IMPORTANCE OF MODEL PARAMETERIZATION IN FINITE FAULT INVERSIONS - APPLICATION TO THE 1974 MW 8.0 PERU EARTHQUAKE

The spatial and temporal slip distributions for the October 3, 1974 (M (W) = 8.0), Peru subduction zone earthquake and its largest aftershock on November 9 (M(S) = 7.1) are calculated and analyzed in terms of th e inversion parameterization and tectonic significance. Teleseismic, l ong-period World-Wide Standard Seismograph Network, P and SH waveforms are inverted to obtain the rupture histories. We demonstrate that err oneous results are obtained if a parameterization is used that does no t allow for a sufficiently complex source, involving spatial variation in slip amplitude, risetime, and rupture time. The inversion method u tilizes a parameterization of the fault that allows for a discretized source risetime and rupture time. Well-located aftershocks recorded on a local network have the same general pattern as teleseismically dete rmined hypocenters and help to constrain the geometry of the subductio n zone. For the main shock a hinged fault is preferred having a shallo w plane with a dip of 11-degrees and a deeper, landward plane with a d ip of 30-degrees. The preferred nucleation depth lies between 11 and 1 5 km. A bilateral rupture is obtained with two major concentrations of slip, one 60 to 70 km to the northwest of the epicenter and a second 80 to 100 km to the south and southeast of the epicenter. For these so urce regions, risetimes vary from 6 to 18 s. Our estimates of risetime s are consistent with the time for the rupture to traverse the dominan t local asperity. The slip distribution for the November 9 aftershock falls within a conspicuous hole in the main shock rupture pattern, nea r the hypocenter of the main shock. The November 9 event has a simple risetime function with a duration of 2 s. Aftershocks recorded by the local network are shown to cluster near the hypocenter of the impendin g November 9 event and downdip from the largest main shock source regi on. Slip during the main shock is concentrated at shallow depths above 15 km and extends updip from the hypocenter to near the plate boundar y at the trench axis. The large amount of slip at shallow depths is at tributed to the absence of any significant accretionary wedge of sedim ents, and the relatively young age and high convergence rate of the su bducted plate, which results in good seismic coupling near the trench axis.