INTRASLAB EARTHQUAKES IN CASCADIA CAUSED BY DEHYDRATION EMBRITTLEMENT

We observe strong wide-angle reflections consistent in time and slowness with reflections off the subducting Juan de Fuca plate from data collected during the 1998 onshore-offshore active-source SHIPS experiment conducted within the inland waterways of NW Washington and SW British Columbia. Our dataset includes 120,000 travel times from active-source and earthquake first arrivals, supplemented by 1200 reflection arrivals. Reflection points are scattered over a broad region underlying the Olympic Peninsula. We develop a non-linear iterative inversion scheme that simultaneously inverts for earthquake locations, smooth 3-D P-wave velocity structure and smooth reflector geometry based on first arrival and reflected travel times. The well known trade-off between reflector depth and velocity structure is greatly reduced in our analysis by the combination of simultaneous inversion and high-quality data with adequate crossing paths. We interpret the reflector to be the Moho of the subducting Juan de Fuca plate. The intraslab earthquakes separate into two groups. Those that lie up-dip of the 40km depth contour of the Moho occur below the reflector in material whose velocity exceeds 7.5km/s, placing them within the subducted mantle. Those down-dip of this contour occur primarily above the reflector in places where velocities are 6.8-7.5km/s, consistent with them occurring within the subducted crust. We suggest that the intraslab earthquakes result from dehydration embrittlement. The first group is consistent with serpentine dehydration within the subducted mantle, while the second group is consistent with the onset of the basalt to eclogite transformation within the crust which, for Cascadia, is expected to begin at about 40 km depth. The presence of mantle intraslab earthquakes allows the possibility of significantly larger intraslab events than have previously been observed (M7) to occur in Cascadia. The release of fluids attendant with the basalt to eclogite transformation have observable consequences which may be related to recent discoveries of slow slip events, tremor events, and a serpentinized mantle wedge.