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LEGG, MARK R., ACTA Inc., Torrance, CA

ABSTRACT: Seismogenic Potential of the South Coast Region

We consider the location, geometry, slip rate, and character of active faults that may generate damaging earthquakes that affect the South Coast region. Although characteristic earthquakes may appropriately model earthquake potential along some major faults in California, such events are generally undefined for the South Coast region. Instead, we consider events that affect facilities with a "design" life of about 50-100 yr. Although the "design" earthquakes we define are considered the largest Previous HitmagnitudeNext Hit events that may be expected within a 100-yr interval, these events generally do not have 100-yr recurrence intervals. For the longest and most continuous faults in the region (i.e, the San Andreas and San Clemente faults), a design Previous HitmagnitudeNext Hit of 7.5-8.0 is used. For other major faults, with well-defined traces that extend a few tens of kilometers, a design Previous HitmagnitudeTop of 6.5-7.5 is relevant. All other active faults are considered to have design magnitudes in the range of 5.0 to 6.5 based upon fault length and character (thrust faults have larger magnitudes for a given length). Besides the regional earthquake potential, relative liquefaction potential for the South Coast region must also be considered. In general, Holocene flood plains and stream channels have the highest liquefaction potential, with the older, more consolidated alluvial deposits having lower potential. Considering the regional earthquake potential, maps of maximum expected "design" shaking intensity are developed, including the attenuation of strong shaking with distance and the effects of local site amp ification. Specific earthquake scenarios can also be generated in this fashion, and the relative liquefaction potential determined considering both shaking intensity (opportunity) and liquefaction susceptibility. Other earthquake related hazards that can be mapped include seismic slope stability (relative stability) and, for the offshore region, relative potential for generating local tsunamis.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.