At the Cascadia margin the Juan de Fuca plate is subducting beneath the Nor
th America plate, causing active seismicity within both plates. Earthquakes
occur down to a maximum depth of 80 km within the descending oceanic plate
and to about 30 km in the overriding continental plate. We use a method of
seismic tomography to invert 28,230 P wave arrival times from 2666 local e
arthquakes that occurred in and around Vancouver Island from 1970 to 1990.
The tomography model uses about 30 km horizontal and 12-19 km vertical grid
spacing and assumes that the seismic velocity perturbations vary continuou
sly between grid points. Velocity structures can be obtained to a depth of
65 km. The obtained tomographic image shows an extensive low velocity zone
above the subducted slab at about 45 km depth and patches of low velocities
at shallower depths just seaward of the volcanic front. The deeper extensi
ve low velocity zone may indicate the presence of partially hydrated mantle
, most likely serpentinite, as a result of slab dehydration associated with
the transformation of metabasalt to eclogite. One of the shallow low veloc
ity patches coincides with an abrupt increase in surface hear flow and may
reflect the presence of partial melts or water in the crust.