HR: 1340h
AN: S43B-1077    [Abstracts]
TI: Seismic Hazard and Fault Length
AU: * Black, N M
EM: nblack@ess.ucla.edu
AF: University of California Los Angeles, 595 Charles Young Dr. East, Los Angeles, CA 90095
AU: Jackson, D D
EM: djackson@ucla.edu
AF: University of California Los Angeles, 595 Charles Young Dr. East, Los Angeles, CA 90095
AU: Mualchin, L
EM: lalliana_mualchin@dot.ca.gov
AF: California Department Of Transportation, 1801 30th St , Sacramento, CA 95816
AB: If m_x is the largest earthquake magnitude that can occur on a fault, then what is m_p, the largest magnitude that should be expected during the planned lifetime of a particular structure? Most approaches to these questions rely on an estimate of the Maximum Credible Earthquake, obtained by regression (e.g. Wells and Coppersmith, 1994) of fault length (or area) and magnitude. Our work differs in two ways. First, we modify the traditional approach to measuring fault length, to allow for hidden fault complexity and multi-fault rupture. Second, we use a magnitude-frequency relationship to calculate the largest magnitude expected to occur within a given time interval. Often fault length is poorly defined and multiple faults rupture together in a single event. Therefore, we need to expand the definition of a mapped fault length to obtain a more accurate estimate of the maximum magnitude. In previous work, we compared fault length vs. rupture length for post-1975 earthquakes in Southern California. In this study, we found that mapped fault length and rupture length are often unequal, and in several cases rupture broke beyond the previously mapped fault traces. To expand the geologic definition of fault length we outlined several guidelines: 1) if a fault truncates at young Quaternary alluvium, the fault line should be inferred underneath the younger sediments 2) faults striking within 45° of one another should be treated as a continuous fault line and 3) a step-over can link together faults at least 5 km apart. These definitions were applied to fault lines in Southern California. For example, many of the along-strike faults lines in the Mojave Desert are treated as a single fault trending from the Pinto Mountain to the Garlock fault. In addition, the Rose Canyon and Newport-Inglewood faults are treated as a single fault line. We used these more generous fault lengths, and the Wells and Coppersmith regression, to estimate the maximum magnitude (m_x) for the major faults in southern California. Then we compared our m_x values with those proposed by CALTRANS, and those assumed in the 2002 USGS/CGS hazard model. To calculate the planning magnitude m_p we assumed a truncated Gutenberg-Richter magnitude distribution with parameters a, b, and m_x. We fixed b and solved for the a-value in terms of m_x, b, and the tectonic moment rate. For many faults m_p is relatively insensitive to m_x and typically falls off at higher magnitudes because the a-value decreases with increasing m_x when the moment rate is constrained. Furthermore, we find that by increasing m_x the cumulative earthquake rate actually decreases for smaller magnitude (5 and 6) events. This suggests that fewer magnitude 5 and 6 earthquakes are required to balance the moment budget if larger, but highly infrequent, earthquakes are allowed to occur.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8010 Fractures and faults
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Seismology [S]
MN: Fall Meeting 2005