1968 Borrego Mountain earthquake
  Las Vegas Los Angeles San Diego Las Vegas  | |
| UTC time | 1968-04-09 02:28:58 |
|---|---|
| ISC event | 823631 |
| USGS-ANSS | ComCat |
| Local date | April 8, 1968 |
| Local time | 6:29 p.m. PST |
| Magnitude | 6.6 Mw 7.0 Ms |
| Depth | 10.0 km 20.0 km |
| Epicenter | 33°10′48″N 116°06′11″W / 33.180°N 116.103°WCoordinates: 33°10′48″N 116°06′11″W / 33.180°N 116.103°W |
| Type | Strike-slip |
| Areas affected | Southern California |
| Total damage | Minor |
| Max. intensity | VII (Very strong)[1] |
| Landslides | Yes |
| Casualties | None |
The 1968 Borrego Mountain earthquake occurred on the evening of April 8, near the unincorporated community of Ocotillo Wells in San Diego County. The moment magnitude 6.6 (7.0 on the surface wave magnitude scale) earthquake was assigned a maximum intensity of VII (Very strong) on the Mercalli intensity scale,[2] causing some damage in the Imperial Valley, but no injuries or deaths. Shaking from this earthquake was widely felt, even in Nevada, and Arizona. It was the largest earthquake in Southern California since the 1952 Kern County earthquake 16 years prior.[3]
Although the earthquake did not have any implications for Southern California, it was a remarkable event for the scientific community, displaying classic afterslip characteristics. It is the first documented earthquake where faults at considerable distances from the event source showed movements. A similar instance occurred along the seismically quiet Garlock Fault after the Kern County earthquake and again after the Ridgecrest earthquake sequence.[4]
Tectonic setting[edit]
The Salton Trough is a pull-apart basin forming due to offsets between the numerous strike-slip faults along its edges. It is a component of the much bigger San Andreas Fault System, joining the San Andreas Fault with the Imperial Fault via the Brawley Seismic Zone. The San Andreas Fault is the main plate boundary that defines the margin between the Pacific and North American Plates in California. However, the plate boundary is slightly more complex; rather than a single fault structure that makes up the boundary, the region is straddled and crisscrossed with numerous shorter faults to accommodate the movement of these two plates. One of these faults is the San Jacinto Fault Zone; a complex, highly segmented, and overlapped 210 km-long fault zone that runs west of the Salton Sea, parallel to that is the San Andreas Fault.[5] It is separated from the San Andreas Fault by the San Jacinto Mountains to its east. It cuts under cities including Hemet, Colton, and San Bernardino along the way, before joining the San Andreas Fault at Devore. Because the fault is so segmented, some branches are given names, although they are considered part of the system of faults. Considered the most active fault in all of Southern California, it has produced at least eleven earthquakes of Mw 6.0 or greater since the late 19th century. Possibly its most destructive and largest earthquake took place in 1812, when 40 people were killed at Mission San Juan Capistrano when the fault produced an Mw 7.0-7.5. The most recent earthquake was the doublet earthquake of 1987, nearly a month after the Whittier Narrows earthquake. The 1987 pair of temblors inflicted heavy damages to Westmorland, and indirectly killed two people outside Mexicali.[6]
Earthquake[edit]
The Mw 6.6 or Ms 7.0 earthquake ruptured bilaterally on a section of the 80 km long Coyote Creek Fault with an almost pure right-lateral (dextral) strike-slip mechanism. The rupture zone and is confined to within California, not crossing into Baja California. The 33-km-long surface rupture was preserved well due to the sparse vegetation, low rainfall, and flat topography. Two sharp discontinuities in the rupture divide the breakage into three sections (north, central, and south), and smaller, isolated ruptures were found 3 km away from the main trace. The surface ruptures consist of en echelon and parallel breaks rather than one consistent trace.[7] The rupture width ranged from 1 meter to as much as 100 meters across throughout its length. Since the entire rupture did not cross any built structures, determining the displacements made by the earthquake was quite difficult. Geologists used channels and tire tracks crossing the fault before the earthquake to measure the horizontal shifts instead.[7] However, strong winds blew sand which buried many parts of the rupture and filled in cracks. Additionally, many vehicles visiting the area crossed and destroyed important evidence to map and determine the lateral movement. A maximum offset of 38 cm was measured at the foothills of Borrego Mountain.[8]
Unusually, there were left-lateral displacements 1 to 2 km from Ocotillo Badlands north of California 78 and at the northern base of Borrego Mountain.[7] The fractures at the base of Borrego Mountain had larger vertical components in then left-lateral. Whether the left-lateral offsets were part of the rupture mechanism or environmental changes unrelated to tectonic processes could not be confirmed as these features faded.[7]
Intensity[edit]
| Modified Mercalli intensities in selected locations | ||
| MMI | Locations | |
|---|---|---|
| VII (Very strong) | Ocotillo Wells, Borrego Mountain | |
| VI (Strong) | Yuma, Mexicali, Homeland, Holtville | |
| V (Moderate) | Huntington Beach, North Hollywood, Beverly Hills, Topock | |
| 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 intensity VII was assigned in the Borrego Mountain and Ocotillo Wells area in San Diego County, although it is thought to be as high as IX,[2] based solely on the ground effects of the earthquake. Within this seismic intensity zone, a small sinistral offset was reported along the Coyote Creek Fault.[9]
Strong shaking (VI) was felt in Anza, Riverside County where it cracked concrete floorings and knocked off plasters from buildings. Many trees, vehicles, and bushes rocked notably during the quake. The same intensity was felt by everyone in locations including Arcadia, Calexico and Brawley. Damage was generally minimal due to the sparse population density when the quake struck. The maximum intensity recorded across the international border was in Mexicali, Baja California where it was assigned VI. In Arizona, the communities of Yuma and Horn were assigned level VI shaking, cracking concrete pavements and driveways.[2]
Moderate to weak shaking (V or lower) was felt in the Los Angeles area. Intensity V was felt in Ehrenberg, Quartzsite and Topock in the state of Arizona. In Las Vegas, the quake resulted in light (IV) shaking.[2]
Foreshock and aftershocks[edit]
The only recorded foreshock was a magnitude 3.7 quake that occurred one minute before the mainshock event. There was no foreshock activity recorded in the hours to weeks before either. In fact, seismic activity in the area was lower than usual in the four months before April.[2]
After the mainshock, a long aftershock sequence lasting up to a year followed. At least 135 aftershocks measuring 3.0 or greater had been recorded a year after the mainshock. Most of the afterhsocks were located near the northwest-southeast trending rupture area, but subparallel to the mapped faults. In the zone of aftershocks, the mainshock epicenter was located in the middle, suggesting a bilateral rupture characteristic. A greater concentration of aftershocks was located southeast of the mainshock than to the northwest. The dominant focal mechanism of the aftershocks was strike-slip in a right-lateral sense. A few dip-slip sense of faulting was also recorded in several aftershocks.[2]
A strong 5.2 aftershock was recorded over an hour after the mainshock. It was centered in San Diego County, west of the border with Imperial County. The aftershock did not result in any damage and was felt with a maximum intensity III (Weak).[10] Near one year later, another 5.8 aftershock was recorded. This shock had its own sequence of aftershocks and was felt VII (Very strong).[11][2]
Impact[edit]
Damage was surprisingly light and confined to the Imperial Valley for an earthquake of this size. California State Route 78 which lay alongside Ocotillo Wells suffered some cracks as a result. Rockfalls were triggered at the Anza-Borrego Desert State Park, as reported by park rangers. This damaged many vehicles belonging to campers at the state park.[9] Rockfalls, slumps and liquefaction took place as a result of the violent ground motion.[1][12]
Cracked and fragmented concrete bridge piers were some of the more serious damage associated with the earthquake. Ocotillo Wells, the closest community to the epicenter, sustained minimal damage. One house, however, had its walls split apart and had the bedroom detached from the remaining structure.[12][13] California 78 near the community suffered cracks, and rockslides blocked off the Montezuma-Borrego Highway.
At Borrego Springs, some minor cracks appeared on the facade of a church. Cracks also appeared in the ground and windows. Furniture moved several inches while swimming pools sloshed. Many stores reported goods and items falling off their shelves, forcing some to shut down. Several concrete pipelines ruptured and a truck nearly overturned in Mecca.[2]
Power lines in San Diego County were severed because of the earthquake while plasters fell off buildings in Los Angeles.[3] The shaking also caused the RMS Queen Mary, which was dry docked at Long Beach to rock for five minutes.[3] In Los Angeles, two structures built before the latest earthquake codes sustained minor damage such as widening or reopening of plaster cracks formed by the 1933 and 1952 earthquakes.[2]
Post-earthquake slips[edit]
Even after the earthquake, the fault displayed a phenomenon known as aseismic creeping, observed only along the central and southernmost section of the rupture. The central section was found creeping several weeks after the earthquake in June of 1968. The creep increased the total horizontal displacement along with the central breakage from 18 cm to 25-30 cm, while vertical displacement increased from 10 cm to 15-23 cm two months after the shock. These were possible documentation of new fractures forming. The new ruptures were discovered on 9 June 1968 by the manager of a motel at Ocotillo Wells who spotted them on a hill, convinced that they weren't present at the time of the earthquake.[14]
While the central rupture was creeping, there were no signs of movement along the southernmost rupture until January 1969, continuing through into December 1970. There was no feasible way of measuring the offsets made by the creep as tracks; the only evidence for measuring the April 1968 displacements had disappeared. It is estimated that the post-quake movements have offset the ground by a further 3-6 cm from the 8 cm during the earthquake.
A pair of earthquakes on the Superstition Hills Fault in 1987 would cause the rupture zone to creep for another 1.5 cm along a 3 km section.[15]
Triggered slips[edit]
A remarkable feature was the occurrence of slip along distant faults and away from the aftershock zone. Several faults displayed this characteristic including the San Andreas, Imperial, and Superstition Hills faults. Movements were first discovered on the Imperial Fault, which prompted checks on other nearby faults. Triggered slips were not observed on other prominent faults such as the San Jacinto Faults north of the Coyote Creek Fault, Superstition Mountains Fault that lies parallel to the Superstition Hills Fault, and the Elsinore Fault Zone.[8]
Imperial Fault[edit]
Evidence of movement along the Imperial Fault was discovered four days after the earthquake on Interstate 8 (I-8) when cracks appeared. The estimated length of creep along this fault is between 22 and 30 km although the actual length was never determined as dunes and developments obstructed any possible rupture trace. The Imperial Fault is estimated to have slipped by 0.8 cm during the earthquake. However, the cracks were not well determined as there were already cracks to the road from an earthquake in March of 1966 (the magnitude 3.6 earthquake is the smallest earthquake associated with a surface rupture[16]).[8]
The Imperial Fault would slip again in 1971, six days after a magnitude 5.0 earthquake on the Superstition Hills Fault.[17] About 1.4 cm of prominent horizontal offset was seen on I-8.
Superstition Hills Fault[edit]
Two and a half centimeters of displacement were measured at Imler Road, crossing the Superstition Hills Fault, which had moved for 23 km. This fault is part of the San Jacinto Fault Zone, together with the Coyote Creek Fault.[8]
San Andreas Fault[edit]
Movement along the Mojave segment of the San Andreas was noted on April 24. This section has not seen any major earthquake since an ~Mw 7.7−7.8 in 1680 and possibly the 1812 earthquakes which happened on December 8 and December 21, respectively.[18] Right-lateral displacement of 1.3 cm was measured, together with many scarps as high as 50 cm. Slippage traced for about 30 km.[8]
See also[edit]
- List of earthquakes in 1968
- List of earthquakes in California
- List of earthquakes in the United States
References[edit]
- ^ a b Carl W. Stover and Jerry L. Coffman (1993). Seismicity of the United States, 1568-1989 (Revised) (1527 ed.). United States Government Printing Office, Washington: U.S. Geological Survey Professional Paper.
- ^ a b c d e f g h i U.S. Geological Survey (1972). "The Borrego Mountain earthquake of April 9, 1968". Professional Paper. Professional Paper. U.S. Govt. Print. Off. 787. doi:10.3133/pp787. Retrieved 7 June 2021.
- ^ a b c "Borrego Mountain Earthquake". Southern California Earthquake Data Center. Retrieved 13 Dec 2020.
- ^ Gajanan, Mahita. "Scientists: Movement Detected Along California Fault That Could Cause an 8-Magnitude Earthquake". Time. Retrieved 6 June 2021.
- ^ Dorsey, R.J. "San Jacinto Fault Zone in Southern California". Quaternary to Recent Basin Development and Neotectonics of the Central San Jacinto Fault Zone, Southern California. Retrieved 13 Dec 2020.
- ^ Singer, Eugene. "Geology of California's Imperial Valley". sci.sdsu.edu. Retrieved 13 December 2020.
- ^ a b c d Clark, Malcolm M. (1972). "Surface rupture along the Coyote Creek fault". Geological Survey Professional Paper. 787: 55–86. doi:10.3133/pp787.
- ^ a b c d e Allen, Clarence R. (1972). "Displacements on the Imperial, Superstition Hills, and San Andreas faults triggered by the Borrego Mountain earthquake". Geological Survey Professional Paper. 787: 87–104.
- ^ a b Coffman, Jerry L.; Cloud, William K. (1984), United States Earthquakes, 1968 (PDF), Open-File Report 84-968, United States Geological Survey, pp. 33–42
- ^ "M 5.2 - 21km NW of Ocotillo, CA". earthquake.usgs.gov. U.S. Geological Survey. Retrieved 15 January 2022.
- ^ Wayne Thatcher; Robert M. Hamilton (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. doi:10.1785/BSSA0630020647.
- ^ a b R. O. Castle and T. L. Youd (1972). "ENGINEERING GEOLOGY". Geological Survey Professional Paper. 787: 158–174.
- ^ Carl W Stover, Jerry L Coffman (1993). "Seismicity of the United States, 1568-1989 (revised)".
- ^ Burford, R. O. (1972). "Continued slip on the Coyote Creek fault after the Borrego Mountain earthquake". Geological Survey Professional Paper. 787: 105–111.
- ^ K. W. Hudnut, M. M. Clark (1989). "New slip along parts of the 1968 Coyote Creek fault rupture, California". Bulletin of the Seismological Society of America. 79: 451–465.
- ^ JAMES N. BRUNE AND CLARENCE R. ALLEN (June 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: 501–514 – via Caltech.
- ^ "M 5.0 - 18km WSW of Westmorland, CA". USGS-ANSS. Retrieved 13 Dec 2020.
- ^ "CALIFORNIA EARTHQUAKE HISTORY". MySafeLA. Retrieved 13 Dec 2020.
External links[edit]
- The International Seismological Centre has a bibliography and/or authoritative data for this event.
- Borrego Mountain Earthquake – Southern California Earthquake Data Center