Modeling earthquakes with local and regional broadband data

[摘要] Waveform modeling techniques are applied to several recent, moderate sized earthquakesrecorded by the broadband TERRAscope array in southern California. Amethod for the determination of source parameters at regional distances with three-component,sparse network data is described. The sensitivity of the method to sourcemislocations and velocity model is investigated. The method is relatively insensitiveto source mislocation. The choice of velocity model can affect the inversion results,but it appears that for a number of paths throughout southern California, a simpleplane layered velocity model derived from travel-time studies explains much of theobserved waveforms.The broadband waveforms of two small earthquakes that occurred in 1988 nearUpland, California are forward modeled to determine Green's functions for the pathto Pasadena, California. The effects of near surface gradients, crustal interface sharpness,and two-dimensional basin-ridge structures were studied. This analysis resultedin a simple plane layered velocity model that best fit the data. The Green's functionsare then used to study the source characteristics of the 1990 Upland mainshock (M_L = 5.2). The long-period body waves are inverted to determine the orientationand seismic moment. Comparisons of the 1990 mainshock with the 1988 events revealedthat the mainshock was a relatively complicated event. Multi-point sourceand distributed finite slip models show that the mainshock ruptured down dip (6km to 9km) with a non-uniform slip distribution in which 30 % of the total seismicmoment was released from a relatively small area at 9 km depth. The overall areaof the mainshock was found to be significantly smaller than the aftershock zone.The source process of the June 28, 1991 Sierra Madre earthquake (M_L = 5.8)is investigated using the broadband data recorded at 6 TERRAscope stations. Thelong-period body waves are inverted to determine the orientation and seismic moment.Ratios of the peak amplitudes of simulated short-period Wood-Anderson andlong-period Wood-Anderson seismograms are compared for the mainshock and thetwo largest aftershocks. The ratios show that stations southwest of the epicenterhave elevated levels of short-period energy relative to stations to the east suggestingthe presence of directivity. The displacement waveforms were forward modeled usingdistributed finite slip models. The best fitting model consists of an updip rupturetoward the west. This model fails however to explain the amplitudes of the short-periodwaves. A non-uniform slip model was developed that better explains theshort-period amplitudes. The results of this analysis indicate that the shorter-periodenergy is controlled more by the patches on the fault that experience the greatestslip, rather than the accumulative motions due to slip on the whole fault surface.

[发布机构] University:California Institute of Technology;Department:Geological and Planetary Sciences

[关键词] Geology