The San Jacinto Fault doesn't command the same name recognition as the San Andreas Fault, but among seismologists, it holds a distinction that should concern every Southern Californian: it is the single most seismically active fault in the state. According to the Southern California Earthquake Center (SCEC), the San Jacinto Fault zone produces more moderate earthquakes (M3.0 and above) than any other fault system in California, including the San Andreas itself.
Running roughly 210 kilometers from the San Gorgonio Pass near Banning to the international border with Mexico, the San Jacinto Fault slices through some of the most densely populated communities in the Inland Empire and beyond. Cities like San Bernardino, Riverside, Hemet, Temecula, and the Anza Valley communities sit directly in the fault's path. While the San Andreas Fault retains a higher probability of producing the largest possible earthquakes (M8+), the San Jacinto's persistent activity and its proximity to population centers make it one of the most consequential seismic hazards in the western United States.
This article examines the geology, history, and future risk posed by the San Jacinto Fault — what it is, what it has done, and what seismologists expect it to do next.
Geography and Fault Trace
The San Jacinto Fault trends northwest-to-southeast through the heart of Southern California's inland valleys. It branches off from the San Andreas Fault system near Cajon Pass, northwest of San Bernardino, and extends southeastward approximately 210 km (130 miles) to the U.S.–Mexico border. South of the border, the fault system continues into Baja California as part of the broader Pacific–North American plate boundary.
The fault runs through or immediately adjacent to the following communities and geographic features:
- San Bernardino and Highland — the northernmost populated area along the fault
- Loma Linda and Redlands — situated near the fault's trace as it curves southeastward
- San Jacinto and Hemet — the San Jacinto Valley, from which the fault takes its name, sits directly within the fault zone
- Anza Valley — a sparsely populated but seismically critical area where several fault segments converge
- Temecula and Murrieta — rapidly growing cities in southwestern Riverside County
- Borrego Springs and the Anza-Borrego Desert — the southeastern portion of the fault traverses this desert landscape before reaching the border
The fault runs roughly parallel to and 15–30 km east of the southern San Andreas Fault. Together, these two fault systems accommodate the majority of the approximately 50 mm/year of total Pacific–North American plate motion in Southern California, according to the USGS.
[MAP: San Jacinto Fault trace from San Gorgonio Pass to the Mexican border, showing individual named segments (San Bernardino, San Jacinto Valley, Anza, Clark, Coyote Creek, Borrego Mountain, Superstition Mountain/Hills), major cities, and the parallel trace of the southern San Andreas Fault.] Data source: USGS Quaternary Fault and Fold Database, SCEC Community Fault Model. Features: Named fault segments in distinct colors, city locations, San Andreas Fault shown for reference, historical earthquake epicenters as scaled circles.
Geology and Fault Mechanics
Fault Classification
The San Jacinto Fault is a right-lateral strike-slip fault, meaning that when viewed from either side, the opposite block moves to the right. This is the same sense of motion as the San Andreas Fault, and for good reason — the San Jacinto is a major subsidiary structure within the San Andreas Fault system. Together with the Elsinore Fault to the west, these three subparallel fault systems collectively transfer plate boundary motion through Southern California.
Slip Rate
Geodetic measurements and paleoseismic studies place the San Jacinto Fault's slip rate at approximately 12–15 mm/year. A 2017 study by Lindsey and Fialko, published in the Journal of Geophysical Research, used InSAR (satellite radar) data to estimate a slip rate of approximately 15.3 mm/year on the central San Jacinto Fault, making it one of the faster-slipping faults in the San Andreas system. For comparison, the southern San Andreas Fault accommodates roughly 20–28 mm/year depending on the segment, and the Elsinore Fault accounts for approximately 5 mm/year.
Segmentation
The San Jacinto Fault is not a single continuous rupture surface. It is divided into multiple named segments and strands, each with its own earthquake history and recurrence characteristics. The primary segments, from northwest to southeast, include:
| Segment | Approximate Length | Key Area |
|---|---|---|
| San Bernardino | ~30 km | San Bernardino, Highland |
| San Jacinto Valley | ~40 km | San Jacinto, Hemet |
| Anza section | ~50 km | Anza, Cahuilla |
| Clark strand | ~55 km | Anza to Clark Valley |
| Coyote Creek | ~40 km | Coyote Creek, Borrego area |
| Borrego Mountain | ~30 km | Borrego Springs |
| Superstition Mountain | ~25 km | Imperial Valley fringe |
| Superstition Hills | ~25 km | Southern terminus near border |
This segmentation is significant because individual segments can rupture independently in moderate earthquakes (M6–6.8), but paleoseismic evidence also suggests that multi-segment ruptures capable of producing M7+ events have occurred in the geologic past.
Seismic Gap: The Anza Section
The Anza section is among the most closely watched fault segments in the world. According to SCEC and USGS researchers, this approximately 50 km stretch has not produced a significant surface-rupturing earthquake in at least 200 years — and possibly much longer. GPS data shows that strain is accumulating along the Anza section at rates consistent with the broader fault slip rate.
A 2006 study by Salisbury et al. in the Bulletin of the Seismological Society of America identified the Anza seismic gap as having accumulated sufficient strain to produce an earthquake of approximately M6.8–7.2, depending on whether the rupture remains confined to the Anza section or propagates into adjacent segments. The Anza area remains one of the highest-priority monitoring targets for SCEC's seismic instrumentation network.
Historical Earthquakes
The San Jacinto Fault has produced a series of damaging earthquakes over the past 150 years. While none have reached the magnitude of the 1906 San Francisco earthquake (M7.9) or the 1857 Fort Tejon earthquake (M7.9) on the San Andreas, the San Jacinto's events have been frequent, destructive, and clearly illustrative of the fault's persistent hazard.
Table: Significant Historical Earthquakes on the San Jacinto Fault
| Date | Magnitude | Name/Location | Segment | Notable Effects |
|---|---|---|---|---|
| December 25, 1899 | M6.4 (est.) | San Jacinto–Hemet | San Jacinto Valley | Widespread damage in San Jacinto and Hemet; six deaths reported; one of the strongest earthquakes in the region's written history |
| April 21, 1918 | M6.8 | San Jacinto | San Jacinto Valley | Significant structural damage in San Jacinto and Hemet; one death; felt from Los Angeles to Yuma, Arizona |
| October 21, 1942 | M6.6 | Borrego Valley | Borrego Mountain/Superstition | Surface rupture observed; moderate damage in desert communities; relatively low population in the area limited impact |
| March 19, 1954 | M6.2 | San Jacinto–Arroyo Salada | Clark/Coyote Creek | Felt across Southern California; limited damage due to remote epicenter |
| April 9, 1968 | M6.6 | Borrego Mountain | Borrego Mountain | 30 km of surface rupture; triggered slip on the Superstition Hills and Imperial faults; minor injuries |
| June 12, 2005 | M5.2 | Anza | Anza section | Moderate shaking; no significant damage; provided valuable data on the Anza seismic gap |
| July 7, 2010 | M5.4 | Collins Valley | Anza/Coyote Creek | Felt widely across Southern California; minor damage in Borrego Springs and Julian |
Several important patterns emerge from this record. The San Jacinto Fault produces M5+ earthquakes roughly every decade and M6+ events roughly every 20–30 years. However, the written historical record covers only about 150 years — a tiny fraction of the fault's seismic cycle.
The 1918 San Jacinto Earthquake
The April 21, 1918 earthquake deserves particular attention. At M6.8, it remains the largest instrumentally recorded earthquake on the San Jacinto Fault. The event caused substantial damage in the towns of San Jacinto and Hemet, collapsing unreinforced masonry buildings and killing one person. Newspaper accounts from the time describe cracked walls, toppled chimneys, and panic across the Inland Empire. The earthquake was felt as far as Los Angeles, approximately 130 km to the northwest.
In 1918, the combined population of the San Jacinto Valley was a few thousand. Today, the Riverside–San Bernardino metropolitan area exceeds 4.6 million people, and communities along the fault trace have grown enormously. A repeat of the 1918 earthquake under modern conditions would affect a dramatically larger population and built environment.
The 1968 Borrego Mountain Earthquake
The M6.6 Borrego Mountain earthquake of April 9, 1968 was a landmark event in earthquake science. It produced approximately 30 km of surface rupture along the Coyote Creek segment and was one of the first California earthquakes to be captured by a dense network of strong-motion instruments. The earthquake also triggered sympathetic surface slip on the Superstition Hills Fault and the Imperial Fault — providing early observational evidence for the phenomenon of triggered fault slip, which has since become a major topic of research.
Paleoseismic Evidence and Future Risk
What the Trenching Record Shows
Paleoseismology — the science of excavating fault trenches to date past earthquakes — has been extensively applied to the San Jacinto Fault. Research sites at Hog Lake (near Anza), Mystic Lake (near San Jacinto), and other locations have revealed a rich record of prehistoric earthquakes.
A landmark 2010 study by Rockwell et al. published in the Bulletin of the Seismological Society of America documented approximately 20 surface-rupturing earthquakes at the Hog Lake site over the past 4,000 years. The average recurrence interval was roughly 150–230 years, with some clusters of events occurring more frequently and some longer gaps. Critically, several of these prehistoric events produced displacements consistent with M7.0 or larger earthquakes.
The most recent large earthquake at Hog Lake was dated to approximately 1795–1800 CE. If this represents the last major event on the Anza section, it has been over 225 years since the last significant rupture — placing the current interval near or beyond the average recurrence time.
UCERF3 Probabilities
The Third Uniform California Earthquake Rupture Forecast (UCERF3), published by the USGS, SCEC, and the California Geological Survey in 2015, provides the most authoritative probabilistic estimates for future earthquakes on California faults. For the San Jacinto Fault zone as a whole, UCERF3 estimates:
- M6.7+ earthquake: approximately 31% probability in 30 years (2014–2043)
- M7.0+ earthquake: approximately 19% probability in 30 years
- M7.5+ earthquake: lower probability, but not negligible — multi-segment ruptures could reach this magnitude
For context, UCERF3 estimates a 93% probability of at least one M6.7+ earthquake somewhere in Southern California during the same period, and 72% for the San Francisco Bay Area. The San Jacinto Fault contributes a meaningful share of the Southern California total.
[CHART: Bar chart — 30-Year Earthquake Probabilities for Major Southern California Faults (UCERF3)] Data: San Andreas (Southern): ~59% M6.7+; San Jacinto: ~31% M6.7+; Elsinore: ~24% M6.7+; Puente Hills Thrust: ~10% M6.7+; Newport-Inglewood: ~15% M6.7+. Source: UCERF3, Field et al. (2015).
Multi-Segment Rupture Scenarios
One of the most concerning findings in recent San Jacinto Fault research is the evidence for past multi-segment ruptures. If the Anza section were to rupture simultaneously with adjacent segments — such as the Clark or San Jacinto Valley segments — the resulting earthquake could reach M7.2–7.5. Such an event would produce strong shaking across the entire Inland Empire, from San Bernardino to Temecula, affecting millions of people.
Dynamic rupture simulations conducted by SCEC researchers, including work by Lozos (2016) published in the Bulletin of the Seismological Society of America, have modeled scenarios in which rupture initiates on the San Jacinto Fault and transfers onto the San Andreas Fault — or vice versa. While the probability of such combined ruptures is difficult to quantify, the physical possibility underscores the interconnected nature of Southern California's fault systems.
Cities and Infrastructure at Risk
Population Exposure
The San Jacinto Fault passes through or immediately adjacent to communities with a combined population exceeding 2 million people. Key population centers include:
- San Bernardino (population approximately 222,000): California's 17th-largest city sits near the northern end of the fault. Much of the city's older building stock predates modern seismic codes.
- Riverside (population approximately 314,000): The county seat is located west of the fault but within the zone of strong shaking for a major event.
- Hemet (population approximately 90,000) and San Jacinto (population approximately 52,000): These communities sit directly within the fault zone in the San Jacinto Valley.
- Temecula (population approximately 110,000) and Murrieta (population approximately 115,000): Among the fastest-growing cities in California, located in the southern portion of the fault system.
Critical Infrastructure
Several major infrastructure corridors cross the San Jacinto Fault, including:
- Interstate 10: Crosses the fault zone near San Bernardino/Loma Linda
- Interstate 15: Crosses the fault near Lake Elsinore and Temecula
- Interstate 215: Runs nearly parallel to the fault for much of its length through the San Jacinto Valley
- California State Route 79: Follows the fault through the Anza Valley
- Water infrastructure: The Metropolitan Water District's Colorado River Aqueduct crosses the fault zone; damage could disrupt water supply to millions of Southern Californians
ShakeOut Scenario Relevance
The 2008 Great California ShakeOut exercise focused on a M7.8 scenario on the southern San Andreas Fault. However, USGS scientists have noted that a major San Jacinto event could produce comparable damage in the Inland Empire due to the fault's closer proximity to population centers. The San Jacinto Fault passes through urban areas that the San Andreas, running through more mountainous and desert terrain, largely bypasses.
How the San Jacinto Compares to the San Andreas
Understanding the relationship between these two faults is essential for assessing Southern California's seismic risk.
| Characteristic | San Jacinto Fault | Southern San Andreas Fault |
|---|---|---|
| Length | ~210 km | ~1,300 km (total system) |
| Slip rate | ~12–15 mm/yr | ~20–28 mm/yr (southern section) |
| Fault type | Right-lateral strike-slip | Right-lateral strike-slip |
| Rate of M3+ earthquakes | Highest in California | Lower than San Jacinto |
| Maximum credible earthquake | ~M7.5 (multi-segment) | ~M8.0+ (multi-segment) |
| Last major earthquake | 1918 M6.8 | 1857 M7.9 (Fort Tejon) |
| Proximity to urban areas | Passes through cities | Mostly mountains/desert |
| UCERF3 30-yr M6.7+ probability | ~31% | ~59% (southern section) |
The San Andreas Fault is longer, slips faster, and is capable of larger earthquakes. But the San Jacinto Fault compensates with higher rates of moderate activity and closer proximity to millions of people. In practical terms, a San Jacinto M7.0 could be more locally destructive than a San Andreas M7.5 centered in a remote desert section.
Monitoring and Research
SCEC Instrumentation
The San Jacinto Fault is one of the most densely instrumented fault zones in the world. The Southern California Earthquake Center (SCEC), in collaboration with the USGS and university partners, maintains a dense network of seismometers, GPS stations, and borehole strainmeters along the fault. The Anza Seismic Network, operated by the University of California, San Diego, provides particularly high-resolution monitoring of the Anza section.
Earthquake Early Warning
The ShakeAlert earthquake early warning system, operated by the USGS in partnership with state agencies and universities, covers the San Jacinto Fault zone. For communities along the fault, ShakeAlert could provide seconds to tens of seconds of warning before strong shaking arrives from a nearby earthquake — enough time to drop, cover, and hold on, and for automated systems to slow trains, open fire station doors, and shut down industrial processes.
Ongoing Research Questions
Active areas of research on the San Jacinto Fault include:
- Anza gap behavior: Will the Anza section rupture independently or in concert with adjacent segments?
- Interaction with the San Andreas: How do stress changes from one fault influence earthquake timing on the other?
- Fault zone structure: High-resolution imaging of the fault's internal structure to understand how rupture propagates through complex fault geometry
- Triggered seismicity: How do San Jacinto earthquakes influence adjacent faults and vice versa?
Preparedness for Communities Along the Fault
Residents and businesses along the San Jacinto Fault face a well-documented seismic hazard. Key preparedness actions recommended by the USGS and California Governor's Office of Emergency Services include:
- Structural assessment: Retrofitting unreinforced masonry buildings and soft-story apartments, which are particularly vulnerable to strike-slip earthquake shaking
- Earthquake kits: Maintaining supplies for at least 72 hours of self-sufficiency
- ShakeAlert enrollment: Downloading the MyShake app for real-time earthquake early warnings
- Drop, Cover, and Hold On: Practicing the recommended protective action during shaking
California earthquake history and risk provides additional context on statewide seismic hazards, and San Diego earthquake information covers the southern extension of the San Jacinto system.