INVERSION FOR SLIP DISTRIBUTION USING TELESEISMIC P WAVEFORMS: NORTH PALM SPRINGS, BORAH PEAK, AND MICHOACAN EARTHQUAKES

We have inverted the teleseismic P waveforms recorded by stations of the Global Digital Seismograph Network for the 8 July 1986 North Palm Springs, California, the 28 October 1983 Borah Peak, Idaho, and the 19 September 1985 Michoacan, Mexico, earthquakes to recover the distribution of slip on each of the faults using a point-by-point inversion method with smoothing and positivity constraints. In the inversion procedure, a fault plane with fixed strike and dip is placed in the region of the earthquake hypocenter and divided into a large number of subfaults. Rupture is assumed to propagate at a constant velocity away from the hypocenter, and synthetic ground motions for pure strike-slip and dip-slip dislocations are calculated at the teleseismic stations for each subfault. The observed seismograms are then inverted to obtain the distribution of strike-slip and dip-slip displacement for the earthquake. Results of the inversion indicate that the Global Digital Seismograph Network data are useful for deriving fault dislocation models for moderate to large events. However, a wide range of frequencies, which includes periods shorter than those within the passband of the long-period Global Digital Seismograph Network instruments, is necessary to infer the distribution of slip on the earthquake fault. Although the long-period waveforms define the size (dimensions and seismic moment) of the earthquake, data at shorter periods provide additional constraints on the variation of slip on the fault. Dislocation models obtained for all three earthquakes are consistent with a heterogeneous rupture process where failure is controlled largely by the size and location of high-strength asperity regions.