Accurate source depths and focal mechanisms of shallow earthquakes in western South America and in the New Hebrides island arc (Vanuatu) [microform]

Synthetic seismograms are matched to long-period P waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms. Accurate depths are obtained only for moderate-sized (6 < Ms < 7) events which produced relatively simple P waveforms. Events which are difficult to match are rejected. The quality of matching between the observed and synthetic seismograms is determined mainly by fitting zero-crossing times (i.e. the points where the trace of the seismogram crosses the time axis) rather than calculating cross-correlation coefficients which are used in most other studies. If the quality of matching is good and if the average velocities used in modelling the region above the source are within (+OR-) 10% of the "true" crustal velocities, then the estimated error in the synthetic seismogram depth will be about (+OR-) 5 km or less. This procedure is applied to two suites of events, one in western South America and one in the New Hebrides.

The accurate depths provide new information on the bending of descending plates near trenches, the seismicity and tectonics of the overriding plate, and the geometry and structure of the zone of contact between the overriding and descending plates.