HR: 0800h
AN: G21C-0684    [Abstracts]
TI: Complex Faulting in the Pacific-North America Transform Offshore Southern California And Implications on Plate Boundary Tectonics and Tsunamigenesis
AU: * Legg, M R
EM: mrlegg@attglobal.net
AF: Legg Geophysical, 16541 Gothard Street, Suite 107, Huntington Beach, CA 92647, United States
AU: Barberopoulou, A
EM: barberop@usc.edu
AF: Tsunami Research Center, KAP-210, Dept. Civil & Environmental Engineering University of Southern California, Los Angeles, CA 90089, United States
AB: Complexity in the tectonic model for Pacific-North America transform motion in the offshore southern California region is demonstrated by earthquakes near San Clemente Island and Fortymile Bank. Observed focal mechanisms show movements opposite to those predicted by the plate tectonic theory for right-slip on NW- trending transform faults and observed in other parts of the California Continental Borderland. Also, there is evidence suggesting that moderate earthquakes in the Inner Borderland have greater magnitudes based on long-period seismic waves than the nominal Richter local magnitudes reported in earthquake catalogs. With better data showing the geologic structure of the area now available, we can try to derive a more complete understanding of this complex tectonic behavior and resulting consequences for local tsunamigenesis. The "backwards" earthquakes suggest the occurrence of plate boundary deformation and/or microplate tectonics with the western side of a block containing Fortymile Bank moving instantaneously faster to the northwest than the adjacent block to the west. Such motions may be consistent with clockwise rotation of blocks in the continental borderland due to the regional dextral shear couple as proposed by Crouch (1978) and Luyendyk and others (1980) based upon paleomagnetic and other geologic data. Alternatively, as initially observed for the 1986 Offshore Oceanside earthquake (MS=5.8) by Hauksson and Jones (1988), the anomalous focal mechanism may be due to an inaccurate model of crustal seismic velocity structure for the offshore region. Use of a refined velocity model may show these anomalous earthquakes to have oblique-reverse mechanisms like the 1986 mainshock. Furthermore, more recent seismicity, located with greater accuracy due to expansion of the Southern California Seismograph Network (SCSN), has apparent NE alignments suggestive of significant active secondary fault structure located off the major NW-trending right-slip faults. Such faulting was also inferred to be significant during the clockwise vertical-axis block rotations of the western Transverse Ranges. Interaction between faults within conjugate systems may enhance vertical movements including subsidence at basins where blocks diverge and uplift where blocks converge, thereby producing local tsunamis during large earthquakes. It was not until recently that local earthquake sources were identified offshore Southern California as potentially damaging tsunami sources. Erroneous magnitude estimate of offshore earthquakes can have serious implications for tsunamigenesis and tsunami warning. A half magnitude error can make the difference between a non-tsunamigenic and a tsunamigenic event especially when a marginal event is considered. Better magnitude estimates using long-period seismographs may be necessary for more accurate identification of potentially tsunamigenic local earthquakes.
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7250 Transform faults
DE: 8106 Continental margins: transform
DE: 8111 Continental tectonics: strike-slip and transform
SC: Geodesy [G]
MN: 2007 Fall Meeting