Historical Gorda Ridge t-phase swarms: relationships to ridge structure and the tectonic and volcanic state of the ridge during 1964-1966

The U.S. Navy's Sound Surveillance System (SOSUS) hydrophone arrays are ext emely efficient receptors of a high-frequency earthquake energy phase known as the t(ertiary)-wave, or t-phase (Fox et al., 1994), After a nearly 30-y ear hiatus in such studies, SOSUS arrays are again being utilized to detect t-phases and to locate seismic and volcanic events occurring along the Gor da seafloor spreading center (Fox et al., 1995; Fox and Dziak, 1998). Earli er, Northrop et al. (1968) also used other military arrays to infer tectoni c structure along the Gorda Ridge. From October 1964 through December 1966, over 600 low-magnitude earthquakes occurred along the Gorda Ridge. Nearly all of these events had magnitudes below the detection thresholds of land-b ased seismic networks. Northrop et al. (1968) interpreted the geographic di stribution of these events as evidence for a nascent fracture zone near the midpoint of the ridge. In the present study, the spatial distributions of these older data and, for the first time, their temporal distributions as w ell, were examined with respect to detailed bathymetry of the ridge that wa s acquired in the early 1980s. This analysis, of 570 on-axis and 74 off-axi s events, led to the following observations: (1) nearly all of the Gorda Ri dge t-phase events occurred in discreet swarms centered about the ridge axi s, (2) most of the events within each of 8 (of 9) observed swarms occurred mainly along single ridge segments, and, (3) reconfirming the earlier North rop et al. (1968) conclusion, most of the events originated in the region o f a major change in the strike of the ridge axis. During the 27-month inter val that the ridge was observed, relatively few t-phase events took place a long the northernmost segment of the Gorda Ridge where the 1996 eruption oc curred. However, a unique sequence of small events which visually resemble the events associated with a Juan de Fuca Ridge eruption in 1993 (Fox et al ., 1995) and a Gorda Ridge eruption in 1996 (Fox and Dziak, 1998) may have been associated with an eruption on the ridge during 1965. (C) 1999 Publish ed by Elsevier Science Ltd. All rights reserved.