San Jacinto Fault Zone

California Coast, Los Angeles to San Diego Bay. San Jacinto Fault Zone is labeled and parallels the Elsinore Fault to the east on the far right of the photo. NASA photo, 2008

The San Jacinto Fault Zone is a major fault zone in Southern California. It runs for 130 miles (210 km) through San Bernardino, Riverside, San Diego and Imperial Counties. It is considered to be the most seismically active fault zone in Southern California. The San Jacinto Fault Zone generally lies west of the larger, more infamous San Andreas Fault while running parallel and to the east of the Elsinore Fault Zone. Both the San Jacinto and Elsinore Fault Zones are classified as part of the San Andreas Fault System, relieving the majority of the stress between the Pacific Plate and North American Plate.

Fault characteristics

The San Jacinto Fault Zone is not a continuous fault, but a series of faults focused along the same general axis. These related faults are dextral, exhibiting right lateral-moving horizontal motion (strike-slip). The San Jacinto Fault Zone accommodates a portion of the deformation caused by the opposing movements between the North American Plate and Pacific Plate. The San Jacinto Fault Zone branches westward from the San Andreas Fault in the Transverse Ranges. The fault zone is characterized by a series of strike-slip faults that cross through the pre-Cretaceous metasedimentary and Cretaceous Plutonic rock of the Peninsular Ranges as well as late Cenozoic sedimentary rocks of the Salton Trough. Throughout the fault zone there are minor compressional, tensional, and oblique zones, but most of the crustal stress is relieved by horizontal, strike-slip motion.

Fault segments

The San Jacinto Fault consists of at least three major segments

• The Claremont Segment begins at the intersection of the San Jacinto and San Andreas Faults and cuts southward through the San Bernardino Basin. Between 1.5 and 1.0 Ma this section has experienced ~25 km of slip, implying a long-term slip rate of 16–25 mm/yr .
• The Clark Section continues southward from the San Bernardino Basin and travels along the flank of the San Jacinto Mountains. 24–25 km of right-lateral slip has been documented , with initiation of slip at 1.5 ± 0.2 Ma, implying 13–18 mm/yr . Rockwell et al. suggest that Late Pleistocene slip rates have been lower .
• The Coyote Creek Section consists of both the Clark and Coyote Creek Faults. The Coyote Creek Fault has an estimated Holocene slip rate of 3–5 mm/yr , and a much larger average slip rate of 11 ± 3 mm/yr since 600 ± 100 ka .

Nearby communities

• Lytle Creek
• San Bernardino
• Colton
• Loma Linda
• Moreno Valley
• San Jacinto
• Hemet
• East Hemet
• Valle Vista
• Idyllwild-Pine Cove
• Anza
• Borrego Springs
• Ocotillo Wells

Notable earthquakes

The San Jacinto Fault Zone has had some notable earthquakes in historic times:

Note: The GPS coordinates listed are approximate in many instances.

• 1890 - Magnitude 6.5 that occurred in the "San Jacinto or Elsinore Fault region". Typing in the GPS coordinates on a map clearly shows that the earthquake occurred on the former fault. GPS Coordinates
• 1892 - Another magnitude 6.5 occurred in the same region as the 1890 earthquake. Like the 1890 earthquake, GPS coordinates show that the San Jacinto Fault was the catalyst, as well. GPS Coordinates
• 1899 San Jacinto Earthquake - Magnitude 6.4 earthquake destroys San Jacinto and Hemet. GPS Coordinates
• 1918 San Jacinto Earthquake - Magnitude 6.9 earthquake strikes the same area that was damaged by an earthquake 19 years earlier, with an epicenter roughly 10 mi NW of the previous earthquake. GPS Coordinates
• 1923 North San Jacinto Fault Earthquake - Magnitude 6.3 earthquake damaged the San Bernardino and Redlands area. Last time the fault, which runs under the I-215/I-10 interchange, ruptured in this area. GPS Coordinates
• 1937 Terwilliger Valley Earthquake - Magnitude 6.0 GPS Coordinates
• 1942 Fish Creek Mountains Earthquake - Magnitude 6.3 GPS Coordinates
• 1954 Arroyo Salada Earthquake - Magnitude 6.2 GPS Coordinates
• 1968 Borrego Mountain Earthquake - Magnitude 6.5 GPS Coordinates
• 1987 Superstition Hills Earthquake - Magnitude 6.6 (Note: some consider it to have occurred on a fault completely unrelated to the San Jacinto Fault Zone) GPS Coordinates
• 2010 Borrego Springs - Magnitude 5.4. This quake was believed by seismologists to have been possibly triggered by the strong earthquake which occurred on Easter of the same year near Calexico.