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San Jacinto Fault Zone
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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 37 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.
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%.
The northernmost primary strand of the SJFZ is the Claremont strand (though subsidiary parallel strands exist). It spans a total of 75 km (47 mi), from its northern endpoint in Cajon Pass to its southern endpoint in the San Jacinto Valley. Within the San Bernardino Valley itself, the SJFZ approaches or intersects the right-lateral strike-slip San Andreas Fault to the north, the oblique normal Crafton Hills Fault Zone to the east, and the Cucamonga Thrust to the west. A series of moderate earthquakes affected this area in the 1890s, though it is uncertain how many of these occurred specifically on the SJFZ. The Claremont strand has not had a major earthquake in the instrumental period, but paleoseismology indicates that its last surface rupturing event occurred in the early 19th century, and that comparable earthquakes occur on average of every 160 to 220 years.
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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 37 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.
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%.
The northernmost primary strand of the SJFZ is the Claremont strand (though subsidiary parallel strands exist). It spans a total of 75 km (47 mi), from its northern endpoint in Cajon Pass to its southern endpoint in the San Jacinto Valley. Within the San Bernardino Valley itself, the SJFZ approaches or intersects the right-lateral strike-slip San Andreas Fault to the north, the oblique normal Crafton Hills Fault Zone to the east, and the Cucamonga Thrust to the west. A series of moderate earthquakes affected this area in the 1890s, though it is uncertain how many of these occurred specifically on the SJFZ. The Claremont strand has not had a major earthquake in the instrumental period, but paleoseismology indicates that its last surface rupturing event occurred in the early 19th century, and that comparable earthquakes occur on average of every 160 to 220 years.
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