TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 53 (No. 4), pp. 285-293, 2001

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

Dapeng Zhao1, Kelin Wang2, Garry C. Rogers2, and Simon M. Peacock3

1Department of Earth Sciences, Ehime University, Matsuyama 790-8577, Japan
2Pacific Geoscience Centre, Geological Survey of Canada, 9860 W Saanich Rd, Sidney, B.C., Canada V8L 4B2
3Department of Geology, Box 871404, Arizona State University, Tempe, AZ 85287-1404, U.S.A.

(Received June 29, 1999; Revised September 23, 2000; Accepted September 25, 2000)

Abstract: 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.

Corresponding author E-mail: zhao@sci.ehime-u.ac.jp

[Full text] (PDF 1.1 MB)