Predictability of strong ground motion in the Imperial Valley: Modeling the M4.9, November 4, 1976 Brawley earthquake
Strong-motion displacements, recorded at 33 km (IVC) and 36 km (ELC) from the November 4, 1976 Brawley earthquake, are modeled using the Cagniard-deHoop technique. The IVC record consists almost entirely of transversely polarized motion, whereas the ELC record contains an approximately equal proportion of transversely and radially polarized motion. A simplified shear-wave velocity model was determined from the compressional wave refraction studies of Biehler, Kovach, and Allen (1964). The epicentral location and focal mechanism computed from P-wave first-arrival studies were used to locate and orient a double-couple point source within the layered half-space. The far-field time function and source depth were the only parameters without good independent constraints. A triangular far-field time function with a duration of 1.5 sec and a source depth of 7 km were sufficient to model the first 25 sec of tangential ground motion. It appears that the effects of velocity structure on the propagation of long-period SH waves are predictable in the Imperial Valley. A study of the synthetic Fourier amplitude spectra indicates that wave propagation effects should be included in studies of source spectra and seismic wave attenuation.
Additional Information© 1978 Seismological Society of America. Manuscript received July 19, 1977. We wish to thank H. Kanamori for bringing the IVC record to our attention and for critically reading the manuscript. We would also like to thank G. Brady of the U.S. Geological Survey for providing copies of the ELC record and also H. Wasson of Imperial Valley College for his assistance with the station IVC. M. Schnapp and G. Fuis provided useful discussion and information regarding the hypocentral location of the earthquake. We are particularly grateful to S. Biehler for supplying important information and figures concerning the velocity structure in the Imperial Valley. This research was supported by the National Science Foundation EVN76-10506. Operating funds for the torsion seismometer at IVC are provided by U.S.G.S. Survey Contract 14-08-001-15893.
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