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Evaluating Event Chronologies on the Raymond Fault, Los Angeles County, California

K. D. Weaver1 and J. F. Dolan2
1William Lettis & Associates, Inc, Valencia, CA
2Department of Earth Sciences, Univ of Southern California, Los Angeles, CA

The Raymond fault (RF) is a left-lateral, strike-slip fault extending for 20 km across the San Gabriel Valley. Our Arboretum site data, in combination with published data, indicate that the most recent RF surface rupture occurred ~1–2 ka. Other studies suggest that the adjacent Hollywood fault ruptured most recently ~6–9 ka, well before the RF’s most recent event (Dolan and others, 2000). Our San Marino trench site records at least five late Pleistocene surface ruptures. Four of these occurred between ~31.5–41.5 ka, yielding a maximum average recurrence interval (RI) of 3,300 yr for this 10 kyr period. Recent 3D trench excavations of a late Pleistocene channel offset by the fault yield a minimum, left-lateral slip rate of at least 1.5 mm/yr (Marin and others, 2000; J. Dolan, unpubl. data). This slip rate indicates that the Raymond fault accumulates ~5 m of strain during a 3,300 yr RI. If this RI and amount of slip are typical of RF earthquakes, it would suggest that the RF ruptures in very large events (Mw~7.5+0.2; Wells and Coppersmith, 1994). Rupture of the entire RF could probably generate an earthquake of only ~Mw 6.7, suggesting that the RF may typically rupture together with other, nearby faults. Thus, while the Raymond fault may not have ruptured with the Hollywood fault most recently, it may have done so in past earthquakes. Alternatively, events may be missing from the paleoseismologic record of the RF, and the true average RI may be much shorter than our maximum 3,300 yr measurement. These studies underscore the value of obtaining paleoseismic slip rate and slip per event data, and how these can help evaluate the completeness of event chronology.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California