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
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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
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SLOW SLIP
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Modes of Strain Release
Discovery of plate boundary “slow slip” requires modification of existing frictional framework
Velocity weakening
Velocity strengthening
LFE
VLFE
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Global Distribution of Slow Slip
ETS
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Modified from Dragert and Rogers [2004]Modified from Obara and Hirose [2005]
Southwest Japan- Hinet and GEONET
Cascadia- Earthscope/PBO
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Shelly et al., 2006
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Kao et al., 2008, preprint
E-layer- high reflectivity and electrical conductivity and low shear velocity- FLUIDS
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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!
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2000-2001
2006-Present
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% Locked
0 20 40 60 80 100
Episodic Aseismic Slip - Locates at frictional transitions between stable sliding and stick slip behavior
2003 & 20072000
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Thermal Modeling by Spinelli and Saffer ( 2004)
300o C isotherm from Harris and Wang (2002)
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MAY 2007 Tremor JD 135-165
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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
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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
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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).
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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?
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Integrated Seismic, Tilt, and Pore Pressure Observatory
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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
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Inversion resultsLa
titud
e
Longitude