Geomorphic and paleoseismic trenching evidence that the Glen Helen fault has not experienced surface rupture in the past 2000 years, and implications for slip transfer between the San Andreas and San Jacinto fault zones.

Nate W. Onderdonk, Drake D. Kerr, & Paula M. Figueiredo

Published August 12, 2020, SCEC Contribution #10436, 2020 SCEC Annual Meeting Poster #024

Paleoseismic data and modeling studies suggest that some ruptures through the Cajon Pass area in the past 2000 years have propagated from the San Jacinto fault zone to the San Andreas fault zone, or vice versa. The San Jacinto fault zone splits into three main fault strands as it approaches the San Andreas fault zone in the Cajon Pass area, with the Glen Helen fault being the strand closest to the San Andreas. The Glen Helen fault parallels the San Andreas for a distance of 10 km in lower Cajon Creek with less than 2 km separating the two faults. The proximity of the Glen Helen fault to the San Andreas makes it a strong candidate for facilitating slip transfer between the San Andreas and San Jacinto fault zones. In an effort to test the idea of coseismic slip transfer between these two fault zones, we conducted a geomorphic and paleoseismic study along the Glen Helen strand to document it’s slip rate and the timing of the last few surface ruptures.

Geomorphic investigations of the Glen Helen fault showed no visible offset of several alluvial fans that cross the inferred fault trace along a linear mountain front in lower Cajon Creek. Apparent fault scarps are present, however, in two of the alluvial fans about 100 m out from the mountain front. We excavated 5 trenches (max depth of 2 m) across these scarps and across a possible lateral offset of a fan edge, but the trenches revealed no evidence for faulting. Stratigraphic relationships in the trenches and radiocarbon dating of detrital charcoal show that the apparent scarps are erosional features from Cajon Creek and that the fans propagated across older fluvial deposits of Cajon Creek around 2000 ybp. Two additional trenches were excavated across the previously-mapped Glen Helen fault at the mountain front and confirmed that these 2000 year old alluvial fans have not been disrupted by fault activity.
These unexpected results suggest that the Glen Helen fault has not accommodated coseismic slip transfer between the San Andreas and San Jacinto fault zones in the past 2000 years. Instead, slip transfer at the surface may have occurred farther to the southeast, possibly involving the Tokay Hill fault, or 10 to 20 kilometers to the northwest where the middle San Jacinto fault strand approaches the San Andreas between Cajon Pass and Wrightwood. Alternatively, slip transfer between the two fault zones during the past 2000 years was accomplished in a manner that did not involve surface rupture.

Key Words
San Andreas fault, San Jacinto fault, earthquake gate, cajon pass

Citation
Onderdonk, N. W., Kerr, D. D., & Figueiredo, P. M. (2020, 08). Geomorphic and paleoseismic trenching evidence that the Glen Helen fault has not experienced surface rupture in the past 2000 years, and implications for slip transfer between the San Andreas and San Jacinto fault zones. . Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)