Paleoseismology of the northern San Jacinto fault, San Bernardino County

Katherine J. Kendrick, & Thomas E. Fumal

Submitted August 15, 2018, SCEC Contribution #8661, 2018 SCEC Annual Meeting Poster #218

The Colton paleoseismic site is located 5 km south of San Bernardino, along the Claremont segment of the San Jacinto Fault, in southern California. The excavations, two parallel trenches oriented orthogonal to the fault strike, exposed a broad, complex fault zone, including a zone of compressional uplift closely juxtaposed with an extensional graben. Sedimentary deposits at this site are characterized by interbedded peat, clay and sand. Radiocarbon dating of detrital charcoal, seeds and other plant parts contained in the peat layers shows that most of the units were deposited between approximately 4000 – 6000 14C yr BP. We found evidence for at least 6 earthquakes in this older part of the stratigraphic section, including filled fissures, deformation, growth strata, upward terminations of faults, and cross-cutting fault relations. The youngest part of the section is within the extensional graben and contains evidence for three additional, younger earthquakes, but datable material in this part of the stratigraphic section is scarce. Radiocarbon dating of a single detrital charcoal sample constrains the most recent rupture to be less than 170 14C yr BP. There is no record of ground rupture associated with historical earthquakes in this area; in this San Bernardino region, the fault system ruptured in 1907 (M 5.8; all magnitudes based on historical reports, after Toppozada, 2002) and 1923 (M 6.2), although there were no reports of ground rupture in the region. The northern San Jacinto Fault system experienced earthquakes in 1899 (M 6.7) and 1918 (M 6.8), but the maximum damage in both of these was much further to the SE, and no damage was reported in the area of the paleoseismic site. A nearby paleoseismic site on the Claremont fault, at Mystic Lake (Onderdonk et al., 2013), located ~ 28 km to the SE, also has evidence suggesting an earthquake within ~150 yr BP, but this feature is instead attributed to subsidence, compaction, liquefaction or creep rather than co-seismic rupture. Here we present evidence for and radiocarbon age constraints for each of the paleoearthquakes.

Key Words
paleoseismology, San Jacinto Fault, Colton

Citation
Kendrick, K. J., & Fumal, T. E. (2018, 08). Paleoseismology of the northern San Jacinto fault, San Bernardino County. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology