PALEOSEISMICITY OF THE NORTHERN SAN JACINTO FAULT, COLTON AND SAN BERNARDINO, SOUTHERN CALIFORNIA; PRELIMINARY RESULTS

The San Jacinto fault is historically the most seismically active fault in the San Andreas fault system in southern California. In spite of this, little is known about the prehistoric activity of the northern section of this fault zone.

Two sites along this portion of the San Jacinto fault were investigated to determine paleo-earthquake timing. Two fault-perpendicular trenches at the Colton site exposed a sequence of interbedded peat, clay and sand. The width of the fault zone is greater than 20 meters with several major fault strands. We found evidence for multiple earthquakes, including upward terminations, differential offsets, and filled fissures. Compressional uplift east of the fault zone in the northern of the two trenches is juxtaposed over a short distance with a filled graben in the southern trench. Preliminary dates on peat and detrital charcoal indicate that multiple earthquakes occurred between approximately 4-5 ka, and two earthquakes occurred within the past ~200 yBP.

The site in San Bernardino is located 5 km to the northwest of the Colton site. The main fault zone here is less than 10 m in width. Tilted and deformed fluvial deposits, dipping up to 90°, are on the SE side of the main fault zone. Numerous secondary faults occur within these deposits, dipping both to the SW and NE, and demonstrating by cross-cutting relations an alternating activity on these two distinct orientations. The sediments on the NE side of the main fault zone are marsh and flood deposits, consisting of clay and sand with abundant detrital charcoal. Paleoseismic evidence at this site includes growth folding of the fine-grained marsh deposits and the occurrence of large liquefaction features. Some of the liquefaction features occur at earthquake horizons determined from growth folding but because this site is only ~ 10 km from the San Bernardino segment of the San Andreas fault, regional seismic triggering of the liquefaction features cannot be ruled out. Preliminary dates on detrital charcoal suggest three earthquakes within an 800 year period, and pending dates will provide the time constraints on more recent faulting. The preliminary recurrence interval at both of these sites is comparable to that obtained for the southern San Jacinto fault, at Hog Lake, averaging 190 years (Rockwell, 2003).