Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 26-8
Presentation Time: 4:10 PM


RASMUSSEN, Gary, PO Box 10575, Sedona, AZ 86339

Most geologic models for the southern San Andreas fault (SAF) indicate a significant bend in the SAF through San Gorgonio Pass (SGP). Several older (non-Quaternary) faults have been shown at various locations through SGP. Most Quaternary (Q) models show the combination of the SGP fault zone (SGPFZ), the Garnet Hill, and Banning fault (B) strands as the Q SAF through SGP. Numerous factors exist that indicate the SAF continues as a straight line between Cottonwood Canyon (CC) and Burro Flats (BF): (1) black and white imagery at a small scale and satellite imagery show a strong lineament between Cottonwood Canyon (CC) and BF; (2) groundwater barriers in the bottom of several canyons exist along the exact location and strike of the B fault in that segment; (3) a dextral kink in several of these canyons exists at these same locations; (4) earthquakes show that the depth of hypocenters in SGP decrease from around 30km in the middle of the pass to about 12km at this location with strike-slip focal mechanisms; (5) an exact replica of the relationship between the SAF and the SGPFZ exists near the site of the largest offset along the Lavic Lake fault (LLF) during the 1999 Hector Mine earthquake where a small thrust fault veers off from the main offset and then rejoins it. The thrust fault resulted in a small vertical separation compared to the larger dextral offset on the nearby (LLF). Enlarging this to the scale of the SAF and the SGPFZ makes a perfect fit. The small thrust occurred during the Hector Mine earthquake and was not the result of an independent earthquake along a thrust fault.

Any one of these factors indicates the presence of a fault. However, a combination of all of these factors coinciding at the same location strongly suggests that the SAF exists along a straight line between CC and BF. Observation of imagery from the air, satellites, and on the ground indicate the presence of hundreds of large and small-scale landslides in the “bedrock” between CC and BF. Multiple and pervasive landsliding has likely partially obscured direct surface expression of the SAF in that area.