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Pelayo, A.M., 1990

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Bibliographic Reference

Pelayo, A.M., 1990, Earthquake source parameter inversion using body and surface waves: Applications to tsunami earthquakes and to Scotia Sea seismotectonics: Saint Louis, Missouri, Washington University, Ph.D. dissertation, 336 p.

Abstract

Seismic body waveform and surface wave spectral amplitude modeling techniques are applied to the study of tsunami earthquakes and the seismotectonics of the Scotia Sea region. The rare occurrence of a tsunami earthquake, defined as a tsunamigenic earthquake whose moderate-size MS considerably underrates its destructive tsunami potential, can effectively undermine a tsunami warning system which uses the standard MS for its forecast. Four tsunami earthquakes originating beneath the accretionary wedges in northern Peru, the southern Kuriles and the eastern Aleutians are studied. All the P-waveforms from these earthquakes show complicated PwP reverberations that obscure depth and stopping phases on the seismograms. The SH-waveforms are simpler and source time histories deconvolved from these waveforms are generally consistent with the surface wave spectral amplitudes. Results suggest that tsunami earthquakes involve a slow source process (rupture velocity on the order of about 0.5 km/s) along shallowly dipping faults in the shallow part of accretionary prisms. The seismic moments we determined are much larger than expected based on the magnitudes reported for the earthquakes and are generally consistent with the observed tsunamis. The long source durations of the tsunami earthquakes render the MS scale an inappropriate measure of the size of these events. Another earthquake, which occurred in the northeastern Philippines and was previously classified as a tsunami earthquake, was also studied and found to be more typical of a fast rupture. We do not consider this Philippine event a tsunami earthquake. Focal mechanisms determined from P- and SH-waveform and amplitude inversion of 20 shallow earthquakes along the poorly understood tectonic boundaries in the Scotia Sea provide constraints on the relative plate motions in the region. Slip vectors from eight of the earthquakes studied were used in a 4-plate model to quantify the relative rates of motions along boundaries. Left-lateral transcurrent motion with a component of compression is predicted at a rate of 0.6 cm/yr along the North Scotia Ridge between the Scotia and South American plates. Left-lateral transcurrent motion with a component of extension is predicted at a rate of 1.0 cm/yr along the South Scotia Ridge between the Scotia and Antarctic plates.

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