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

Primary Goal of CAFE Role of water in Cascadia subduction Role of water in Cascadia subduction How does water get into the subducting plate? How does water get into the subducting plate? Intraslab Earthquakes: Dehydration embrittlement Intraslab Earthquakes: Dehydration embrittlement Serpentine in mantle wedge Serpentine in mantle wedge Role of water in episodic tremor and slip Role of water in episodic tremor and slip Role of water in Cascade volcanism Role of water in Cascade volcanism

CAFE July September 2008

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

Episodic Tremor and Slip

Array Analysis of Tremor McCausland

CAFE Cascadia Arrays For Earthscope is all about water

Primary Goal of CAFE Role of water in Cascadia subduction Role of water in Cascadia subduction How does water get into the subducting plate? How does water get into the subducting plate? Intraslab Earthquakes: Dehydration embrittlement Intraslab Earthquakes: Dehydration embrittlement Serpentine in mantle wedge Serpentine in mantle wedge Role of water in episodic tremor and slip Role of water in episodic tremor and slip Role of water in Cascade volcanism Role of water in Cascade volcanism

Faults Never Die: They Just Get Reactivated!

Local mantle hydration by pseudofaults Propagating Juan de Fuca ridge creates offsets in isochrones called pseudofaults Propagating Juan de Fuca ridge creates offsets in isochrones called pseudofaults Pseudofaults may hydrate the uppermost mantle producing 25% serpentine (McClymont and Clowes, 2005) Pseudofaults may hydrate the uppermost mantle producing 25% serpentine (McClymont and Clowes, 2005) Subsequent dehydration of this serpentine as it heats up during subduction appears to cause intraslab earthquakes in the mantle lithopshere Subsequent dehydration of this serpentine as it heats up during subduction appears to cause intraslab earthquakes in the mantle lithopshere Wilson, 2002 Washington Oregon British Columbia JDF ridge pseudofaults

Primary Goal of CAFE Role of water in Cascadia subduction Role of water in Cascadia subduction How does water get into the subducting plate? How does water get into the subducting plate? Intraslab Earthquakes: Dehydration embrittlement Intraslab Earthquakes: Dehydration embrittlement Serpentine in mantle wedge Serpentine in mantle wedge Role of water in episodic tremor and slip Role of water in episodic tremor and slip Role of water in Cascade volcanism Role of water in Cascade volcanism

Lower Continental Crust Mantle Wedge Basaltic 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 Seismicity Largest intraslab earthquakes are in south sound region at base of microseismicity Largest intraslab earthquakes are in south sound region at base of microseismicity Deepest earthquakes are downstream from large events Deepest earthquakes are downstream from large events Intraslab seismicity is virtually absent north and south of arch Intraslab seismicity is virtually absent north and south of arch Mantle events coincide with subducted pseudofault Mantle events coincide with subducted pseudofault Intraslab focal mechanisms are widely scattered but generally are in-plane tension Intraslab focal mechanisms are widely scattered but generally 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 arrivals from active source experiments (e.g. SHIPS) ~92,000 first arrivals from active source experiments (e.g. SHIPS) 1200 wide-angle reflection times from SHIPS 1200 wide-angle reflection times from SHIPS

Local mantle hydration by pseudofaults Propagating Juan de Fuca ridge creates offsets in isochrones called pseudofaults Propagating Juan de Fuca ridge creates offsets in isochrones called pseudofaults Pseudofaults may hydrate the uppermost mantle producing 25% serpentine (McClymont and Clowes, 2005) Pseudofaults may hydrate the uppermost mantle producing 25% serpentine (McClymont and Clowes, 2005) Subsequent dehydration of this serpentine as it heats up during subduction appears to cause intraslab earthquakes in the mantle lithopshere Subsequent dehydration of this serpentine as it heats up during subduction appears to cause intraslab earthquakes in the mantle lithopshere Wilson, 2002 Washington Oregon British Columbia JDF ridge pseudofaults

subducted pseudofault Subducted Pseudofault Hydrated upper mantle along pseudofault manifests as upper mantle seismicity during subduction Below Slab Moho Above Slab Moho Medema, 2006

Cascadia Intraslab Earthquakes Velocity at reflector varies from 7 to 8 km/s Velocity at reflector varies from 7 to 8 km/s West of 45-km Moho contour all events are at or below subducted Moho; wavespeeds are km/s West of 45-km Moho contour all events are at or below subducted Moho; wavespeeds are km/s East of 45-km contour, nearly all events are at or above reflector; wavespeeds are km/s East of 45-km contour, nearly all events are at or above reflector; wavespeeds are km/s

Primary Goal of CAFE Role of water in Cascadia subduction Role of water in Cascadia subduction How does water get into the subducting plate? How does water get into the subducting plate? Intraslab Earthquakes: Dehydration embrittlement Intraslab Earthquakes: Dehydration embrittlement Serpentine in mantle wedge Serpentine in mantle wedge Role of water in episodic tremor and slip Role of water in episodic tremor and slip Role of water in Cascade volcanism Role of water in Cascade volcanism

Magnetic Gravity Strongly magnetic rocks (left hatched) correspond to low density rocks (right hatched). Serpentine Wedge (Blakeley et al., 2005)

Primary Goal of CAFE Role of water in Cascadia subduction Role of water in Cascadia subduction How does water get into the subducting plate? How does water get into the subducting plate? Intraslab Earthquakes: Dehydration embrittlement Intraslab Earthquakes: Dehydration embrittlement Serpentine in mantle wedge Serpentine in mantle wedge Role of water in episodic tremor and slip Role of water in episodic tremor and slip Role of water in Cascade volcanism Role of water in Cascade volcanism

September, 2005 ETS (Thompson, Bennett and Johnson, 2006)

(Melbourne, 2006)

Kao et al., 2005

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

Primary Goal of CAFE Role of water in Cascadia subduction Role of water in Cascadia subduction How does water get into the subducting plate? How does water get into the subducting plate? Intraslab Earthquakes: Dehydration embrittlement Intraslab Earthquakes: Dehydration embrittlement Serpentine in mantle wedge Serpentine in mantle wedge Role of water in episodic tremor and slip Role of water in episodic tremor and slip Role of water in Cascade volcanism Role of water in Cascade volcanism

Dragert et al., Science, 2001

Tremor events start in center and migrate NW and S

Rogers et al., Science, 2003

Primary Goal of CAFE Role of water in Cascadia subduction Role of water in Cascadia subduction How does water get into the subducting plate? How does water get into the subducting plate? Intraslab Earthquakes: Dehydration embrittlement Intraslab Earthquakes: Dehydration embrittlement Serpentine in mantle wedge Serpentine in mantle wedge Role of water in episodic tremor and slip Role of water in episodic tremor and slip Role of water in Cascade volcanism Role of water in Cascade volcanism

Dragert et al., 1994

Peacock et al., 2002

Notice disappearance of Moho boundary