A Study of the Strong Ground Motion of the Borrego Mountain, California, Earthquake by Thomas

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 Et Centro. The faulting appears to slow down approaching the surface. The moment is calculated to be approximately 7 X 1025 dyne-cm which is somewhat smaller than the moment found by Burdick and Mellman (1976). I N T R O D U C T I O N Understanding the nature of strong ground motion is a problem of importance to both seismologists and earthquake engineers. Generally, the engineering community has been particularly interested in the shorter periods whereas the former investigators usually study the more coherent longer-period motions. However, in recent times their domains of interest appear to overlap in the frequency band from 0.1 to 10 Hz because man-made structures have become larger and because seismologists have become increasingly interested in the details of faulting that can only be obtained from a combination of teleseismic and local observations. In many situations, the seismic waves recorded in the local field travel more nearly horizontal paths than waves which are recorded at teleseismic distances. This allows us to sample waves from earthquakes -~bich leave the source area in directions which are inaccessible to researchers studyin~ the wave forms of teleseismic records. Unfortunately, the fact that energy in the 1,,~ al field travels nearly horizontal paths implies that reflections from horizontal crustal layers are both large and complicated. In the local field, a clear distinction between body waves and surface waves is not possible. Thus, in many respects, interpreting the relative effects of source and earth structure is a more tractable problem for teleseismic modeling than for local field modeling. Yet, as we will show in this paper, it is possible to model local observations of earthquakes with realistic source and crustal structure models. Obviously, it is important to construct earthquake source models which are compatible with both local and teleseismic wave forms. This test of compatibility is especially important with respect to the assumed Q structure of the Earth which must be used when correcting teleseismic observations. Thus, the