Modeling earthquakes with local and regional broadband data

Citation

Dreger, Douglas S. (1993) Modeling earthquakes with local and regional broadband data. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/es5j-ag55. https://resolver.caltech.edu/CaltechTHESIS:02172011-095833318

Abstract

Waveform modeling techniques are applied to several recent, moderate sized earthquakes recorded by the broadband TERRAscope array in southern California. A method for the determination of source parameters at regional distances with three-component, sparse network data is described. The sensitivity of the method to source mislocations and velocity model is investigated. The method is relatively insensitive to 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 simple plane layered velocity model derived from travel-time studies explains much of the observed waveforms. The broadband waveforms of two small earthquakes that occurred in 1988 near Upland, California are forward modeled to determine Green's functions for the path to Pasadena, California. The effects of near surface gradients, crustal interface sharpness, and two-dimensional basin-ridge structures were studied. This analysis resulted in a simple plane layered velocity model that best fit the data. The Green's functions are 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 orientation and seismic moment. Comparisons of the 1990 mainshock with the 1988 events revealed that the mainshock was a relatively complicated event. Multi-point source and distributed finite slip models show that the mainshock ruptured down dip (6 km to 9km) with a non-uniform slip distribution in which 30 % of the total seismic moment was released from a relatively small area at 9 km depth. The overall area of 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. The long-period body waves are inverted to determine the orientation and seismic moment. Ratios of the peak amplitudes of simulated short-period Wood-Anderson and long-period Wood-Anderson seismograms are compared for the mainshock and the two largest aftershocks. The ratios show that stations southwest of the epicenter have elevated levels of short-period energy relative to stations to the east suggesting the presence of directivity. The displacement waveforms were forward modeled using distributed finite slip models. The best fitting model consists of an updip rupture toward the west. This model fails however to explain the amplitudes of the short-period waves. A non-uniform slip model was developed that better explains the short-period amplitudes. The results of this analysis indicate that the shorter-period energy is controlled more by the patches on the fault that experience the greatest slip, rather than the accumulative motions due to slip on the whole fault surface.

Thesis Files