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Analysis of the ground accelerations radiated by the 1980 Livermore Valley earthquakes for directivity and dynamic source characteristics

U.S. GEOLOGICAL SURVEY OFFICE OF EARTHQUAKE STUDIES, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

The strong motion accelerograph recordings of the 24 January 1980 main shock and the 27 January 1980 aftershock of the Livermore Valley earthquake sequence are analyzed for systematic variations with azimuth or station location. The variation of the peak accelerations with epicentral azimuth is apparently reversed for the two events: the main shock accelerations are larger to the south, and the aftershock accelerations are larger to the northwest. We eliminate the site effects by forming the ratio of the peak accelerations recorded at the same station, after correcting for the epicentral distance. This analysis indicates that source directivity caused a total variation of a factor of 10 in the peak accelerations. Comparison of this variation with the spatial extent of the aftershock sequences suggests that the strong directivity in the radiated accelerations is the result of unilateral ruptures in both events.

The accelerograms recorded at 10 stations within 35 km of the events were digitized to analyze the azimuthal variation of the rms acceleration, the peak velocity, and the radiated energy flux. The variation of rms acceleration correlates almost exactly with the variation of the peak accelerations. This correlation is analyzed using both deterministic and stochastic models for the acceleration waveforms. The peak velocities, corrected for epicentral distance, vary with azimuth by a factor of 5 for both events, while the radiated energy flux varies by a factor of 30 for the main shock and 15 for the aftershock. The peak velocities are strongly correlated with the radiated energy flux. The radiated seismic energies are estimated to be 2.6 ± 0.9 x 10²⁰ dyne-cm for the main shock and 1.5 ± 0.3 x 10²⁰ dyne-cm for the aftershock.

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