Paper No. 10
Presentation Time: 11:15 AM


ONDERDONK, Nate, Department of Geological Sciences, Cal State Long Beach, 1250 Bellflower Blvd, Long Beach, CA 90840, MCGILL, Sally, Geological Sciences, California State University, San Bernardino, 5500 University Parkway, San Bernardino, CA 92407, WARBRITTON, Matthew J., Earth and Atmospheric Science Department, Saint Louis University, Saint Louis, MO 63103 and ROCKWELL, Thomas K., Geological Sciences, San Diego State University, 5500 Campanile Dr, San Diego, CA 92182,

The Mystic Lake paleoseismic site is located within a small (.4-km-wide) step over within the Claremont fault zone in the northern San Jacinto Valley. Trenches excavated at the site in 2009 and 2010 provided exposures of seven ground-rupturing earthquakes in the upper 1.5 m of strata. A 4-meter deep trench excavated in 2012 revealed evidence for four older earthquakes, bringing the total to eleven events since about A.D. 300, yielding an average recurrence interval of 140-160 years. The record reveals fairly regularly repeating earthquakes about every 110 years from AD ~300-1000 (events 6-11), followed by a gap of at least 200 years, with no earthquakes between AD1000-1200. This was followed by five events between about AD1250-1800, and the present open interval of ~200 years. Comparison of the Mystic Lake paleoseismic record to that at Hog Lake, on the Clark fault, farther southeast along the San Jacinto fault zone, shows that 5 out of 11 events at Mystic Lake overlap such that they may be the same earthquake (or closely timed events) as those at Hog Lake. Six out of eleven Mystic Lake events do not overlap with events at Hog Lake. Thus, some past events have jumped the step-over between the Claremont and Casa Loma-Clark faults, whereas others have not.

The dry winter of 2012-13 allowed us to excavate a 5-meter deep trench in June 2013. This trench exposed additional evidence for the events discovered in prior trenches, as well as revealing evidence for three, and possibly four additional, older earthquakes. The events were expressed as upward terminations of faults and clay seams, fissure fills, folding and angular unconformities, thickening of units across faults, and pinching of units against paleoscarps. We collected 143 detrital charcoal samples, which will be used to constrain the timing of the new events and to refine the timing of the previously recognized events. We anticipate that this longer record will allow better comparisons of the earthquake history at Mystic Lake with paleoseismic records from other nearby sites to help evaluate patterns of strain distribution along the San Jacinto fault in the late Holocene.