Reflections on the Cascadia Slab: Dehydration and Intraslab Earthquakes

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. The times of 1200 reflected waves as well as 120,000 direct waves from both earthquakes and active sources are used in a non-linear iterative inversion scheme that simultaneously inverts for earthquake locations, smooth 3-D P-wave velocity structure and smooth reflector geometry. 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 45km depth contour of the Moho occur below the reflector in material whose velocity exceeds 7.7km/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. If intraslab earthquakes were confined only to the oceanic crust, this would geometrically limit them to about M7. Indeed, the largest intraslab events in Cascadia, such as the 2001 M6.8 Nisqually event, are near this limit. Although no reflections are observed in the vicinity of these large events, wave speeds suggest they nucleate near the subducted Moho. In Cascadia the presence of both subducted mantle and crustal intraslab earthquakes geometrically allows the possibility of significantly larger events. Additionally, the large amplitude of the reflected arrivals relative to direct arrivals suggest that wide-angle slab reflections should be included in strong ground motion calculations from megathrust events. 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.