Paper No. 7
Presentation Time: 10:30 AM


WOLFF, L.1, YULE, D.2, SCHARER, K.3, WITKOWSKY, R.3, DESJARLAIS, I.2, CARDONA, J.2, HUERTA, B.2, MCGUIRE, R.2 and VADMAN, M.J.4, (1)Geological Sciences, California Stat University, Northridge, 18111 Nordhoff Street, Northridge, CA 91330, (2)Department of Geological Sciences, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330, (3)U.S. Geological Survey, 525 South Wilson Ave, Pasadena, CA 91106, (4)Department of Geological Sciences, California State Polytechnic University, Pomona, 3801 West Temple Avenue, Pomona, CA 91768,

Previous paleoseismic research at the San Gorgonio Pass Fault zone (SGPFZ) shows two large, surface-rupturing earthquake occurred in the last ~1200 years, an average interval that is twice as long as neighboring sections of the San Andreas fault (SAF). Unlike the rest of the SAF, the SGPFZ consists of short, interconnected fault segments that could present geometric limits to ruptures through the Pass. To test if large, infrequent earthquakes are the norm for the SGPFZ, we excavated a mega-trench (9 m deep, 45 m long, 30 m wide) in Summer 2013, 2 km NW of Cabazon, CA. Deposits in the trench are derived from two sediment sources, Lion Canyon, a large drainage that taps the southern edge of the San Bernardino Mountains, and a small unnamed side canyon that debouches directly across the trench site. Lion Canyon sediment packages are grey alluvium varying from silty sand layers to well-sorted, lenticular lenses and planar bedded, pebble and gravel channel-fill sequences. Conversely, the unnamed canyon sediments are dominated by tan to orange gravel and include medium to thick massive tabular layers of muddy, sandy gravels with poorly sorted gravels. The deposits are interpreted as a mixing of hillslope colluvium from the side canyon with fluvial and debris flow deposits issuing from both sources.

We identified 5 surface-rupturing events using fault terminations and angular unconformities between sub-horizontal, on-lapping deposits that bury deformed hanging wall and footwall strata. Two primary thrust fault zones separated by ~20 m, oriented N45°E, 30° NW. The SE fault zone ruptured during the most recent earthquake with a vertical separation of ~60 cm. At lower trench depths, the SE fault zone has additional splays and an abrupt increase in vertical separation (~100 cm), which we interpret as evidence for an older event. The NW fault zone places middle Pleistocene Cabazon Formation over Holocene gravels with vertical offsets of 20-60 cm and shows evidence of 3 earthquakes on several fault splays. Initial radiocarbon dates indicate the base of the stratigraphic section is ~6000 years old. In-progress radiocarbon age data and further stratigraphic and structural analysis will provide additional constraints on the rupture history of the San Andreas Fault system in the San Gorgonio Pass.