Holocene Earthquakes and Slip Rates within the San Gorgonio Pass and implications for the ShakeOut Scenario 2.0

Richard V. Heermance, Katherine M. Scharer, & Bryan A. Castillo

Submitted August 30, 2026, SCEC Contribution #15176, 2026 SCEC Annual Meeting Poster #TBD

The San Gorgonio Pass (SGP) represents a structurally complex region of the southern San Andreas fault (SAF), where it diverges into multiple strands and has produced a region of low hills. Over the last decade, at least six published studies provide new information on Holocene earthquakes and slip rates through the SGP that inform the ShakeOut scenario rupture. This abstract summarizes the current understanding of earthquake history and slip rates through the SGP. The Burro Flats (BF) site is located near the western edge of the SGP along the eastern end of the San Bernardino strand. This site has experienced 9 earthquakes over the last ~1800 years, with a recurrence interval (R.I.) of ~200 years, similar to SAF sites to the northwest. The Millard and Cabazon trenches are located ~10 km southeast of BF within the San Gorgonio Pass Fault Zone (SGPFZ) but contain evidence for far fewer earthquakes– at least 7 earthquakes on the southern SGPFZ strand since 8600 y.b.p. (R.I. ~1250 yrs) and at least 4 earthquakes on the northern SGPFZ strand since 2700 y.b.p. (R.I. ~700 yrs). The SGPFZ merges with the Banning strand to the east, where the 18th Ave trench provides evidence for 8 earthquakes since 6300 y.b.p. (R.I. ~490 yrs). Published Holocene slip rate studies provide rates between 2 and 6 mm/yr for each fault within the SGP, including the western end of the Mission Creek strand near Burro Flats, SGPFZ, and Banning strand of the San Andreas Fault zone. In contrast, the Mill Creek strand within the northern SGP and the Mission Creek strand within the Mission Creek Preserve at the northeast corner of the SGP have no documented Holocene record of slip, despite evidence for Late Pleistocene offsets and earthquakes. These data provide evidence for only 2 ruptures within the SGP over the last 1600 years (~1200, ~600 C.E.), both along the southern SGP, and only the 600 C.E. rupture appears to cross from the Coachella Valley to the San Bernardino segments of the SAF. In summary, data suggest that the SGP acts as a structural barrier to through-going rupture, with 1 in 4 earthquakes penetrating into the SGP from the outside and 1 in 8 earthquakes cutting through the SGP. Geologic slip rates are anomalously low on all fault traces throughout the SGP relative to the San Andreas fault outside the SGP, suggesting slip (at depth) may be lost in off-fault deformation, blind structures, or dispersed and accommodated as uplift within the hills of the SGP.

Key Words
Earthquake, San Gorgonio Pass, Recurrance, paleoseismology, rates

Citation
Heermance, R. V., Scharer, K. M., & Castillo, B. A. (2026, 08). Holocene Earthquakes and Slip Rates within the San Gorgonio Pass and implications for the ShakeOut Scenario 2.0. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology