PRELIMINARY RESULTS FROM MYSTIC LAKE: A NEW PALEOSEISMIC SITE ALONG THE NORTHERN SAN JACINTO FAULT ZONE

We present preliminary results from a new paleoseismic site along the Claremont strand of the San Jacinto Fault Zone in southern California. The site is located along the northeast edge of the ephemeral Mystic Lake at the north end of the San Jacinto Valley step-over. A small-scale releasing step-over along the Claremont fault has created a sag depression that can be seen in early aerial photography. We excavated a 1.5-2 m deep, 400 m-long trench across the full width of this sag, exposing multiple faults and a structural depression filled with excellent shallow lake stratigraphy. Evidence for at least six surface ruptures is recorded in the stratigraphy, and preliminary radiocarbon dating indicates that all six events occurred in the past 1600 years.

Most of the rupture events recorded in the stratigraphy are recognized along the fault that bounds the southwest edge of the sag. This fault is expressed in the trench as a zone of progressively folded and displaced strata. The stratigraphic relationships suggest a model in which each surface rupture results in subsidence of the sag, followed by clay deposition as the sag is filled in, culminating in a weak surface soil. Upward terminations, fissures, folding, and growth stratigraphy provide evidence for the six events. Preliminary ¹⁴C dating of 19 samples (out of over 200 that were collected) indicates that the most recent rupture occurred after AD 1706-1719, and two events have occurred since ca. AD 1200. There is evidence of a cluster of three events between about AD 1020 and 1220, with a 6^th event between AD 260-560. A similar event cluster between AD 1050 and 1400 was observed at the Hog Lake paleoseismic site located 50 km to the south along the Clark fault (Rockwell, 2008). This observation suggests that either some large San Jacinto events may jump across the San Jacinto Valley releasing step-over that separates the northern and central fault segments, or that stress triggering along one segment causes the other to fail in close succession. Additional work will test these models, as well as whether ruptures along the San Andreas fault identified at Wrightwood may have ruptured down the San Jacinto, which would help explain the significant reduction in slip-rate southward along the San Andreas fault in the San Bernardino area (McGill et al., 2008).