Sonobuoy array measurements of active faulting on the Gorda Ridge

Abstract

The observed mismatch between the topography and the epicenter pattern associated with the Gorda Ridge spreading center was studied with two small arrays of four sonobuoys each deployed over the rift valley at 42°37′N and 43°N. Observed microearthquake activity originates from the crestal region and supports earlier suggestions of systematic mislocation of teleseismically determined epicenters on the Gorda Ridge. The observed seismic activity, which includes swarm events, averaged 3.5 events per hour over a total array recording time of 19.3 hours. Located microearthquakes originated from the median valley floor, valley walls, and crestal mountains. Other events, whose location could not be computed, appeared to originate from the surrounding crestal mountains with a predominance of events from west of the intersection of the Gorda Ridge and the Blanco fracture zone. Of 69 events detected, 5 were suitable for the calculation of focal depth. Focal depths at the intersection of the Gorda Ridge and Blanco fracture zone are 6.5–10 km below a 3.5‐km datum, while those farther to the south at a linear portion of the ridge range in depth from 2.5 to 6.5 km below datum. This may imply more rapid cooling near the fracture zone. A composite fault plane solution for three events on the eastern valley wall indicates movement on a high angle, with the inner wall moving upward with respect to the crestal mountains. This is the first direct evidence for uplift of median valley walls, a process which must occur if median valleys are steady state features of slowly spreading ridges.