Cascadia

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

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

3. July 2006- September 2008 CAFE

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

9. Episodic Tremor and Slip

12. Array Analysis of TremorMcCausland

15. CAFE Cascadia Arrays For Earthscope is all about water

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

17. Faults Never Die: They Just Get Reactivated!

18. Local mantle hydration by 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) • Subsequent dehydration of this serpentine as it heats up during subduction appears to cause intraslab earthquakes in the mantle lithopshere British Columbia “pseudofaults” JDF ridge Washington Oregon Wilson, 2002

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

20. Lower Continental Crust Mantle Wedge Basaltic Oceanic Crust Transforming to eclogite Serpentine dehydration Oceanic Mantle • 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)

21. Intraslab Seismicity • Largest intraslab earthquakes are in south sound region at base of microseismicity • Deepest earthquakes are downstream from large events • Intraslab seismicity is virtually absent north and south of arch • Mantle events coincide with subducted pseudofault • Intraslab focal mechanisms are widely scattered but generally are in-plane tension

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

23. 3-D Tomographic InversionActive-Source Data • ~92,000 first arrivals from active source experiments (e.g. SHIPS) • 1200 wide-angle reflection times from SHIPS

25. Local mantle hydration by 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) • Subsequent dehydration of this serpentine as it heats up during subduction appears to cause intraslab earthquakes in the mantle lithopshere British Columbia “pseudofaults” JDF ridge Washington Oregon Wilson, 2002

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

27. Cascadia Intraslab Earthquakes • 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 7.7-8.1 km/s • East of 45-km contour, nearly all events are at or above reflector; wavespeeds are 7.0 - 7.6 km/s

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

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

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

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

32. (Melbourne, 2006)

39. Kao et al., 2005

40. Tremor spectra are similar to magnitude 1 earthquakes from 1-5 Hz Tremor are depleted in higher frequency energy Kao et al., 2005

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

44. Dragert et al., Science, 2001

45. Tremor events start in center and migrate NW and S

47. Rogers et al., Science, 2003

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