Coseismic and postseismic velocity changes measured by repeating earthquakes

被引:200
作者
Schaff, DP [1 ]
Beroza, GC [1 ]
机构
[1] Stanford Univ, Dept Geophys, Stanford, CA 94305 USA
关键词
velocity change; nonlinearity; Loma Prieta;
D O I
10.1029/2004JB003011
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
Repeating earthquakes that rupture approximately the same fault patch and have nearly identical waveforms are a useful tool for measuring temporal changes in wave propagation in the Earth's crust. Since source and path effects are common to all earthquakes in a repeating earthquake sequence (multiplet), differences in their waveforms can be attributed to changes in the characteristics of the medium. We have identified over 20 multiplets containing between 5 and 40 repeating events in the aftershock zones of the 1989 Loma Prieta and 1984 Morgan Hill, California, earthquakes. Postmain shock events reveal delays of phases in the early S wave coda of as much as 0.2 s relative to premain shock events. The delay amounts to a path-averaged coseismic velocity decrease of about 1.5% for P waves and 3.5% for S waves. Since most of the multiplets are aftershocks and follow Omori's law, we have excellent temporal sampling in the immediate postmain shock period. We find that the amplitude of the velocity decrease decays logarithmically in time following the main shock. In some cases it returns to the premain shock values, while in others it does not. Similar results are obtained for the Morgan Hill main shock. Because the fractional change in S wave velocity is greater than the fractional change in P wave velocity, it suggests that the opening or connection of fluid-filled fractures is the underlying cause. The magnitude of the velocity change implies that low effective pressures are present in the source region of the velocity change. Our results suggest that the changes are predominantly near the stations and shallow, but we cannot exclude the possibility that changes occur at greater depth as well. If the variations are shallow, we may be detecting the lingering effects of nonlinearity during main shock strong ground motion. If the variations are deep, it suggests that pore pressures at seismogenic depths are high, which would likely play a key role in the earthquake process.
引用
收藏
页码:B103021 / 14
页数:18
相关论文
共 52 条
[21]  
KANAMORI H, 1976, B SEISMOL SOC AM, V66, P2027
[22]  
KARAGEORGI E, 1992, B SEISMOL SOC AM, V82, P1388
[23]  
Karageorgi E, 1997, B SEISMOL SOC AM, V87, P39
[24]   Seismological studies at Parkfield VIII: Modeling the observed travel-time changes [J].
Korneev, VA ;
McEvilly, TV ;
Karageorgi, ED .
BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA, 2000, 90 (03) :702-708
[25]   FREQUENCY-DEPENDENT CRUSTAL SCATTERING AND ABSORPTION AT 5-160 HZ FROM CODA DECAY OBSERVED AT 2.5 KM DEPTH [J].
LEARY, P ;
ABERCROMBIE, R .
GEOPHYSICAL RESEARCH LETTERS, 1994, 21 (11) :971-974
[26]   Evidence of shallow fault zone strengthening after the 1992 M7.5 Landers, California, earthquake [J].
Li, YG ;
Vidale, JE ;
Aki, K ;
Xu, F ;
Burdette, T .
SCIENCE, 1998, 279 (5348) :217-219
[27]   Postseismic fault healing on the rupture zone of the 1999 M 7.1 Hector Mine, California, earthquake [J].
Li, YG ;
Vidale, JE ;
Day, SM ;
Oglesby, DD ;
Cochran, E .
BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA, 2003, 93 (02) :854-869
[28]   Healing of the shallow fault zone from 1994-1998 after the 1992 M7.5 Landers, California, earthquake [J].
Li, YG ;
Vidale, JE .
GEOPHYSICAL RESEARCH LETTERS, 2001, 28 (15) :2999-3002
[29]  
LINDH AG, 1978, B SEISMOL SOC AM, V68, P721
[30]  
Mavko G, 1998, ROCK PHYS HDB TOOLS