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A 7000 year record of paleoearthquakes on the Central Garlock Fault, near El Paso Peaks, California
Dawson, Timothy E.
Rockwell, Thomas K.
Day, Steven M.
Short, Donald R., Jr.
A trench excavated across the central Garlock fault, on a small playa near the El Paso Mountains, has revealed a remarkable record of paleoearthquakes. The trench contains event evidence in the form of buried fissures and scarps for six well-resolved earthquakes, designated as events W, U, R, Q, K, and F, within the last 7000 years. The trench and auxiliary exposures that we excavated did not reveal evidence for three poorly-resolved events reported in a previous study and in some cases, we provided alternative interpretations for what was previously observed. Additionally, one paleoearthquake identified in the previous study and thought to be a well-resolved event in the original trench was misinterpreted and is not an event. The multiple exposures allow us a qualitative measure of our interpretation uncertainties, and we believe that our event record is not over-interpreted, although it may be under-interpreted. Radiocarbon dates of detrital charcoal, combined with a technique of interpolating the event ages with sedimentation rates, provide constraints on the timing of faulting events. The most recent earthquake, event W, occurred between A.D. 1450-1640, with a preferred age of A.D. 1530. The penultimate event occurred between A.D. 560-800 and has a preferred age of A.D. 680. Event R has a preferred age of A.D. 325, and occurred between A.D. 225-450. Event Q, which has a preferred age of A.D. 100 and occurred between 40 B.C. - A.D. 240, appears to have more vertical deformation associated with it, indicating it may have been larger event. Event K, which occurred after 3490 B.C. and before 2910 B.C. has a preferred age of 3120 B.C. The oldest identified paleoearthquake, event F, occurred between 5320-4050 B.C., with a preferred age of 4880 B.C. The event ages indicate that earthquake recurrence is highly irregular at the El Paso Peaks site, with individual intervals ranging from as little as 225 years to as much as 3220 years. This suggests that the central Garlock fault follows a Poissonian model of recurrence behavior rather than quasi-periodic behavior. Our analysis shows that, while the irregular recurrence may be an artifact of the geologic evolution of the trench site, there may exist a more fundamental physical process underlying the behavior of the central Garlock fault. This poorly understood process may be related to the temporal and spatial clustering of earthquakes observed for faults in the Eastern California Shear Zone.
San Diego State University
Master of Science (M.S.) San Diego State University, 2000
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