Active Faults in Southern California

Kinematic analysis and slip rate study of the Elsinore fault, northern Santa Ana Mountains

The Elsinore fault is one of several active strike-slip faults, west of the San Andreas fault, that accommodates Pacific-North American plate boundary strain.  The fault is also one of the largest faults within this system and extends ~200 km from the margin of the Los Angeles basin to the U.S.-Mexico border, where is links with the Laguna Salada fault in northern Mexico. This USGS-supported study explores the geometry, kinematics, and slip history of the Elsinore fault in the northern Santa Ana Mountains, near Corona, California through 1:12,000 scale geological mapping and Quaternary dating of faulted landforms and deposits (Fig. 1). Collaborators: Dan Sturmer and Dylan Ward (UC).

Figure 1. (a) Google Earth™ satellite image showing the Santa Ana Mountains study area and the Elsinore, Whittier, and Chino faults. The current study focuses on the Glen Ivy section of the fault. (b) Northern Santa Ana Mountain current map area (white polygon).  Cyan lines are USGS 7.5’ quadrangle boundaries. Faults from USGS Quaternary fault and fold database.

Mapping and slip rate study of the Coachella Valley segment of the San Andreas fault

The Coachella Valley segment of the San Andreas fault (SAF) transitions from a single strand—the Indio Fault—into three distinct strands: the Mission Creek, Banning, and Garnet Hills faults, as it approaches the San Bernardino Mountains (Fig. 2a). Previous studies have proposed that the slip rate along the Mission Creek fault decreases westward, from 14–17 mm/year at Biskra Palms Oasis (Behr et al., 2010), to 2–6 mm/year at Thousand Palms Canyon (Fumal et al., 2002), and further down to 0–3 mm/year in the San Bernardino Mountains (Field et al., 2015), as strain is transferred to the Banning and Garnet Hills faults and Eastern California shear zone. However, a recent study by Blisniuk et al. (2021) reported slip rates of 21.6 ± 2 mm/year for the Mission Creek fault and 2.5 ± 1 mm/year for the Banning fault in the southern Indio Hills. This USGS-supported study investigates the geometry, kinematics, and slip histories of these fault strands through detailed 1:12,000-scale geologic mapping and Quaternary dating of faulted landforms and deposits to reconcile these competing models and datasets for SAF slip (Fig. 2b).

Figure 2. (a) Google EarthTM satellite image showing Indio Hills study area and Mission Creek and Banning faults and (b) the proposed map area. Cyan lines are USGS 7.5-minute quadrangle boundaries. Faults from USGS Quaternary fault and fold database.