1968 Borrego Mountain earthquake
1968 Borrego Mountain earthquake is located in California
Las Vegas
Las Vegas
Los Angeles
Los Angeles
San Diego
San Diego
Las Vegas
Las Vegas
1968 Borrego Mountain earthquake
UTC time1968-04-09 02:28:58
ISC event823631
USGS-ANSSComCat
Local dateApril 8, 1968
Local time6:28 p.m. PST
Magnitude6.6 Mw
Depth11.1 km (6.9 mi)
Epicenter33°10′48″N 116°06′11″W / 33.180°N 116.103°W / 33.180; -116.103
TypeStrike-slip
Areas affectedSouthern California
Total damageMinor
Max. intensityVII (Very strong)
CasualtiesNone

The 1968 Borrego Mountain earthquake occurred on April 8, at 18:28 PST in the geologically active Salton Trough of Southern California. The Salton Trough represents a pull-apart basin formed by movements along major faults. This region is dominated by major strike-slip faults one of them being the San Jacinto Fault which produced the 1968 earthquake. The mainshock's epicenter was near the unincorporated community of Ocotillo Wells in San Diego County. The moment magnitude (Mw ) 6.6 strike-slip earthquake struck with a focal depth of 11.1 km (6.9 mi). The zone of surface rupture was assigned a maximum Modified Mercalli intensity (MMI) of VII (Very strong).

Despite being the largest earthquake to strike California since the 1952 Kern County earthquake, structural damage was limited due to the region being sparsely populated. Damage mostly comprised fallen plaster from building facades and material losses. Rockfalls were widely reported in the epicenter region. There was also no deaths or injuries as a result Some minor damage also occurred in Arizona and Baja California. In the aftermath, many nearby faults displayed afterslip which became the subject of scientific interest.

Tectonic setting

1968 Borrego Mountain earthquake is located in Salton Trough
A simplified tectonic boundary map of the Salton Trough area. Red lines show major faults and arrows indicate fault motion.

The San Andreas Fault (SAF) is the main plate boundary that defines the margin between the Pacific and North American Plates in California. It is believed to have formed during the Oligocene. The fault has a length of 1,200 km (750 mi),[1] of which, it is visible for 1,005 km (624 mi) from the Salton Sea to Point Arena.[2] Divided into four distinct segments, it displays right-lateral strike-slip movement.[1] It accommodates 20–75 percent of plate motion between the Pacific and North American plates its segments.[3] The tectonic boundary in Southern California is complex—plate motion is accommodated by the SAF and a network of subparallel faults. The SAF terminates at the Salton Trough, a transtensional zone (pull-apart basin) that separates it from the Imperial Fault in the south.[4] The exact measurement of slip across faults in this zone is poorly understood.[1]

California Borderland

West of the Salton Trough is a largely offshore fault system that accommodates approximately 20 percent of the plate motion.[5] Faults such as the Rose Canyon Fault and the Newport-Inglewood Fault pose large risks to unprepared coastal communities such as San Diego and Los Angeles.[6]

San Jacinto Fault Zone

East of the California Borderland, the plate boundary is a complex zone of faults that run parallel to the SAF. The two main faults are the San Jacinto Fault Zone (SJFZ) and Elsinore Fault Zone.[2] The SJFZ is a complex, highly segmented, and overlapped fault zone that runs parallel to the San Andreas Fault,[7] but separated by the San Jacinto Mountains.[2] It is located on the eastern Salton Trough, and runs 290 km (180 mi) directly beneath the cities of San Bernardino, Colton, San Jacinto and Hemet.[2] Segments of the SJFZ are given names despite being one fault system.[8] These segments include the Coyote Creek (CCF), Superstition Hills (SHF), and Superstition Mountain (SMF) faults. The CCF is estimated to be 44 km (27 mi)-long and displays right-lateral strike-slip displacement.[8] Considered the most active fault in Southern California, 36 notable earthquakes have been associated with it since 1857. Between 1915 and 1954, five earthquakes of magnitude 6.0 or greater were damaging. Seismic activity on the SJFZ is greater than on the SAF.[2]

Eastern California Shear Zone

Further east, near the border with Nevada and Arizona, the Eastern California Shear Zone (ECSZ) takes up to 25 percent of the plate motion.[9] The ECSZ consists of north–west trending right-lateral faults in the Mojave Desert, and Walker Lane, which lies at the western margin of the Basin and Range Province.[3] Large earthquakes associated with the ECSZ occurred in 1872, 1992, 1999 and 2019.[10]

Earthquake

The San Jacinto Fault Zone was involved in the earthquak.

The only recorded foreshock had a magnitude of 3.7, occurring one minute before the mainshock. There was no foreshock activity recorded in the hours to weeks before the mainshock; seismic activity in the area was lower than usual in the four months before April 1968. The mainshock which measured 6.6 Mw ,[11] was the result of shallow strike-slip faulting which initiated from the hypocenter at 11.1 km (6.9 mi) depth. It ruptured bilaterally along the CCF and displayed an almost pure right-lateral focal mechanism. A focal mechanism analysis indicated the rupture plane had a northwest strike and dipped steeply (80°) to the south.[12]

Surface rupture

A 33 km (21 mi)-long surface rupture through Quaternary alluvium and lake bed sediments was revealed. At its northern extreme, the rupture sliced through the Palm Spring Formation. The rupture zone consisted of two northwest-oriented segments, separated by a 2 km (1.2 mi)-wide discontinuity.[13] Smaller, isolated ruptures were found 3 km (1.9 mi) away from the main trace.[14] A maximum horizontal offset of 38 cm (15 in) was measured along the northern rupture located 5 km (3.1 mi) northwest of Ocotillo Wells, at the foothills of Borrego Mountain.[15] Along the southern rupture, the maximum offset was 20 cm (7.9 in), measured 10 km (6.2 mi) southeast of Ocotillo Wells. Vertical offsets of up to 20 cm (7.9 in) were also recorded.[13]

There were also left-lateral displacements 1–2 km (0.62–1.24 mi) from Ocotillo Badlands north of Highway 78 and at the northern base of Borrego Mountain. Whether these left-lateral offsets were part of the rupture mechanism or environmental changes unrelated to tectonic processes could not be determined.[14]

Aftershocks

A one-year-long aftershock sequence followed; at least 135 aftershocks measuring 3.0 or greater was recorded.[12] Most aftershocks were located 2–3 km (1.2–1.9 mi) away[12] and subparallel to the northwest-southeast trending rupture. The mainshock epicenter was located in the middle of the aftershock zone. The concentration of aftershocks was greater southeast of the mainshock than to the northwest. These aftershocks were predominantly right-lateral strike-slip events. Several aftershocks had dip-slip focal mechanisms. There were aftershocks reported close to the SMF and SHF at the southeastern extremity of the rupture.[16] On the day of the mainshock, a magnitude 5.2 aftershock occurred at 19:03. Forty-five minutes later, a magnitude 4.7 aftershock caused minor damage in Calexico.[17] Nearly a year later, a 5.8 aftershock was recorded. This shock had its own sequence of aftershocks.[16][18]

Intensity

Modified Mercalli intensities in selected locations
MMILocations
VII (Very strong) Ocotillo Wells, Borrego Mountain, Brea, Chula Vista, Desert Center
VI (Strong) Yuma, Mexicali, Homeland, Holtville
V (Moderate) Huntington Beach, North Hollywood, Beverly Hills, Ehrenberg, Quartzsite
IV (Light) Glendale, Tehachapi, Anaheim, Las Vegas
III (Weak)–II (Weak) Fresno, Yosemite Valley, Santa Barbara, Goleta
Coffman & Cloud 1984, pp. 33–42

The maximum peak ground acceleration (pga) recorded by a seismometer at El Centro was 0.14 g. A maximum MMI of VII (Very strong) was assigned in the Borrego Mountain–Ocotillo Wells area (northeastern San Diego County) where surface rupturing occurred. Based on the study of ground effects alone, the MMI may be as high as IX (Violent). Severe damage was restricted to a 2,331 km2 (900 sq mi) area, but the earthquake was felt for 160,000 km2 (62,000 sq mi).[19] In the meizoseismal area, small offsets occurred along the Coyote Creek Fault and Highway 78 near Ocotillo Wells cracked. At Anza-Borrego Desert State Park, large boulders toppled. At Split Mountain, falling rocks damaged many parked vehicles. Large boulders also blocked the Montezuma-Borrego Highway.[17] Rockfalls, slumps and liquefaction took place in response to the strong ground motion.[20]

Heavier damage included cracked and fragmented concrete bridge piers. There was minor damage in Ocotillo Wells; the only documented serious damage was to a house that had walls split apart and bedroom detached from the main structure.[21][20] A storage tank beside the house spilled 3,600 gallons of water over the porch, toppling its posts. Ground cracks appeared at the airport and roads. Residents were temporarily without water because of a damaged community well pump. A motel 4.8 km (3.0 mi) west of Ocotillo Wells sustained broken water and sewerage pipes, cracked tiles and its lower floor was flooded by water from the swimming pool. Large transformers 5.1 km (3.2 mi) from Ocotillo Wells were displaced, snapping anchor bolts and X-bracings.[17]

A strong ground motion map show shaking intensity.

A long rolling motion lasting up to 30 seconds was described. Swimming pools sloshed about for 10 minutes. Plaster fell from the walls and ceilings on the second level of the Balboa Hotel in El Centro. In Calexico, the ceiling at a Safeway supermarket partially collapsed. The upper brick wall of a laundromat in Westmorland collapsed while another building was cracked.[19]

MMI VI (Strong) shaking frightened many residents.[17] Damage was minimal due to the area's sparse urban planning. Shifting furnitures; falling objects; rocking vehicles, trees and bushes; and minor cracks were reported. Minor rockslides occurred, including some at Barrett Dam. At Borrego Springs, minor cracks appeared in a church façade. Cracks also appeared in the ground and on windows. Furniture moved several inches while swimming pools sloshed. Goods and items fell off shelves in stores, forcing some to shut down. In Mecca, several concrete pipelines ruptured and a truck nearly overturned. In Anza concrete floorings cracked and plaster fell from buildings. In a public library at Imperial, 7,500 books fell from shelves. Cracked concrete pavements and driveways occurred in Yuma and Horn, Arizona.[17]

At San Diego, a small crack appeared in a 2.74 m (9 ft 0 in) concrete retaining wall. Roughly 3.2 km (2.0 mi) north of downtown San Diego, furnitures were displaced. Broken windows, severed powerlines, and sloshing of swimming pools were widely reported. Cracks appeared along Sunset Cliffs Boulevard and plaster detached from building. At Mexicali, Baja California, windows broke and lights went out at a sheriff office. Landslides occurred at Sage. Grocery stores in Riverside suffered huge losses from fallen bottled products.[17]

The RMS Queen Mary, which was docked at Long Beach rocked for five minutes.[22] In Los Angeles, two structures built before revised earthquake codes suffered widening or reopening of plaster cracks formed by the 1933 and 1952 earthquakes.[23] Plaster fell off some buildings in the city.[22]

Post-earthquake slips

After the earthquake, the fault displayed a phenomenon known as aseismic creep, observed only along the central and southernmost section of the rupture. It was discovered on June 9 by the manager of a motel at Ocotillo Wells. Aseismic creep increased the total horizontal displacement from 18 cm (7.1 in) to 25–30 cm (9.8–11.8 in), and vertical displacement from 10 cm (3.9 in) to 15–23 cm (5.9–9.1 in), two months after the mainshock.[24]

While the central section of the CCF rupture experienced aseismic creep, there was no movement along the southernmost rupture until from January 1969 to December 1970. There was no feasible way of measuring these new offsets as tire tracks, used for measuring, had disappeared. It is estimated that the post-earthquake slip produced 3–6 cm (1.2–2.4 in) of additional displacement, in addition to the 8 cm (3.1 in) during the earthquake.[24] After a pair of earthquakes in 1987, new surface ruptures with 1.5 cm (0.59 in) of displacement were observed. These new surface ruptures measured 3 km (1.9 mi).[25]

Triggered slips

A remarkable feature was slip along faults located far from the epicenter area. The SAF, Imperial, and Superstition Hills faults displayed this phenomenon. Field observations revealed 1–2.5 cm (0.39–0.98 in) of right-lateral displacement occurred along these faults. Triggered slips were not observed on other prominent faults such as segments of the SJFZ north of the CCF, the SMF that lies parallel to the SHF, and the Elsinore Fault Zone. Geologists postulated the slip was shaking-induced, ruling out stress transfer as a cause.[15] Slip continued until 1972.[23]

Evidence of movement along the Imperial Fault (70 km (43 mi) from epicenter) was discovered on April 13 on Interstate 8 when cracks appeared. However, the cracks were not well determined as there were already cracks to the road from an earthquake in March 1966 (the magnitude 3.6 earthquake is the smallest earthquake associated with a surface rupture[26]). The Imperial Fault was the first to be discovered creeping which prompted checks on other faults. Creep occurred for 22 km (14 mi), although its actual length is unknown as dunes and urban developments obstructed any possible rupture trace. It produced an estimated 0.3 cm (0.12 in) of right-lateral slip.[15]

Along the SHF, 2.5 cm (0.98 in) of displacement was measured at Imler Road (45 km (28 mi) from epicenter). A 23 km (14 mi) section of the SHF moved.[15] The SHF also produced creep after the 1987 earthquakes.[1] Movement along the Southern California segment of the SAF (50 km (31 mi) northeast of the epicenter) was documented on April 24. Right-lateral displacement of 1.3 cm (0.51 in) and vertical scarps as high as 50 cm (20 in) were measured. Slip occurred for 30 km (19 mi).[15] The last major earthquake on this section of the SAF occurred in 1680, with an estimated magnitude of 7.8.[27]

See also

References

  1. 1 2 3 4 Van Zandt, Afton Jade (March 19, 2009). Southern Coyote Creek fault to Superstition Hills fault: New insight to the San Jacinto fault system (Masters of Science thesis). San Diego State University.
  2. 1 2 3 4 5 Singer, Eugene (1998), Geology of California's Imperial Valley: A Monograph by Eugene Singer, archived from the original on March 24, 2011, retrieved December 13, 2020
  3. 1 2 Liu, Mian; Wang, Hui; Li, Qiang (July 7, 2010). "Inception of the eastern California shear zone and evolution of the Pacific‐North American plate boundary: From kinematicsto geodynamics". Journal of Geophysical Research: Atmospheres. 115 (B7). Bibcode:2010JGRB..115.7401L. doi:10.1029/2009JB007055.
  4. ↑ Ye, Jiyang; Liu, Mian; Wang, Hui (March 16, 2015). "A numerical study of strike-slip bend formation with application to the Salton Sea pull-apart basin". Geophysical Research Letters. 42 (5): 1368–1374. Bibcode:2015GeoRL..42.1368Y. doi:10.1002/2015GL063180. S2CID 15569067.
  5. ↑ Maloney, Jillian; Legg, Mark; Nicholson, Craig; Rockwell, Thomas. "White Paper: The California continental borderland" (PDF). Southern California Earthquake Center. Retrieved August 6, 2022.
  6. ↑ Monroe, Robert (March 7, 2017). "Fault System off San Diego, Orange, Los Angeles Counties Could Produce a Magnitude 7.3 Quake". UC San Diego News Center. Retrieved August 6, 2022.
  7. ↑ Dorsey, Rebecca J. (2002). "Stratigraphic record of Pleistocene initiation and slip on the Coyote Creek fault, lower Coyote Creek, southern California". Contributions to Crustal Evolution of the Southwestern United States. Vol. 365. pp. 251–269. doi:10.1130/0-8137-2365-5.251. ISBN 9780813723655. {{cite book}}: |journal= ignored (help)
  8. 1 2 Treiman, J.A.; Lundberg, M.; compilers (1999). "Fault number 125d, San Jacinto fault, Coyote Creek section, in Quaternary fault and fold database of the United States". United States Geological Survey. Retrieved August 19, 2022.
  9. ↑ Liu, Mian; Wang, Hui; Li, Qingsong (July 7, 2010). "Inception of the eastern California shear zone and evolution of the Pacific-North American plate boundary: From kinematics to geodynamics". Journal of Geophysical Research. 115 (B7). Bibcode:2010JGRB..115.7401L. doi:10.1029/2009JB007055. Retrieved August 6, 2022.
  10. ↑ Chen, Kejie; Avouac, Jean-Philippe; Aati, Saif; Milliner, Chris; Zheng, Fu; Shi, Chuang (January 7, 2020). "Cascading and pulse-like ruptures during the 2019 Ridgecrest earthquakes in the Eastern California Shear Zone". Nature Communications. 11 (22): 22. Bibcode:2020NatCo..11...22C. doi:10.1038/s41467-019-13750-w. PMC 6946662. PMID 31911581. Retrieved August 6, 2022.
  11. ↑ ISC (June 27, 2022), ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009), Version 9.1, International Seismological Centre
  12. 1 2 3 Allen, Clarence R.; Nordquist, John M. (1972). Foreshock, Main Shock, and Larger Aftershocks of the Borrego Mountain Earthquake. In: The Borrego Mountain Earthquake of April 9, 1968 (Report). U.S. Geological Survey Professional Paper (787 ed.). Washington, DC: United States Government Printing Office. pp. 16–23. doi:10.3133/pp787.
  13. 1 2 Allen, C. R.; Grantz, A.; Brune, J. N.; Clark, M. M.; Sharp, R. V.; Theodore, T. G.; Wolfe, E. W.; Wyss, M. (June 1, 1968). "The Borrego Mountain, California, earthquake of 9 April 1968: A preliminary report" (PDF). Bulletin of the Seismological Society of America. 58 (3): 1183–1186. Bibcode:1968BuSSA..58.1183A. doi:10.1785/BSSA0580031183.
  14. 1 2 Clark, Malcolm M. (1972). "Surface rupture along the Coyote Creek fault. In: The Borrego Mountain Earthquake of April 9, 1968". Geological Survey Professional Paper (787 ed.). Washington, DC: United States Government Printing Office: 55–86. doi:10.3133/pp787.
  15. 1 2 3 4 5 Allen, Clarence R. (1972). Displacements on the Imperial, Superstition Hills, and San Andreas faults triggered by the Borrego Mountain earthquake. In: The Borrego Mountain Earthquake of April 9, 1968 (Report). U.S. Geological Survey Professional Paper (787 ed.). Washington, DC: United States Government Printing Office. pp. 87–104. doi:10.3133/pp787.
  16. 1 2 Hamilton, R. M. (1972). Aftershocks of the Borrego Mountain Earthquake from April 12 to June 12, 1968. In: The Borrego Mountain Earthquake of April 9, 1968 (Report). U.S. Geological Survey Professional Paper (787 ed.). Washington, DC: United States Government Printing Office. pp. 31–54. doi:10.3133/pp787.
  17. 1 2 3 4 5 6 Coffman, Jerry L.; Cloud, William K. (1984), United States Earthquakes, 1968 (PDF), Open-File Report 84-968, United States Geological Survey, pp. 33–42
  18. ↑ Wayne Thatcher; Robert M. Hamilton (April 1, 1973). "Aftershocks and source characteristics of the 1969 Coyote Mountain earthquake, San Jacinto Fault zone, California*". Bulletin of the Seismological Society of America. 63 (2): 647–661. Bibcode:1973BuSSA..63..647T. doi:10.1785/BSSA0630020647. S2CID 132696468.
  19. 1 2 National Oceanic and Atmospheric Administration (1972). Intensity distribution and field effects, strong-motion seismograph records, and response spectra. In: The Borrego Mountain Earthquake of April 9, 1968 (Report). U.S. Geological Survey Professional Paper (787 ed.). Washington, DC: United States Government Printing Office. pp. 141–157. doi:10.3133/pp787.
  20. 1 2 Stover, Carl W.; Coffman, Jerry L. (1993). Seismicity of the United States, 1568-1989 (Revised). Professional Paper (1527 ed.). United States Government Printing Office, Washington: U.S. Geological Survey Professional Paper. doi:10.3133/pp1527. hdl:2027/uc1.31822016474439.
  21. ↑ Castle, R. O.; Youd, T. L. (1972). Engineering geology. In: The Borrego Mountain Earthquake of April 9, 1968 (Report). U.S. Geological Survey Professional Paper. Vol. 787. Washington, DC: United States Geological Survey. pp. 158–174. doi:10.3133/pp787.
  22. 1 2 "Borrego Mountain Earthquake". Southern California Earthquake Data Center. California Institute of Technology. Retrieved December 13, 2020.
  23. 1 2 U.S. Geological Survey 1972.
  24. 1 2 Burford, R. O. (1972). Continued slip on the Coyote Creek fault after the Borrego Mountain earthquake (Report). U.S. Geological Survey Professional Paper. Vol. 787. Washington, DC: United States Government Printing Office. pp. 105–111. doi:10.3133/pp787.
  25. ↑ Hudnut, K. W.; Clark, M. M. (April 1, 1989). "New slip along parts of the 1968 Coyote Creek fault rupture, California". Bulletin of the Seismological Society of America. 79: 451–465.
  26. ↑ Brune, James N.; Allen, Clarence R. (June 2, 1967). "A low-stress-drop, low-magnitude earthquake with surface faulting: The Imperial, California, earthquake of March 4, 1966" (PDF). Bulletin of the Seismological Society of America. 57 (3): 501–514. Bibcode:1967BuSSA..57..501B. doi:10.1785/BSSA0570030501.
  27. ↑ "California earthquake history". MySafeLA. Archived from the original on January 18, 2021. Retrieved December 13, 2020.

Sources

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