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Science 5 May 1989:
Vol. 244. no. 4904, pp. 557 - 559
DOI: 10.1126/science.244.4904.557
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Articles

Microearthquake Imaging of the Parkfield Asperity

P. E. MALIN 1, S. N. BLAKESLEE 1, M. G. ALVAREZ 1, and A. J. MARTIN 1

1 Institute for Crustal Studies, University of California, Santa Barbara, CA 93106.

Microearthquake data from a downhole seismometer network on the San Andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the Parkfield characteristic earthquakes. The source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most. Furthermore, the magnitude-frequency statistics suggest that at local magnitude 0.6, the cumulative number of small events begins to fall off the usual Gutenberg-Richter (b = -1) relation, in which the number of events increases exponentially with decreasing magnitude. The downhole seismometer data establish a baseline from which the evolution of the earthquake process at Parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical Gutenberg-Richter relation may be operating for the smallest of Parkfield microearthquakes.

Submitted on November 14, 1988
Accepted on March 27, 1989


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
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Study of the 1999 M 7.1 Hector Mine, California, Earthquake Fault Plane by Trapped Waves.
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Fault-zone guided waves from explosions in the San Andreas fault at Parkfield and Cienega Valley, California.
Y.-G. Li, W. L. Ellsworth, C. H. Thurber, P. E. Malin, and K. Aki (1997)
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Microearthquake S-wave observations from 0 to 1 km in the Varian well at Parkfield, California.
D. Jongmans and P. E. Malin (1995)
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Stress Diffusion Along the San Andreas Fault at Parkfield, California.
P. E. Malin, P. E. Malin, and M. G. Alvarez (1992)
Science 256, 1005-1007
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High-frequency site effects at two Parkfield downhole and surface stations.
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Bulletin of the Seismological Society of America 81, 332-345
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Seismological studies at Parkfield. I. Simultaneous inversion for velocity structure and hypocenters using cubic B-splines parameterization.
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M. FEHLER and W. S. PHILLIPS (1991)
Bulletin of the Seismological Society of America 81, 553-575
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San Andreas Fault Zone Head Waves Near Parkfield, California.
Y. Ben-Zion, Y. BEN-ZION, and P. MALIN (1991)
Science 251, 1592-1594
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Fault zone trapped seismic waves.
Y.-G. LI and P. C. LEARY (1990)
Bulletin of the Seismological Society of America 80, 1245-1271
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Seismic Trapped Modes in the Oroville and San Andreas Fault Zones.
Y.-G. Li, Y.-G. Li, P. Leary, K. Aki, and P. Malin (1990)
Science 249, 763-766
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