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Description
Trenches excavated in Late Holocene marsh deposits across the San Jacinto fault within the Anza seismic gap reveal clear evidence of repeated surface rupture. This evidence is preserved in a well-stratified section that is characterized by interbedded silty clay, sand, and peat layers. Hog Lake, which was interpreted as a sag pond resulting from a right-step in the fault trace, is actually the product of a prograding Holocene alluvial fan across the local drainage. The local geomorphology indicates that the fault zone is locally transpressive and exposures of the fault within trenches suggests that most of the slip is carried by the trace that traverses the center of the marsh. Evidence of three prehistoric earthquakes along this trace of the fault were documented in successive lateral excavations. Chronological constraint for each of these events was provided by radiocarbon ages derived from the peats. All of the 14C samples from the exposed section were dated using high-precision techniques. The resultant radiocarbon ages were dendro-chronologically AD age corrected and include two sigma errors. Based on these data, some important implications regarding the late Holocene behavior of the fault system can be made. Lateral excavations along the fault through the deepest portion of the lake exposed vertically displaced strata across two distinct traces. This vertical movement resulted from the transpression produced in a minor left bend in the fault trace. Progressively greater vertical separation and incremental increases in the degree of folding of successively older stratigraphic units, intervals of liquefaction phenomenon, and the upward truncation of fault strands provided evidence of repeated surface rupture. The occurrence of these events was consistent between all of the successive exposures along the fault. Evidence for lateral slip associated with each of these events could not be attained. The most recent of these events, which ruptured to within 15 centimeters of the surface, is constrained by the youngest peat unit, which yielded a dendrocorrected age of 1774+24-28 AD. This highest well-formed peat is extremely convoluted, but does not display any distinct displacement. This deformation is believed to be the product of the intense ground shaking (Modified Mercalli VII-IX) that resulted from either the 1899 or 1918 San Jacinto earthquakes. Based on the chronological control provided by the radiocarbon ages on event constraining peats within Hog Lake, the timing for each event can be estimated. While the error in determining this timing is ultimately controlled by the chronological constraint, stratigraphic relationships suggest that events occurred at approximately 1210, 1530, and 1750 AD. This recurrent movement shows a relatively systematic occurrence for ground rupturing earthquakes. From these data, an average recurrence interval of about 220-320 years is suggested. If interpretations of the faulting record in Hog Lake are accurate, then the last event occurred approximately 300 years ago. Assuming steady and uniform strain accumulation over a 40 kilometer long segment within the seismic gap since the occurrence of the last event, a minimum accumulated seismic moment of 2.9 x 1026 dyne-centimeters is suggested. This accumulated seismic moment equates to about a M 6.9 earthquake.
