A study of the strong ground motion of the Borrego Mountain, California, earthquake
Several synthetic models are constructed to fit the first 40 sec of the transversely polarized displacement, as recorded at El Centro, of the April 9, 1968 Borrego Mountain earthquake. The modeling is done in the time domain using the response computed for a distributed set of point shear dislocations embedded in a layered half-space. The beginning 10 sec of the observed record is used to model the spatial and temporal distribution of faulting whereas the remaining portion is used to determine the upper crustal structure based on surface-wave periodicity. A natural depth criterion was provided by comparing the amplitude of the direct arrival with the surface-wave excitations. Trade-offs are found to exist between source models and velocity structure models. Within the framework of a layer over a half-space model, faulting of finite vertical extent is required, whereas the horizontal dimensions of faulting are not resolvable. A model which is also consistent with the teleseismic results of Burdick and Mellman indicates massive faulting near a depth of 9 km with a fast rise time producing a 10-cm displacement pulse of 1 sec duration at El Centro. The faulting appears to slow down approaching the surface. The moment is calculated to be approximately 7 × 10^(25) dyne-cm which is somewhat smaller than the moment found by Burdick and Mellman (1976).
Additional Information© 1977 Seismological Society of America. Manuscript received June 10, 1976. We wish to thank G. Mellman, H. Kanamori, and C. Langston for critically reading the manuscript. We also would like to thank the California Institute of Technology, EERL, and G. Brady of the U.S. Geological Survey for allowing us access to the original strong-motion record. This research was supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Office of Scientific Research under Contract F44620-72-C-0083 and by National Science Foundation ENV76-10506.
Published - 315.full.pdf