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 42ø37'N and 43øN. Observed microearthquake activity originates from the crestal region and supports earlier suggestions of systematic mislocation of teleseismica!!y 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-kin 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.

Multiple‐event relocation of historic earthquakes along Blanco Transform Fault Zone, NE Pacific

“…Epicenters determined using teleseismically recorded P‐phase arrivals for earthquakes along the Juan de Fuca Ridge system are routinely located east or northeast of the active plate boundary [ Northrop , 1970]. While some of these earthquakes are intra‐plate events related to deformation [ Stoddard and Woods , 1990], many are plate‐boundary events with inaccurate initial locations caused by poor azimuthal distribution of seismograph stations, inadequate crustal velocity models for earthquakes along the Juan de Fuca Ridge system, or random recording errors [ Northrop et al , 1968; Northrop , 1970; Johnson and Jones , 1978; Jones and Johnson , 1978].…”

Complex P waveforms from a Gulf of Aden earthquake

“…Mendiguren 1971 ;Duschenes & Solomon 1977) have previously identified reflections from the sea surface, and this modelling of their characteristic appearance offers an accurate way of fixing the focal depths of shallow undersea earthquakes, since the arrival time of pP can be deduced once sea surface reflections have been identified with certainty. Focal depth determinations for shallow earthquakes beneath the oceans have normally only been possible using arrays of ocean bottom seismographs, or sonobuoys; depths determined by those methods have been restricted to microearthquakes (Prothero et al 1976;Jones & Johnson 1978). Teleseismic location studies are unable to constrain the depths of shallow earthquakes, and identification of surface reflections on individual seismograms is often unambiguous or uncertain.…”

Microearthquake activity on the Orozco Fracture Zone: Preliminary results from Project ROSE

“…A number of microearthquake surveys in oceanic transform fault and ridge crest areas have been conducted with sonobuoys and with ocean bottom seismometers [Francis and Porter, 1973;Reid et al, 1973Reid et al, , 1977Reid and Macdonald, 1973;Spindel et al, 1974;Macdonald and Mudie, 1974;Prothero et al, 1976;Reichle et al, 1976;Reichle and Reid, 1977;Solomon et al, 1977;Francis et al, 1977Francis et al, , 1978Lilwall et al, 1977Lilwall et al, , 1978Johnson and Jones, 1978;Jones and Johnson, 1978]. Because all such previous studies have been conducted with at most a few stations, the epicentral location capabilities have been quite limited, and the focal depth resolution generally poor.…”