HR: 17:00h
AN: G14A-05 INVITED     [Abstracts]
TI: Long-Range and Long-Term Fault Interactions in Southern California
AU: * Dolan, J F
EM: dolan@usc.edu
AF: Dept. of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 United States
AU: Bowman, D D
EM: dbowman@fullerton.edu
AF: Department of Geological Science, California State University at Fullerton, Fullerton, CA 92834-6850 United States
AU: Sammis, C G
EM: sammis@usc.edu
AF: Dept. of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 United States
AB: Paleoseismological data reveal four clusters of large earthquakes in the Los Angeles region during the past 12,000 years. The historic period is part of an ongoing, >=1,000-year-long lull between clusters. These Los Angeles-region clusters have occurred during the lulls between similar clusters observed on the eastern California shear zone (ECSZ) in the Mojave Desert, which is now seismically active. A kinematic model in which the faults of the San Andreas system suppress activity on faults in the ECSZ, and vice versa, can explain the switching of activity between the two fault networks. Interestingly, available geologic and geodetic rate data suggest that the interseismic loading rates of faults with relatively slow recent seismic slip rates (e. g., SAF Big Bend, Garlock, and LA-region faults) are slower than the long-term slip rates of those faults. Conversely, the geodetically determined loading rate of the ECSZ in the Mojave, currently in the midst of a seismic cluster, appears to be faster than the long-term rate suggested by available paleoseismological data. These observations suggest that when the upper crust is seismically most active the lower crust beneath the major faults is also deforming more rapidly, whereas in regions where the upper crust is less seismically active the loading rate of these faults is also relatively slow. Thus, either rapid ductile slip in the lower crust (and upper mantle?) beneath faults loads the upper crust, resulting in a seismic cluster, or rapid seismic displacement along upper crustal faults increase the ductile loading rate of the downward extensions of the faults. At present, we cannot distinguish the relative importance of these competing hypotheses. Similar behavior at other plate boundaries suggests that alternating clusters of large earthquakes may be the expected mode of seismicity when two fault systems accommodate the same plate boundary motion and slip on one system suppresses slip on the other.
DE: 7230 Seismicity and seismotectonics
DE: 8107 Continental neotectonics
DE: 1206 Crustal movements--interplate (8155)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting