Slow slip an ubiquitous yet poorly understood mode of strain release
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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

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Slow slip an ubiquitous yet poorly understood mode of strain release
Slow Slip: An Ubiquitous yet Poorly Understood Mode of Strain Release

Susan Y. Schwartz

Department of Earth and Planetary Sciences

UC Santa Cruz


Objectives
OBJECTIVES Strain Release

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 Strain Release


Modes of Strain Release Strain Release

Velocity weakening

Velocity strengthening

VLFE

LFE

Discovery of plate boundary “slow slip” requires modification of existing frictional framework



Cascadia- Earthscope/PBO Strain Release

Southwest Japan- Hinet and GEONET

Modified from Dragert and Rogers [2004]

Modified from Obara and Hirose [2005]


Shelly et al., 2006 Strain Release


E-layer- high reflectivity and electrical conductivity and low shear velocity- FLUIDS

Kao et al., 2008, preprint


OBSERVATIONS: low shear velocity- FLUIDS

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!


2006-Present low shear velocity- FLUIDS

2000-2001


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

2003 & 2007

2000

0 20 40 60 80 100

% Locked


Thermal Modeling by Spinelli and Saffer ( 2004) between stable sliding and stick slip behavior

300o C isotherm from Harris and Wang (2002)


MAY 2007 Tremor JD 135-165 between stable sliding and stick slip behavior


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


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)


Fundamental things we do not know about slow slip and tremor
Fundamental Things We Do Not Know About Slow Slip and Tremor 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?



From Ide et al. Nature, 2007 tremor

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


Inversion results

Latitude tremor

Inversion results

Longitude


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