Open Access

Tomographic image of low P velocity anomalies above slab in northern Cascadia subduction zone

  • Dapeng Zhao1Email author,
  • Kelin Wang2,
  • Garry C. Rogers2 and
  • Simon M. Peacock3
Earth, Planets and Space201453:BF03352385

DOI: 10.1186/BF03352385

Received: 29 June 1999

Accepted: 25 September 2000

Published: 18 June 2014


At the Cascadia margin the Juan de Fuca plate is subducting beneath the North 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 earthquakes 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 continuously 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 extensive 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 velocity patches coincides with an abrupt increase in surface heat flow and may reflect the presence of partial melts or water in the crust.