Late Quaternary activity of the Waterman Canyon Fault, Devil Canyon, San Bernardino Mountains

Chad W. Carlson, Scott C. Lindvall, Andre Mere, Chelsea M. Blanton, Michael J. DeFrisco, Thomas K. Rockwell, Kathleen Rodrigues, Brian J. Swanson, Kristina Calderon, & Benjamin Anderson

Submitted August 30, 2026, SCEC Contribution #15643, 2026 SCEC Annual Meeting Poster #TBD

The Waterman Canyon fault (WCF) is a westerly striking Quaternary active fault in the San Bernardino Mountains (SBM) that appears to splay from the San Andreas fault (SAF) about 10 km ESE of its junction with the San Jacinto fault. Its close proximity to California State Water Project facilities in Devil Canyon was the motivation for this study. The WCF and similar faults accommodate a fraction of plate boundary strain within the southern flank of SBM. We conducted desktop mapping with LiDAR and aerial imagery, field mapping, roadcut and backhoe exposure logging, soil analysis in hand dug pits, geomorphic analyses, and IRSL dating of 14 samples from Quaternary deposits to characterize late Quaternary fault activity.

In Devil Canyon, the WCF is composed of three left-stepping, en-echelon strands, and to the west bifurcates with the NW-striking northern branch apparently the most active since the latest Pleistocene. Holocene erosion rates within the steep bedrock terrain of the southwestern San Bernardino Mountains are generally moderate to high and range from ~0.3 mm/yr to >1.0 mm/yr. Together, high rates of bedrock erosion and a presumably low dextral slip rate of the WCF explain the lack of late Quaternary coseismic displacements in the landscape except where the fault traverses terrain mantled by alluvial surfaces.

The WCF has ruptured multiple times since c. 31 – 35 ka, and the most recent surface rupturing earthquake may be as young as latest Holocene based on an age of 4.88 ± 0.33 ka from a thick, cumulic A horizon that appears to be displaced. The WCF accommodates right-lateral strike-slip with a subordinate component of up-to-the-south dip-slip displacement. Available data does not allow a net slip rate estimate. However, detailed geomorphic analysis and new geochronological constraints allow us to constrain a local vertical-separation rate of ~0.2 – 0.4 mm/yr since c. 31 ka.

IRSL ages of coarse alluvial fan deposits (Qyf, Qof, and Qvof) range from about 10 to 65 ka within Devil Canyon. Relict Qyf surfaces, constrained between ages of 9.58 ± 0.7 ka and 4.88 ± 0.33 ka, appear to grade to a common Devil Canyon paleo-valley floor. This early to middle Holocene aggradation was followed by a major pulse of late Holocene incision that created the deeply incised canyons and drainages in the watershed. This rapid incision provided many of the exposures in Quaternary sediments exploited for dating in this study.

Key Words
Waterman Canyon fault, Paleoseismology, Late Quaternary tectonics, San Bernardino Mountains, San Andreas fault system

Citation
Carlson, C. W., Lindvall, S. C., Mere, A., Blanton, C. M., DeFrisco, M. J., Rockwell, T. K., Rodrigues, K., Swanson, B. J., Calderon, K., & Anderson, B. (2026, 08). Late Quaternary activity of the Waterman Canyon Fault, Devil Canyon, San Bernardino Mountains. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology