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Evidence for seven surface ruptures in the past 1600 years on the Claremont fault at Mystic lake, northern San Jacinto fault zone, California

Nathan W. Onderdonk, Thomas K. Rockwell, Sally F. McGill, & Gayatri I. Marliyani

Published February 2013, SCEC Contribution #1503

We present data from a new paleoseismic site along the Claremont segment of the northern San Jacinto fault zone. The site is located within a sag formed between two fault strands that pass through the eastern side of Mystic Lake in the San Jacinto Valley. Trenches excavated across the sag exposed faulted and folded lacutrine and alluvial strata that record at least 7 ground-rupturing earthquakes over the past 1600 years. Evidence for past surface deformation includes upward terminating faults with associated fissure fills, folding, angular unconformities, and pinching of strata against a paleoscarp. Most of the event horizons occur at the tops of paleosols and are overlain by lacustrine clay layers. We interpret this pattern to represent development of soils at the surface between earthquakes that are buried when fault rupture causes subsidence and renewed filling of the depression with lacustrine sediments. The ages of the events are constrained by 50 radiocarbon dates determined from detrital charcoal. The recurrence interval for the past 7 events ranges from 159 to 210 years and the most recent event occurred sometime between AD 1738 and AD 1850 based on radiocarbon ages trimmed by historical data. Some of the event ages at Mystic Lake overlap in time with events recorded at Hog Lake on the Clark strand of the San Jacinto fault zone to the south, suggesting that these events may have jumped the San Jacinto Valley releasing step-over, or that events on one fault triggered closely timed events on the adjacent fault.

Citation
Onderdonk, N. W., Rockwell, T. K., McGill, S. F., & Marliyani, G. I. (2013). Evidence for seven surface ruptures in the past 1600 years on the Claremont fault at Mystic lake, northern San Jacinto fault zone, California. Bulletin of the Seismological Society of America, 103(1), 519-541. doi: 10.1785/0120120060.