Paleoseismology of the Colton Site, Northern San Jacinto Fault, San Bernardino County, Southern California

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The Colton paleoseismic site is 5 km south of San Bernardino, along the Claremont segment of the San Jacinto Fault. Two parallel trenches oriented orthogonal to the fault strike exposed a 20m-wide, complex fault zone, including compressional uplift and folding juxtaposed with an extensional graben. This site shows edimentary deposits of 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 and 6000 ¹⁴C yr BP, with a poorly-constrained younger section, dated near the top by two detrital charcoal samples at 1815+/-35 and 170+/-35 ¹⁴C yr BP. We found evidence for at least six earthquakes in the older part of the stratigraphic section, including filled fissures, growth strata, upward fault terminations, and cross-cutting faults. The youngest part of the section, which is younger than 4500 y BP, is within the extensional graben and contains evidence for three additional, younger earthquakes. The uppermost detrital charcoal sample constrains the most recent two ruptures to have occurred since 170 ¹⁴C yr BP. In the San Bernardino region, the fault system produced earthquakes in 1907 (M 5.8) and 1923 (M 6.2), although there were no reports of ground rupture for these events in the area [all magnitudes based on historical reports, after Toppozada, 2002]. 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 farther to the SE, and no damage was reported in the area of the Colton 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 the last ~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.

Presenting Author: Katherine Kendrick

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