San Jacinto Fault Zone
The San Jacinto Fault Zone (SJFZ) is a major strike-slip fault zone that runs through San Bernardino, Riverside, San Diego, and Imperial Counties in Southern California. The SJFZ is a component of the larger San Andreas transform system and is considered to be the most seismically active fault zone in the area. Together they relieve the majority of the stress between the Pacific and North American tectonic plates.
The SJFZ itself consists of many individual fault segments, some of which have only been individualized as recently as the 1980s, but activity along the line of faults has been documented since the 1890s. One segment of the SJFZ, the Anza seismic gap, has not experienced any major activity since instrumental records have been kept. Each segment was evaluated for its seismic risk and was assigned a probability for the occurrence of a large rupture for the thirty year period starting in 1995.
While several of the large earthquakes along the SJFZ have not resulted in significant property damage or loss of life (due to their remote location) the cities of Hemet and San Jacinto were both heavily damaged in two significant events in 1899 and 1918. The recurrence interval for a series of large earthquakes starting in 1899 (including the 5.9 1937 Terwilliger Valley earthquake) was 18, 5, 14, 5, 12, 14, and 19 years, yet there has not been a strong earthquake for 25 years (since the 1987 Superstition Hills and Elmore Ranch sequence).
The San Jacinto Fault Zone and the San Andreas Fault (SAF) accommodate up to 80% of the slip rate between the North American and Pacific plates. The extreme southern portion of the SAF has experienced two moderate events in historical times, while the SJFZ is one of California's most active fault zones and has repeatedly produced both moderate and large events. The locations of earthquakes before the 1954 Arroyo Salada earthquake are not precisely known, but the events' effects place them on the SJFZ and not on the SAF. The 1923 North San Jacinto Fault earthquake struck the Inland Empire area of southern California at a time of relatively low population, and a repeat event in modern times would result in heavy property damage and loss of life.
|San Bernardino||35 km||1890|
|San Jacinto||42 km||1918|
|Coyote Creek||40 km||1892|
|Borrego Mtn||29 km||1968|
|Superstition Mtn||23 km||1430|
|Superstition Hills||22 km||1987|
|Working Group on California Earthquake Probabilities |
A 1995 report by the Working Group on California Earthquake Probabilities identified seven individual segments of the SJFZ. The group consisted of more than three dozen seismologists, including Keiiti Aki and C. Allin Cornell, and was organized by the Southern California Earthquake Center for the USGS and the California Office of Emergency Services. The 1995 paper was the third in a series of reports that was set in motion following the 1992 Landers earthquake in southern California with the intention of updating the data and the approach for calculating the probabilities for large earthquakes along the southern San Andreas and San Jacinto Fault zones. Both these fault zones were grouped together as having adequate paleoseismic data to assign conditional probabilities for future damaging earthquakes.
The original Working Group in 1988 had identified five segments of the fault zone. From north to south, the segments were labeled the San Bernardino Valley, San Jacinto Valley, Anza, Borrego Mountain, and Superstition Hills. The 1995 group then added the Coyote Creek and Superstition Mountain segments, defined the Anza segment to include the Clark and Casa Loma faults, and updated the slip rates for each segment. The three northern sections (San Bernardino, San Jacinto, and Anza) were assigned 12 mm per year of slip and the four remaining sections were given 4 mm of slip, and error rates were half the total estimated slip for each segment (±6 mm and ±2 mm respectively) with the exception of the Anza segment which had slightly exaggerated rates of +7 mm and -5 mm.
Thirty year probabilities for segment-rupturing earthquakes were estimated using three separate models then a preferred weighted result was presented for each segment. While the San Bernardino (37%) and San Jacinto (43%) segments both saw large increases since the 1988 report, due in part to increased estimates for slip rates and decreased estimates for inherent displacement, the Anza segment (17%) was determined to have a decreased probability, based on an increased segment length. The Coyote Creek (18%), Superstition Mountain (9%), and Superstition Hills (2%) segments received first time estimates (none were assigned in 1988) and the Borrego Mountain segment received a more specific value of 6%.
San Bernardino Valley 
The northernmost segment of the SJFZ includes the Claremont fault (though other parallel strands exist) and spans a total of 35 km (22 mi).
San Jacinto Valley 
Moving southward, this 45 km (28 mi) segment includes the parallel Casa Loma and Claremont faults with the southern limit at the point where both of these faults unite to form the Clark fault. The April 1918 event most likely occurred on this segment.
While the 1988 Working Group included the Clark, Coyote Creek, and Buck Ridge faults, the 1995 Working Group limited the segment to just the 90 km (56 mi) Clark fault. A paleoseismic investigation on this segment at Hog Lake indicated three historical surface-rupturing events occurred around 1210, 1530, and 1750 with an average recurrence period for a magnitude 7.0 – 7.5 earthquake of 250 years.
Coyote Creek 
With a recurrence period of 175 (+158 / -95) years, no surface-rupturing event has occurred on this 40 km (25 mi) segment since 1892.
Borrego Mountain 
The extent of this segment is based on the surface rupture of the 1968 Borrego Mountain earthquake and shares a recurrence interval of 175 years.
Superstition Hills 
The 1988 Working Group defined the segment as two parallel strands, the Superstition Hills and Superstition Mountain faults, though no slip rate or recurrence interval was known. On November 23, 1987 the Working Group determined that the available information was still not adequate to assign 30-year probabilities. On November 24, 1987 the fault ruptured, along with an unknown fault (later named the Elmore Ranch fault). Kenneth W. Hudnut and Kerry Sieh examined the surface rupture (along with a trench investigation) in 1989 and estimated the slip rate for the prior 330 years to be 2 – 6 mm/yr (±1 mm). The Working Group used these new figures to assign a slip rate of 4 ±2 mm/yr with an average recurrence interval of 250 (+400 / -133) years for the segment.
Superstition Mountains 
Three surface-faulting events were found to have occurred along this newly added segment. A trench investigation by Larry Gurrola and Thomas Rockwell near the north shore of ancient Lake Cahuilla dated the events to 885 – 1440. The slip rate for the Borrego mountain segment (4 ± 2mm/yr) was extrapolated for use along the segment and a recurrence interval of 340 years was established.
Anza seismic gap 
With at least six and as many as ten large events since 1890, the right-lateral strike-slip SJFZ is southern California's most restless fault, with the exception of several sections which have seen less frequent activity. In a 1975 study, one of these (a 40 km (25 mi) stretch) was labeled the "Anza to Coyote Mountain slip gap", and was further refined in a 1984 paper by seismologists Christopher Sanders and Hiroo Kanamori to include only a smaller 20 km (12 mi) section near the town of Anza. By studying several moderate events (and their aftershocks) that occurred in 1967 (4.7L), 1975 (4.8L), and 1980 (5.5L), Sanders and Kanamori determined the seismogenic but locked nature of the gap. Were the entire fault segment to rupture in a single event, this newly modified length limited the potential of the segment to generate a magnitude 6.5 earthquake, similar in size to previous events along the SJFZ. However, if the slip were to extend out of the Anza area, the earthquake could be up to, but not larger than 7.0 in magnitude.
Link to San Andreas Fault 
At least six large ruptures of the San Jacinto Fault Zone are known to have followed the 1857 Fort Tejon earthquake that ruptured the central segment of the San Andreas Fault. These events began with the 1899 San Jacinto earthquake and occurred at intermittent intervals culminating with the 1987 Superstition Hills and Elmore Ranch events. The 1857 rupture spanned a total of 360 kilometers (220 mi) and terminated on the southeast end near the point where the San Jacinto Fault Zone branches away from the San Andreas Fault Zone at the Cajon Pass. In a paper published in the journal Science, Christopher Sanders plotted the earthquakes of the SJFZ by time and location and found that a uniform pattern became apparent. Moving southeastward from the Cajon Pass, the large SJFZ events appear on a line with a slope of 1.7 km per year and Sanders hypothesized that the 1857 earthquake introduced a strain pulse that migrated southeast and triggered large earthquakes as it traversed the SJFZ at that rate.
Sanders, in a 1993 newspaper interview following the publication of the paper, said that "these earthquakes were given that little extra bump over the edge by this strain pulse". Other seismologists had mixed reviews of Sanders' theory. Kerry Sieh (from Caltech) acknowledged that "his hypothesis is not unreasonable." Hiroo Kanamori added that "It's certainly possible that the current seismicity in southern California is still reflecting the effect of 1857."
- February 9, 1890 earthquake
An early morning event measuring 6.3 occurred in northeast San Diego County that was reported with a maximum intensity of MM VI at all Southern Pacific stations between Pomona to Yuma, Arizona. The railroad line runs near both the San Andreas and San Jacinto fault traces northeast of Riverside, and through San Gorgonio Pass (near the Banning Fault). Since the event was felt equally at the railroad stations, Sanders and Kanamori submitted that the earthquake was most likely not a result of slip of any fault strand near the railroad, and that a more probable source of the event would be farther south along the SJFZ near Anza, though this contradicted the report from the 1995 Working Group on California Earthquake Probabilities, which placed the earthquake on the northernmost San Bernardino segment of the SJFZ. ( )
- May 28, 1892 earthquake
Another earthquake with an approximate magnitude of 6.5 occurred in the same region and delivered similar effects as the 1890 shock. Felt reports came from San Diego, Los Angeles, and Yuma, Arizona. Two shocks were reportedly felt at San Bernardino and Ontario at 3:15 and again at 3:20 am with the direction of the vibrations moving from east to west. The first shock was described as being very heavy and that it had broken dishes and stopped clocks. It is possible that it was an aftershock of the February 1892 Laguna Salada earthquake. ( )
- 1899 San Jacinto earthquake
On December 25, with a maximum intensity of MM IX, this magnitude 6.6 earthquake destroyed San Jacinto and Hemet and six were killed by adobe walls that collapsed at Saboba (just east of San Jacinto). A 46 m (151 ft) fissure, which may have been surface rupture of the San Jacinto Fault, ran under a house that was severely damaged near Hemet. The effects of the early morning earthquake were severe. Not all the buildings in San Jacinto were completely destroyed by the thirty seconds of shaking, but most of the brick buildings' second floors were heavily damaged. ( )
- 1918 San Jacinto earthquake
On April 21, a magnitude 6.6 earthquake caused major damage in the same area as the 1899 San Jacinto earthquake, with several injuries and one death occurring there. Many of the buildings in San Jacinto's business district were of poor construction and all but one frame building and one concrete building collapsed though high quality structures did not experience serious damage. Minor damage to other buildings within 160 kilometers (99 mi) of San Jacinto also occurred. Roadways and irrigation canals also sustained damage and small sand blows were seen on a farm near San Jacinto. ( )
The earthquake occurred on a Sunday afternoon when most of the businesses in San Jacinto were closed and void of customers. According to a 1918 report by Sidney Townley, it was there in the business district that damage was the greatest, though Hemet was also severely damaged. The area was surveyed for three days about a week following the event, and Townley acknowledged that damage to chimneys, windows, and plaster walls occurred to buildings and structures within one hundred miles of San Jacinto, but not all locations with damage were surveyed. Some of the damage that was inspected included landslides, partially collapsed buildings, and damaged irrigation canals and roads. Ground cracks were observed near the banks of the San Jacinto river and sand blows were seen on a farm northwest of San Jacinto.
- 1923 North San Jacinto Fault earthquake
On July 23, a magnitude 6.0 earthquake damaged numerous buildings in the area surrounding San Bernardino and Redlands. The San Bernardino County Courthouse and Hall of Records was damaged beyond repair and was ultimately rebuilt. The large brick building housing the Patton State Hospital (also in San Bernardino) was closed then demolished because of the severe damage. No one was killed in the quake but two were critically injured. ( )
- 1937 Terwilliger Valley earthquake
On March 25, a magnitude 5.9 earthquake occurred in San Diego County. Damage was limited as a result of the epicenter being located away from population centers, but some damage to chimneys and windows was found in Anza, Hemet, and Palm Springs. The maximum felt intensity was MM VI. ( )
- 1942 Fish Creek Mountains earthquake
On October 21, a magnitude 6.0 event occurred near Borrego Valley. Landslides damaged a railroad bridge 19 kilometers (12 mi) north of Jacumba. Minor damage was also reported in several towns in the area and several aftershocks were felt. ( )
- 1954 Arroyo Salada earthquake
On March 19, an early morning magnitude 6.2 earthquake rattled much of southern California. Damage included broken water pipes, cracked walls and swimming pools, and broken windows. The earthquake was felt over a wide area including western Arizona and southwestern Nevada. Several aftershocks followed with the strongest occurring later the same day. The event occurred at 2 am local time and was described as two separate motions that took place within 10–15 seconds. Though the quake was reportedly felt in adjacent states the Kern County Sheriff stated that the rolling movements were not felt there. Kern County was the scene of the 1952 Kern County earthquake. ( )
- 1968 Borrego Mountain earthquake
On April 9, a magnitude 6.4 ML earthquake with a maximum perceived intensity of MM VII hit the extreme eastern San Diego County area and created a 31 km (19 mi) surface break along the Coyote Creek Fault. California State Route 78 was damaged with cracks near Ocotillo Wells and large boulders blocked the Montezuma-Borrego Springs Highway. Other rockslides occurred at Palm Canyon and Split Mountain in Anza-Borrego Desert State Park. One house was split apart in Ocotillo Wells with one bedroom becoming detached from the rest of the home. The mainshock was felt in Arizona and Nevada and the largest aftershock damaged a theater's walls in Calexico near the United States - Mexico border.
According to a report in the Los Angeles Times, this was the strongest earthquake to affect southern California since the Tehachapi earthquake fifteen years earlier. Taller buildings swayed in both Los Angeles and San Diego and power outages affected numerous areas, primarily in the cities of Imperial Valley. Power failures along with disruption to telephone service caused problems in the Hemet Valley area, and smaller power outages in Los Angeles and Orange Counties also occurred. A brick wall collapsed at a laundromat in Westmorland (in the El Centro Metropolitan Area) but no one was injured, and in the seaside neighborhoods of San Diego county several hundred windows were broken. Charles Richter, a Caltech seismologist, stated that the earthquake was centered near Ocotillo Wells about 120 mi (190 km) southeast of Los Angeles. The mainshock occurred at 6:28 pm and aftershocks continued through the evening, but were tapering off by 10 pm. ( )
- 1987 Superstition Hills and Elmore Ranch earthquakes
Two earthquakes in late November caused property damage totaling three million in Imperial County. The two events were separated by eleven hours and were located in the western Imperial Valley on the Superstition Hills Fault and a previously unknown fault. Damage in Westmoreland, Imperial, and El Centro consisted of collapsed chimneys, broken windows, and damaged highways. The Worthington Road bridge, at the New River, failed due to liquefaction and at the Desert Test Range Control Center, water tanks toppled into the building and other equipment crashed through a window. Activities were suspended there for several days due to the damage. The Southern California Irrigation District estimated damage to be $600,000 – $750,000. The initial shock produced a small amount of deformation in the canal's liner while the second main shock caused considerable damage to thousands of feet of canal lining in the northwest section of the valley.
Several foreshocks preceded the main shocks and a series of aftershocks included two in the range of magnitude five. On the Mexican side of the border, 50 injuries and two deaths were reported, and 44 were treated for their injuries in California. According to the spokesperson for the state of Baja California, a motor vehicle accident east of Mexicali that claimed the lives of a mother and her four year old son was blamed on the earthquake. Thomas H. Heaton, a USGS seismologist, stated that the faults in the area are difficult to track down because of the sediment deposited in the valley, which had been an intermittent drainage basin of Colorado River.
The Superstition Hills fault (SHF) lies between the Coyote Creek fault that ruptured during the 1968 event and the Imperial Fault that ruptured during the 1940 El Centro earthquake and the 1979 Imperial Valley earthquake. To the northeast are several cross faults that trend northeast. One of these faults ruptured during a large aftershock of the 1979 event and another slipped as the smaller of the two shocks during the November 1987 sequence. The first shock (on what became known as the Elmore Ranch fault) measured 6.2 Ms and the shock 11.4 hours later on the SHF measured 6.6 Ms. ( & )
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