Active Tectonics Projects II

Quaternary geologic slip rate along the Banning strand of the San Andreas fault, southern California

Background: Present-day Pacific-North American relative plate motion in southern California is shared primarily between the San Jacinto and San Andreas faults. At the north end of the Coachella Valley, the San Andreas fault splits into the Banning and Mission Creek strands, which are sub-parallel to each other within the Indio Hills but diverge farther to the west (Box 1, Figure 1). The Banning strand more obviously disrupts Quaternary alluvium in Morongo wash, and thus is commonly assumed to be the more active of the two strands. Understanding how slip is partitioned between these two strands is critical to southern California earthquake forecasting efforts, however, geologic slip rates along both strands remain unconstrained, so evaluating their relative contributions to regional seismic hazard has been challenging.

Progress: Using the B4 LiDAR as a base, we have mapped the extents of three truncated and offset alluvial fan deposits near Dever’s Hill, which we have differentiated based on both field and remote (LiDAR- and air photo-based) observations of texture: in particular, the distribution of different clast sizes, pavement and soil development, color, and appearance (Figure 2). To confirm across-fault correlation of the displaced deposits, we have measured 26 cosmogenic Be-10 ages from boulders and cobble samples taken from each of the three fan surfaces on both sides of the fault. The most promising offset surface is a distinct bouldery debris flow deposit (Q2b) that could only have been sourced in Morongo Wash to the north, but has since been offset 1-1.6 km to the southeastern tip of the San Gorgonio Mountains (Figure 3). Be-10 ages for this surface are pending, though we estimate an age of ~80 ka based on similarities in surface weathering and pavement development as observed on nearby surfaces for which exposure ages have already been measured. Together, these measurements indicate a preliminary late Quaternary slip rate for the Banning strand of the San Andreas fault of about 12-20 mm/yr, suggesting that slip along the San Andreas south of San Gorgonio Pass may be more focused on the Banning strand than the Mission Creek strand.

Figure 1: Fault map of the southern San Andreas fault system. Box 1 shows the location of the Dever’s Hill project (Part 1). Box 2 shows the location of the Agua Blanca-San Miguel-Vallecitos study area (Part 2). Black and white circles indicate locations of published geologic slip rates and paleoseismic trenches, respectively.

Figure 2: Geomorphic map of the Dever’s Hill site along the Banning strand of the San Andreas fault. All mapped surfaces have been dated using Be-10, though the most likely reconstruction to yield a viable slip rate will be based on surface Qoa2b at the west end of the site (see Figure 3).

Figure 3: Maximum and minimum reconstructions of the Qoa2b surface at Dever’s Hill. Be-10 ages are still pending, but a preliminary slip rate estimate from this fan surface places 12-20 mm/yr of right lateral displacement along the Banning strand of the southern San Andreas Fault.