Irregular geometry of the Gorda subduction and deep structure of the Eel River basin determined from teleseismic P delays

Abstract

Nearly 1000 observations of travel-time residuals for teleseismic P arrivals across the Humboldt Bay Seismic Network near Cape Mendocino, California, are analyzed for a study of 3-D geometry of the Gorda plate near Mendocino Triple Junction (MTJ), by the application of a three-dimensional velocity inversion procedure. The area is underlain by the subducted edge of the Gorda plate and the eastward extension of the Mendocino Fracture Zone. The observed variation of the travel-time gradient with azimuth favors either deepening of the slab southward, or southward increase of dilatational velocity across the Mendocino Fracture Zone at depths of more than 50 km. The topographically corrected mean travel-time delays at individual stations suggest that the Eel River basin is deep and tectonically controlled. Further support for this view is provided by the inversion results, which point to the possible extension of the low-velocity material through the entire crustal section. These results also suggest that from the south, the Eel River basin may be confined at depth by a steeply northward-dipping sliver of high-velocity material (possibly a detached oceanic slab). This feature may be responsible for the intense seismic activity reported beneath the eastern end of the Mendocino Fracture Zone (MFZ). Thus, the role of the Eel River basin in accommodating the relative motion between the adjacent plates, may be significant. The leading edge of the downgoing slab appears irregular and at some latitudes may not extend beyond the coast line. The estimated velocity anomalies vary by as much as 20% in the near-surface blocks and nearly half as much at depth. The pattern may result from intense tearing and deformation of the slab, producing buckling, folding and compression ridges.