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 Discovery of plate boundary “slow slip” requires modification of existing frictional framework Velocity weakening Velocity strengthening LFE VLFE

6. Global Distribution of Slow Slip ETS

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

9. Shelly et al., 2006

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

11. Basalt to eclogite transformation releases fluid 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- 550 o C 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. 2000-2001 2006-Present

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

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

15. MAY 2007 Tremor JD 135-165

16. Costa Rica (2007) CascadiaSW Japan (LT) SW Japan (ST) Guerrero, Mexico New Zealand Surface slip 1.2 cm2-6 mm3 cm2-5 mmUp to 6 cm2-3 cm Slip on fault 10 cm2-3 cm5-30 cm1-4 cm9-30 cm18-50 cm Depth of slip 20-30 km within Seismogenic Zone 30-45 km at downdip frictional transition 30-45 km at downdip frictional transition 35-45 at downdip frictional transition 30 km at downdip frictional transition <15 km and 30-50 km at variable downdip frictional transition Duration 30 days40-50 days6 mo-1 yr4-7 days5-6 mo10-550 days Equivalent magnitude ~6.7-6.86.2-6.86.7-7.05.7-6.27.1-7.5~7.0 Tremor Character Intermittent Short duration ETS IntermittentNONE Thermal State COOLHOT WARMCOOL

17. Hot or warm subduc- tion zones have slow slip at downdip frictional transition Cascadia SW Japan Mexico Cooler subduction zones or with thin overriding crust have slow slip shallower than frictional transition NE Japan (afterslip) Boso Japan Costa Rica 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

18. Hot subduction zones have slow slip closely associated with tremor Cascadia SW Japan 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) Seismic tremor associated with slow slip may be facilitated by fluids generated from dehydration reactions (baslate-eclogite).

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. T = c M o c ~ 10 -12 - 10 -13 s/Nm From Ide et al. Nature, 2007 Low Stress Drop Model Diffusional Model ASSUMES: D  LD = c M o  L 3 M o  L 2  = c (~10 kPa)   L -1 V r  L -2 V r  L -1

22. Inversion results Latitude Longitude