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Modeling of ground motion from a 1994 Northridge aftershock using a tomographic velocity model of the Los Angeles Basin

Haase, Jennifer S. and Hauksson, Egill and Vernon, Frank and Edelman, Adam (1996) Modeling of ground motion from a 1994 Northridge aftershock using a tomographic velocity model of the Los Angeles Basin. Bulletin of the Seismological Society of America, 86 (1B). S156-S167. ISSN 0037-1106.

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The 1994 Northridge mainshock and its aftershocks show a complex pattern of peak accelerations at stations located in the Los Angeles Basin. The waveforms contain multiples of body-wave phases and extensive surface waves at frequencies mostly below 1 Hz. In particular, for stations at distances greater than 18 km, secondary arrivals show larger accelerations than the direct S-wave arrivals. The mainshock waveforms are further complicated by irregularities of the source rupture. We use 2D finite difference to evaluate the effect of lateral variations in seismic velocity on the amplitude of shear-wave energy and to distinguish the effects of source and propagation path. We model waveforms from one aftershock recorded at nine stations deployed along a 60-km-long profile extending into the Los Angeles Basin. We use a two-dimensional slice through the 3D tomography model of the Los Angeles Basin in the 2D finite-difference calculations. These synthetic waveforms fit the aftershock waveforms significantly better than corresponding waveforms determined from simple 1D velocity models. With the addition of a thin low-velocity surface layer above the tomography model, the finite-difference synthetics reproduce most of the important features of the recorded data, in particular, the large-amplitude arrivals 7 to 10 sec following the direct S arrival. These arrivals correspond to the SS arrival, which is sharply refracted at the basin edge, and the S-wave with multiple legs trapped by the dipping near surface gradient. For large earthquakes located either inside or outside the basin, these phases can be the cause of the largest and hence potentially most hazardous shaking in the Los Angeles Basin.

Item Type:Article
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Hauksson, Egill0000-0002-6834-5051
Additional Information:© 1996 Seismological Society of America. Manuscript received 9 January 1995. We would like to thank the quick and effective team of the SCEC seismologists and technicians who deployed the instruments that provided the data for this work. We thank Dave Wald and Craig Scrivner who provided critical reviews that greatly improved this article. We would like to thank Don Heimberger and Doug Dreger for their helpful discussions. We would also like to thank Craig Scrivner and Xi Song for their help in using the programs to explore the range of possibilities for modeling the data. We thank Eric Calais for his help with the graphics. This work was supported by NSF Grant Number EAR 9416119 and the Southern California Earthquake Center. This is SCEC Contribution Number 181 and Caltech Division Publication 5510.
Group:Seismological Laboratory
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Southern California Earthquake Center (SCEC)UNSPECIFIED
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences5510
Southern California Earthquake Center (SCEC)181
Issue or Number:1B
Record Number:CaltechAUTHORS:20130306-140743520
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37351
Deposited By: Tony Diaz
Deposited On:07 Mar 2013 16:06
Last Modified:03 Oct 2019 04:46

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