Yang Shen, Woods Hole Oceanographic Institution, Woods Hole, MA
Donald W. Forsyth, James Conder, Brown University, Providence, RI
LeRoy M. Dorman, Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0215, USA.Abstract
Between February 1991 and May 1992, 33 intraplate earthquakes having body wave magnitudes between 4.3 and 6.0 were located on the west flank of the Southern East Pacific Rise by the International Seismological Center (ISC). Seven Months after the last teleseismic event, we deployed four ocean-bottom seismometers at the site of the teleseismic swarm. During the 16-day deployment, 192 earthquakes were located using records from three or more instruments. Most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains. In addition, several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis. The focal depths of the microearthquakes range from 1 to 15 km, and most are between 5 and 12 km, similar to the depth range of the teleseismic events [Hung and Forsyth, 1996]. The composite P-wave polarities indicate that the microearthquakes had a variety of focal mechanisms. We developed a new grid-search, inversion technique that utilizes the P-wave polarities and the empirically corrected ratios of P and S wave amplitudes to find the focal mechanisms of individual events. Normal faulting is found in the ridge-parallel seismicity line. The thrust and strike-slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events. There is no apparent depth dependence of fault types. None of the existing models of the sources of stress (ridge push, thermoelastic stresses, loading by topographic features, caldera collapse, and north-south extension of the Pacific Plate) provides a satisfactory explanation for both the teleseismic swarm and microearthquakes. We propose a new tectonic scenario. In this scenario, the lithosphere is prestressed by the cooling of the plate. Magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere, which is already under extensional, thermal stress, producing the larger, teleseismically detected events. Once the tip of the dikes propagate into the lithosphere, the region surrounding the dikes behind the tips is compressed by the overpressure of magma. Depending on the geometry of the dikes, local orientations of the minimum principal stress, and local weaknesses in the lithosphere,, thrust of strike-slip faulting ( the microearthquakes) may occur.