| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Seismological Laboratory 252-21 Caltech, Pasadena, California 91125
Abstract
The proximity of several recent earthquakes to the Los Angeles sedimentary basin provides an opportunity to isolate the effects of the basin on wave propagation. The 4 October 1987 aftershock (ML = 5.3) of the Whittier Narrows sequence and the 28 June 1991 Sierra Madre mainshock (ML = 5.8) are on a similar azimuth to stations overlying the deepest part of the basin. A distinctive feature of records from basin stations recording the 4 October after-shock is the large amplitude of multiple S, SS, etc. The multiples have up to twice the amplitude of the direct S phase on the tangential component. At such a short range, less than 25 km, a horizontal seismic velocity gradient is needed to turn rays rapidly enough for large-amplitude multiples to form. A forward modeling approach is employed, using finite-difference numerical techniques that produce double-couple point-source solutions. A model based on a recent geologic cross section constructed for the east edge of the Los Angeles Basin generates more phases than are seen in the seismic records. Simpler models, based on dipping layers with low shear velocities in the top few layers, fit the data better. The seismic velocity, depth, and dip of the layers are varied to fit the timing between the direct P, the direct S, and the first S multiple. The timing and amplitude of the direct and first multiple S pulses are well modeled, though the phase of the first multiple does not match the data. Including a steeply dipping west edge in the basin model has little effect on the synthetic waveforms, except at distances near that basin edge. The amplitude of SS is greatest in the deepest part of the basin, where it is two times larger than direct S. The coda duration increases from 8 sec to more than 20 sec from the NE to the SW. The Sierra Madre mainshock occurred about 25 km to the NE of the Whittier Narrows sequence. The model for Whittier Narrows was extended to this distance, with a shallow basin between Whittier and the Sierra Madre hypocenter to simulate the San Gabriel sedimentary basin. Phases generated by the edge of the deep basin continue to dominate the synthetic waveforms, but this model generates a lengthy coda. This study shows that specific phases with frequencies up to 1 Hz that have traveled through deep sedimentary basins can be explained by two-dimensional seismic velocity models.
This article has been cited by other articles:
|
|
 |
|
  A. Rodgers, N. A. Petersson, S. Nilsson, B. Sjogreen, and K. McCandless Broadband Waveform Modeling of Moderate Earthquakes in the San Francisco Bay Area and Preliminary Assessment of the USGS 3D Seismic Velocity Model Bulletin of the Seismological Society of America, April 1, 2008; 98(2): 969 - 988. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Modeling the Jiyang Depression, Northern China, Using a Wave-Field Extrapolation Finite-Difference Method and Waveform Inversion Bulletin of the Seismological Society of America, June 1, 2004; 94(3): 988 - 1001.
|
|
|
|
|
|
B. Savage and D. V. Helmberger Site Response from Incident Pnl Waves Bulletin of the Seismological Society of America, February 1, 2004; 94(1): 357 - 362. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Basin Structure Estimation by Waveform Modeling: Forward and Inverse Methods Bulletin of the Seismological Society of America, August 1, 2000; 90(4): 964 - 976.
|
|
|
|
|
|
C. W. Scrivner and D. V. Helmberger Finite-difference modeling of two aftershocks of the 1994 Northridge, California, earthquake Bulletin of the Seismological Society of America, December 1, 1999; 89(6): 1505 - 1518. [Abstract] [PDF]
|
|
|
|
|
|
R. W. Graves Three-dimensional finite-difference modeling of the San Andreas fault: Source parameterization and ground-motion levels Bulletin of the Seismological Society of America, August 1, 1998; 88(4): 881 - 897. [Abstract] [PDF]
|
|
|
|
|
|
D. J. Wald and R. W. Graves The seismic response of the Los Angeles basin, California Bulletin of the Seismological Society of America, April 1, 1998; 88(2): 337 - 356. [Abstract] [PDF]
|
|
|
|
|
|
A. Pitarka, K. Irikura, T. Iwata, and H. Sekiguchi Three-dimensional simulation of the near-fault ground motion for the 1995 Hyogo-Ken Nanbu (Kobe), Japan, earthquake Bulletin of the Seismological Society of America, April 1, 1998; 88(2): 428 - 440. [Abstract] [PDF]
|
|
|
|
|
|
L. Wen and D. V. Helmberger Propagational corrections for basin structure: Landers earthquake Bulletin of the Seismological Society of America, June 1, 1997; 87(3): 782 - 787. [Abstract] [PDF]
|
|
|
|
|
|
E. H. Field Spectral amplification in a sediment-filled Valley exhibiting clear basin-edge-induced waves Bulletin of the Seismological Society of America, August 1, 1996; 86(4): 991 - 1005. [Abstract] [PDF]
|
|
|
|
|
|
K. B. Olsen and R. J. Archuleta Three-dimensional simulation of earthquakes on the Los Angeles fault system Bulletin of the Seismological Society of America, June 1, 1996; 86(3): 575 - 596. [Abstract] [PDF]
|
|
|
|
|
|
K.-H. Yoon and G. A. McMechan 3D eight-order elastic finite-difference modeling of refraction and strong-motion data from the Coyote Lake region, California Bulletin of the Seismological Society of America, June 1, 1996; 86(3): 616 - 626. [Abstract] [PDF]
|
|
|
|
|
|
C. W. Scrivner and D. V. Helmberger Preliminary work on an early warning and rapid response program for moderate earthquakes Bulletin of the Seismological Society of America, August 1, 1995; 85(4): 1257 - 1265. [Abstract] [PDF]
|
|
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
