Cascadia Ken Creager, Steve Malone, Geoff Abers, Stephane Rondenay, Brad Hacker and Tim Melbourne Earth and Space Sciences University of Washington

CascadiaCascadia

Ken Creager, Ken Creager,

Steve Malone, Steve Malone,

Geoff Abers, Geoff Abers,

Stephane Rondenay, Stephane Rondenay,

Brad Hacker and Brad Hacker and

Tim MelbourneTim MelbourneEarth and Space SciencesUniversity of Washington

Primary Goal of CAFEPrimary Goal of CAFE

Role of water in Cascadia subductionRole of water in Cascadia subduction How does water get into the subducting How does water get into the subducting

plate?plate? Intraslab Earthquakes: Dehydration Intraslab Earthquakes: Dehydration

embrittlementembrittlement Serpentine in mantle wedgeSerpentine in mantle wedge Role of water in episodic tremor and slipRole of water in episodic tremor and slip Role of water in Cascade volcanismRole of water in Cascade volcanism

CAFECAFEJuly 2006-September 2008

Rondenay et al., 2001; Bostock et al., 2002; Nicholson et al., 2004

Episodic Tremor and Slip

Array Analysis of TremorArray Analysis of TremorMcCauslandMcCausland

CAFECAFECCascadia ascadia AArrays rrays FFor or EEarthscopearthscope

is all about wateris all about water





Faults Never Die: They Just Get Faults Never Die: They Just Get Reactivated!Reactivated!

Local mantle hydration by Local mantle hydration by

pseudofaultspseudofaults Propagating Juan de Propagating Juan de

Fuca ridge creates Fuca ridge creates offsets in isochrones offsets in isochrones called pseudofaultscalled pseudofaults

Pseudofaults may Pseudofaults may hydrate the uppermost hydrate the uppermost mantle producing 25% mantle producing 25% serpentine (McClymont serpentine (McClymont and Clowes, 2005)and Clowes, 2005)

Subsequent Subsequent dehydration of this dehydration of this serpentine as it heats serpentine as it heats up during subduction up during subduction appears to cause appears to cause intraslab earthquakes intraslab earthquakes in the mantle in the mantle lithopsherelithopshere

Wilson, 2002

Washington

Oregon

British Columbia

JDF ridge

“pseudofaults”





Lower Continental Crust

Mantle WedgeBasaltic Oceanic Crust

Transforming to eclogite

Oceanic Mantle

Serpentine dehydration

Before the slab Moho reaches a depth of 45 km most intraslab earthquakes occur within the subducting mantle, consistent with temperature induced serpentinite dehydration.East of this contour most intraslab earthquakes occur within the subducting crust, consistent with pressure induced basalt-to-eclogite dehydration reactions. (Preston et al., 2003)

Intraslab Intraslab Seismicity Seismicity

Largest intraslab earthquakes Largest intraslab earthquakes are in south sound region at are in south sound region at base of microseismicitybase of microseismicity

Deepest earthquakes are Deepest earthquakes are downstream from large downstream from large eventsevents

Intraslab seismicity is Intraslab seismicity is virtually absent north and virtually absent north and south of arch south of arch

Mantle events coincide with Mantle events coincide with subducted pseudofault subducted pseudofault

Intraslab focal mechanisms Intraslab focal mechanisms are widely scattered but are widely scattered but generally are in-plane tensiongenerally are in-plane tension

The largest intraslab earthquakes tend to occur at the base of the seismic zone.

3-D Tomographic Inversion

Active-Source Data

~92,000 first ~92,000 first arrivals from arrivals from active source active source experiments (e.g. experiments (e.g. SHIPS)SHIPS)

1200 wide-angle 1200 wide-angle reflection times reflection times from SHIPSfrom SHIPS

Local mantle hydration by Local mantle hydration by

pseudofaultspseudofaults Propagating Juan de Propagating Juan de

Fuca ridge creates Fuca ridge creates offsets in isochrones offsets in isochrones called pseudofaultscalled pseudofaults

Pseudofaults may Pseudofaults may hydrate the uppermost hydrate the uppermost mantle producing 25% mantle producing 25% serpentine (McClymont serpentine (McClymont and Clowes, 2005)and Clowes, 2005)

Subsequent Subsequent dehydration of this dehydration of this serpentine as it heats serpentine as it heats up during subduction up during subduction appears to cause appears to cause intraslab earthquakes intraslab earthquakes in the mantle in the mantle lithopsherelithopshere

Wilson, 2002

Washington

Oregon

British Columbia

JDF ridge

“pseudofaults”

subducted pseudofaultSubducted Pseudofault

Hydrated upper mantle along pseudofault manifests as upper mantle seismicity during subduction

Below Slab MohoAbove Slab Moho

Medema, 2006

Cascadia Intraslab Cascadia Intraslab EarthquakesEarthquakes

Velocity at reflector varies from 7 to 8 Velocity at reflector varies from 7 to 8 km/skm/s

West of 45-km Moho contour all events West of 45-km Moho contour all events are at or below subducted Moho; are at or below subducted Moho; wavespeeds are 7.7-8.1 km/swavespeeds are 7.7-8.1 km/s

East of 45-km contour, nearly all East of 45-km contour, nearly all events are at or above reflector; events are at or above reflector; wavespeeds are 7.0 - 7.6 km/swavespeeds are 7.0 - 7.6 km/s





Magnetic GravityStrongly magnetic rocks (left hatched) correspond to low density rocks (right hatched).

Serpentine Wedge

(Blakeley et al., 2005)





SeptembeSeptember, 2005r, 2005

ETSETS

(Thompson, Bennett (Thompson, Bennett and Johnson, and Johnson, 2006)2006)

(Melbourne, 2006)

Kao et al., 2005

Kao et al., 2005

Tremor spectra are similar to magnitude 1 earthquakes from 1-5 HzTremor are depleted in higher frequency energy





Dragert et al., Science, 2001

Tremor Tremor events start events start

in center in center and migrate and migrate NW and SNW and S

Rogers et al., Science, 2003





Dragert et al., 1994

Peacock et al., 2002

Notice disappearance of Moho boundary

Documents

Cascadia Ken Creager, Steve Malone, Geoff Abers, Stephane Rondenay, Brad Hacker and Tim Melbourne Earth and Space Sciences University of Washington