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Finite-difference modeling of two aftershocks of the 1994 Northridge, California, earthquake

Scrivner, Craig W. and Helmberger, Donald V. (1999) Finite-difference modeling of two aftershocks of the 1994 Northridge, California, earthquake. Bulletin of the Seismological Society of America, 89 (6). pp. 1505-1518. ISSN 0037-1106.

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Recent evidence for nonlinear behavior of strong motions generated by the Northridge, California, earthquake relies explicitly on the analysis of aftershock data. Thus, correcting these observations for propagational and source effects becomes a crucial step in these studies. Here, we demonstrate that the dipping structure near the edges of the San Fernando Valley can strongly alter the local site responses, especially for shallow events. We model two aftershocks with similar epicenters, but with shallow (4 km) and deep (16 km) hypocentral depths. Waveforms from 12 portable instruments deployed across the basin and recording these events were processed and studied in the 0.3-4-Hz bandpass. For the shallow event, distinctive features are a broad direct S phase and large-amplitude surface waves at the basin stations, and high-frequency S phases at stations in the Santa Monica Mountains. The deep event is less strongly affected by the basin but displays strong evidence of the source radiation pattern. An array analysis indicates 2D propagation for most of the strong arrivals. Some of these features are explained by a simple 2D basin model containing a strong shallow contrast, a factor of 2 jump in seismic velocity at a depth of 1 km within the basin, and a moderate deeper gradient.

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Additional Information:© 1999 by the Seismological Society of America. Manuscript received 4 November 1997. The authors thank Sue Hough and Harold Magistrale for their thoughtful reviews that significantly improved this article. The authors acknowledge the Southern California Earthquake Center (SCEC) for access to the portable instrument data from the Northridge aftershock sequence. This research was supported by SCEC under Contract Number 89-20136 and by the USGS (00HQGR0035). The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily the official policies of the U.S. Government. Division of Geological and Planetary Sciences Contribution Number 8665.
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Southern California Earthquake Center (SCEC)89-20136
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Caltech Division of Geological and Planetary Sciences 8665
Issue or Number:6
Record Number:CaltechAUTHORS:20121211-135914948
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35917
Deposited By: Tony Diaz
Deposited On:12 Dec 2012 22:32
Last Modified:03 Oct 2019 04:33

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