M4.9 - Indio, CA

Tectonic Summary

Earthquakes in southern California

Southern California's high level of seismic activity is a result of tectonic motion, which causes the northward progress of the Pacific plate (on the west) relative to the North American plate (on the east). The main plate-boundary fault is the San Andreas Fault, which stretches 1,200 km (745 mi) from the Salton Sea in the south to offshore Cape Mendocino in the north. The southern San Andreas Fault crosses through the largest mountains in southern California and runs near communities including Parkfield, Frazier Park, Palmdale, Wrightwood, San Bernardino, Banning, and Indio. This right-lateral strike-slip fault moves at different rates along its trace, from about 35 mm per year (1.4 in/yr) near Parkfield to a low of about 18 mm/year (0.7 in/yr) in the south. The San Andreas Fault can generate the region's largest magnitude earthquakes (up to about M8.2). The most recent 'Big One' on this fault in southern California was the M7.9 Fort Tejon earthquake of 1857, which was strongly felt throughout Los Angeles.

To the west of the San Andreas, several other right-lateral, strike-slip faults accommodate plate-boundary motion. The San Jacinto, Elsinore, Newport-Inglewood, and Rose Canyon Faults are the most prominent. There are also right-lateral faults offshore, including the faults of the San Diego Trough Fault Zone. Among these faults, the San Jacinto moves the fastest, at a rate of about 14 mm/year (0.6 in/yr), but all of these faults are capable of major earthquakes. There have been large, historical earthquakes on the San Jacinto (1918, M6.7) and the Newport-Inglewood Faults (1933, M6.4). The Rose Canyon Fault is an important source for seismic hazard in San Diego because of its proximity to the city. Just south of the Salton Sea, deformation from the San Jacinto and San Andreas Faults merge onto a single fault, known as the Imperial Fault. This fault has a high slip rate (>30 mm/yr, 1.2 in/yr) and two large historic earthquakes in 1940 (M6.9) and 1979 (M6.5).

The faults of the Eastern California Shear Zone (ECSZ) accommodate about a quarter of the Pacific-North American tectonic plate boundary motion. This network of faults runs north from the southern San Andreas Fault to the California-Nevada border. The ECSZ has hosted nearly all of the major earthquakes in southern California over the past few decades, including the M7.2 1992 Landers earthquake, M7.1 1999 Hector Mine earthquake, and the M7.1 2019 Ridgecrest earthquake sequence. Most faults of the ECSZ exhibit strike-slip motion.

Many of the mountains of southern California are bounded by reverse faults. Between Palm Springs and Santa Barbara, the San Andreas Fault bends to the west, and plate-boundary motion creates compressive forces. Reverse fault slip accommodates these forces and the Transverse Range mountains, including the San Bernadino, San Gabriel, and Santa Monica Mountains, are lifted as a result. The Cucamonga, Sierra Madre, and Hollywood Faults are prominent examples, but not all of these faults reach Earth's surface. The damaging M6.7 Northridge earthquake in 1994 occurred on a blind fault beneath greater Los Angeles and highlighted the potential hazard from faults that have no visible scarp. The Puente Hills Fault is an important blind fault that has been identified since the 1994 Northridge earthquake. Although many southern California reverse faults are short compared to the San Andreas, they are still quite dangerous. Moderate earthquakes on these faults, including the 1994 Northridge and M6.6 1971 Sylmar, have caused over 100 deaths and thousands of casualties. Moreover, multiple faults may rupture together in a major earthquake in close proximity to population centers.

Very few southern California faults exhibit primarily left-lateral slip. The Garlock Fault is an example that is highly visible in the landscape and capable of large earthquakes. The Garlock fault slips at about 8 mm/year (0.3 inches/year). Other left-lateral faults tend to be much less active