Analysis and Modeling of Seismic Ground Motions in Heterogeneous Structures in Southern California

Citation

Scrivner, Craig William (1998) Analysis and Modeling of Seismic Ground Motions in Heterogeneous Structures in Southern California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/m61x-wq81. https://resolver.caltech.edu/CaltechTHESIS:07142023-185900244

Abstract

This thesis contains studies of seismic data from the 1995-1996 Ridgecrest earthquake sequence, an aftershock of the 1987 Whittier Narrows earthquake and the 1991 Sierra Madre mainshock, and aftershocks of the 1994 Northridge earthquake. The Ridgecrest data set spans southern California, including stations in the Los Angeles area basins. The Whittier Narrows/Sierra Madre and Northridge data sets consist of stations in the Los Angeles and San Fernando Valleys, respectively, and record earthquakes occurring directly adjacent to these sedimentary basins. The studies examine the variability of ground motions in the crust and details of seismic propagation from the crust into sedimentary basins. In the Ridgecrest study, amplitudes of synthetic waveforms from a 1D model are compared with data amplitudes at rock, soil, and basin sites. At rock sites, the data amplitudes are within a factor of 2 of the synthetic amplitudes. At basin and soil sites, the data are within a factor of 3 of the synthetic amplitudes. Stations beyond the trailing edge of sedimentary basins are affected by leaked basin surface waves. In the Whittier Narrows/Sierra Madre study, waveform phases generated by the edge of the Los Angeles basin are identified and modeled with a 2D structure. In the data, multiples of the direct shear wave, reflected from the surface and turned by the basin edge, are up to two times the amplitude of the direct arrival. A simple, smooth, 2D basin edge model produces the correct timing and relative amplitude of basin-trapped phases. In the Northridge study, we contrast waveforms from a shallow and a deep event. The waveforms from the shallow event include basin-generated surface waves in the basin, and a phase-shift in the direct shear wave outside the basin. A model with a strong velocity contrast at about 1 km depth in the upper basin, a depth for the entire basin just above the shallow source depth, and a gradient beneath the basin produces synthetic waveforms that match the distinctive features in the data set.

Thesis Files