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

Susan Y. Schwartz

Department of Earth and Planetary Sciences

UC Santa Cruz

OBJECTIVESGlobal 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 30-40 km depth (the downdip edge of the seismogenic zone) where T~ 450-550oC

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!

2000-2001

2006-Present

% Locked

0 20 40 60 80 100

Episodic Aseismic Slip - Locates at frictional transitions between stable sliding and stick slip behavior

2003 & 20072000

Thermal Modeling by Spinelli and Saffer ( 2004)

300o C isotherm from Harris and Wang (2002)

MAY 2007 Tremor JD 135-165

Costa Rica (2007)

Cascadia SW Japan (LT)

SW Japan (ST)

Guerrero, Mexico

New Zealand

Surface slip

1.2 cm 2-6 mm 3 cm 2-5 mm Up to 6 cm 2-3 cm

Slip on fault

10 cm 2-3 cm 5-30 cm 1-4 cm 9-30 cm 18-50 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 days 40-50 days 6 mo-1 yr 4-7 days 5-6 mo 10-550 days

Equivalent magnitude

~6.7-6.8 6.2-6.8 6.7-7.0 5.7-6.2 7.1-7.5 ~7.0

Tremor Character

Intermittent

Short duration

ETS ETS ETS Intermittent NONE

Thermal State

COOL HOT HOT HOT WARM COOL

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 Mo c ~ 10-12 - 10-13 s/Nm

From Ide et al. Nature, 2007

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

Inversion resultsLa

titud

e

Longitude