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

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

SLOW SLIP

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

Global Distribution of Slow Slip ETS

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

Shelly et al., 2006

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

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 km depth (the downdip edge of the seismogenic zone) where T~ 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!

Present

% Locked Episodic Aseismic Slip - Locates at frictional transitions between stable sliding and stick slip behavior 2003 &

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

MAY 2007 Tremor JD

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 km within Seismogenic Zone km at downdip frictional transition km at downdip frictional transition at downdip frictional transition 30 km at downdip frictional transition <15 km and km at variable downdip frictional transition Duration 30 days40-50 days6 mo-1 yr4-7 days5-6 mo days Equivalent magnitude ~ ~7.0 Tremor Character Intermittent Short duration ETS IntermittentNONE Thermal State COOLHOT WARMCOOL

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

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).

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?

Integrated Seismic, Tilt, and Pore Pressure Observatory

T = c M o c ~ 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

Inversion results Latitude Longitude