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Published November 2003 | Published
Journal Article Open

Stress drops and radiated energies of aftershocks of the 1994 Northridge, California, earthquake

Abstract

We study stress levels and radiated energy to infer the rupture characteristics and scaling relationships of aftershocks and other southern California earthquakes. We use empirical Green functions to obtain source time functions for 47 of the larger (M ≥ 4.0) aftershocks of the 1994 Northridge, California earthquake (M6.7). We estimate static and dynamic stress drops from the source time functions and compare them to well-calibrated estimates of the radiated energy. Our measurements of radiated energy are relatively low compared to the static stress drops, indicating that the static and dynamic stress drops are of similar magnitude. This is confirmed by our direct estimates of the dynamic stress drops. Combining our results for the Northridge aftershocks with data from other southern California earthquakes appears to show an increase in the ratio of radiated energy to moment, with increasing moment. There is no corresponding increase in the static stress drop. This systematic change in earthquake scaling from smaller to larger (M3 to M7) earthquakes suggests differences in rupture properties that may be attributed to differences of dynamic friction or stress levels on the faults.

Additional Information

Copyright 2003 by the American Geophysical Union. Received 1 December 2000; revised 11 May 2003; accepted 25 June 2003; published 28 November 2003. Data used in this study were provided by the SCEC Data Center. J. M. acknowledges support from the USGS during this study. We appreciated many helpful comments from John Boatwright, Art McGarr, and Allan Rubin, which significantly improved the paper.

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August 22, 2023
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October 18, 2023