Evidence for Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake, Northern San Jacinto Fault Zone, California

We present new data from the Mystic Lake 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 lacustrine and alluvial strata that record at least seven ground-rupturing earthquakes during the past 1600 yr. Evidence for past surface deformation includes upward terminating faults with associated fissure fills, folding, angular unconformities, and pinching of strata against a paleoscarp. All of the event horizons occur at the tops of paleosols and are overlain by massive lacustrine clay units. 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 seven events ranges from 159 to 210 yr, and the most recent event occurred sometime between A.D. 1738 and 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 stepover, or that events on one fault triggered closely timed events on the adjacent fault.