone) where T~ 450-550oC Basalt to eclogite transformation releases fluid IMPLICATIONS: Both slow slip and tremor involve fluids released from the basalt to eclogite transformation. Cooler subduction zones may not exhibit slow slip and tremor WRONG!

12. 2006-Present 2000-2001

13. Episodic Aseismic Slip - Locates at frictional transitions between stable sliding and stick slip behavior 2003 & 2007 2000 0 20 40 60 80 100 % Locked

14. Thermal Modeling by Spinelli and Saffer ( 2004) 300o C isotherm from Harris and Wang (2002)

15. MAY 2007 Tremor JD 135-165

17. Hot or warm subduc-tion zones have slow slip at downdip frictional transition Cascadia SW Japan Mexico Deep slow slip may require frictional transition at shallow depth (low pressure) or where downgoing plate is in contact with the the crust of the overlying plate Cooler subduction zones or with thin overriding crust have slow slip shallower than frictional transition NE Japan (afterslip) Boso Japan Costa Rica

18. Hot subduction zones have slow slip closely associated with tremor Cascadia SW Japan Seismic tremor associated with slow slip may be facilitated by fluids generated from dehydration reactions (baslate-eclogite). Cooler subduction zones have slow slip and intermittent or no tremor NE Japan (afterslip) Boso Japan (no tremor) Mexico (intermittent) Costa Rica (intermittent tremor) New Zealand (no tremor)

19. Fundamental Things We Do Not Know About Slow Slip and Tremor • Do dominant weakening mechanisms between fast (changes in coefficient of friction) and slow slip differ? • Is slow slip always constrained to occur at frictional stability transitions? • Is tremor shear slip on the plate interface or fluid migration, both or neither?

20. Integrated Seismic, Tilt, and Pore Pressure Observatory

21. From Ide et al. Nature, 2007 T = c Mo c ~ 10-12 - 10-13 s/Nm Low Stress Drop Model Diffusional Model ASSUMES: D  L D = c Mo L3 Mo L2  = c (~10 kPa)   L-1 Vr L-2 Vr L-1

22. Latitude Inversion results Longitude