1979 Coyote Lake earthquake

Last updated

1979 Coyote Lake earthquake
Relief map of California.png
Green pog.svg
Reno
Green pog.svg
Healdsburg
Green pog.svg
Twain Harte
Green pog.svg
Visalia
Green pog.svg
Santa Barbara
Bullseye1.png
UTC  time1979-08-06 17:05:24
ISC  event 659922
USGS-ANSS ComCat
Local dateAugust 6, 1979 (1979-08-06)
Local time10:05:24
Magnitude5.7 Mw [1]
Depth10 km (6.2 mi) [1]
Epicenter 37°06′N121°36′W / 37.1°N 121.6°W / 37.1; -121.6 [1]
Fault Calaveras Fault
Type Strike-slip [2]
Areas affected South Bay
Northern California
United States
Total damage$500,000 [3]
Max. intensity MMI VII (Very strong) [3]
Peak acceleration0.44 g [4]
Casualties16 injured [3]

The 1979 Coyote Lake earthquake occurred at 10:05:24 local time on August 6 with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII (Very strong). The shock occurred on the Calaveras Fault near Coyote Lake in Santa Clara County, California and resulted in a number of injuries, including some that required hospitalization. Most of the $500,000 in damage that was caused was non-structural, but several businesses were closed for repairs. Data from numerous strong motion instruments was used to determine the type, depth, and extent of slip. A non-destructive aftershock sequence that lasted throughout the remainder of the month was of interest to seismologists, especially with regard to fault creep, and following the event local governments evaluated their response to the incident.

Contents

Tectonic setting

The San Andreas Fault system (SAF) is a network of right-lateral strike-slip faults that form a portion of a complex and diffuse plate boundary. The faults span on and off shore along the California portion of the Pacific Rim, and in the area near San Francisco Bay, the extent of the various fault strands are limited to about 80 kilometers (50 mi) wide from east to west. This system of faults terminates in the north at the Mendocino Triple Junction where the north-northwest trending SAF meets the east trending Mendocino Fracture Zone. It terminates in the south in a more gradual fashion at the Salton Sea, where displacement transitions to a series of transform faults and spreading centers along the Gulf of California Rift Zone. [5]

Several strands of the SAF in the eastern region of the San Francisco Bay Area are the Hayward–Rodgers Creek and Calaveras Faults. The Hayward Fault exhibits fault creep, but it also has potential for large earthquakes, like the 1868 Hayward earthquake that occurred on its southern segment. The northern Calaveras Fault meets the Hayward Fault near the Calaveras Reservoir and can also produce large earthquakes. Except for a large shock that occurred in 1911, the central and southern segments might only produce smaller events and fault creep. [6]

Earthquake

The earthquake occurred on the Calaveras Fault near Coyote Lake in Santa Clara County. Although the Hayward Fault Zone is also nearby, making this an area of regular seismic activity, no observable foreshocks occurred within the preceding three months. The mainshock was felt throughout the San Francisco Bay Area, but damage was mainly limited to the nearby towns of Gilroy and Hollister. A large number of strong motion stations recorded the event, including an array of units along the rupture zone, and instruments at the Berkeley Seismological Laboratory. The two closest stations at Coyote Creek and Gilroy, as well as the Berkeley stations, were used to refine the overall fault length, slip, and depth of faulting. The records revealed that strike-slip motion occurred over 8.7 mi (14 km) on a vertical fault, and that the total amount of slip varied with depth, with more slip occurring in the shallower regions. [2] [7]

Damage

In Gilroy and Hollister, sixteen people were injured, and damage totaled $500,000. Chimneys fell (especially on older homes in the downtown area of Gilroy) and glass was broken, but in Gilroy, structural damage afflicted five buildings. A wall was cracked at city hall, and a court room ceiling collapsed. Damaged structural components at a Ford's Department Store forced its closure. In Hollister, a J. C. Penney had a hole and cracks in its ceiling and a parapet collapsed at a law office. At Casa de Fruta, a service station sustained structural damage, as did a fire station at Pacheco Pass. [3]

USGS ShakeMap for the event USGS Shakemap - 1979 Coyote Lake earthquake.jpg
USGS ShakeMap for the event

An early estimate by the Small Business Administration put total damage in Gilroy at twice the amount of what was seen in Hollister. Ten people were brought to Hazel Hawkins Memorial Hospital in Hollister for treatment of lacerations, a cardiac problem, and anxiety. In Gilroy, the Wheeler Hospital saw six similar cases and a patient with a fractured hip. Most of those that sought care (including four that were transported by ambulance) were treated and released, but six were admitted. [8]

Intensity

At the most extreme points of its perceptibility, it was felt with intensity II or III (Weak) at Healdsburg in the north and Santa Barbara in the south, Visalia in the central valley and Twain Harte in the Sierra Nevada foothills. It was also listed as having been felt (with no specific intensity level given) by residents in high-rise buildings in Reno, Nevada. Closer to the epicenter, several instances of damage were aligned with the highest observed intensity. The structural effects to the gas station's walls and roof at Casa de Fruta, the beam damage on the second floor of the Ford's department store in Gilroy, and the caved-in roof from the fallen parapet at the law office in Hollister were all considered by the United States Geological Survey to be consistent with intensity VII (Very strong) shaking. [4] [7]

Strong motion

The event was captured on seismographs at distances of up to 71 miles (114 km), including Richmond, with the unit 56 ft (17 m) "down hole" in bay mud. Other underground instruments on the BART Transbay Tube, as well as the Richmond site, showed accelerations that were very low. In San Juan Bautista, the U.S. Route 101/State Route 156 overpass saw peak acceleration of 0.12 g on the ground and 0.29 g on the structure. Instruments on the gymnasium roof diaphragm at the campus of West Valley College in Saratoga provided records of interest. Of a number of dams that had instruments installed, the San Luis Dam at San Luis Reservoir had the strongest response. The highest acceleration of 0.44 g was seen at the San Ysidro School in Gilroy. [4] [9]

Aftershocks

The United States Geological Survey operated a network of seismograph stations in the region where the shock occurred since 1969. A survey of the aftershock activity used data from these stations, along with a custom crustal velocity model, to narrow epicenter locations to within several tens of meters. The study indicated that the mainshock and the aftershocks were aligned with the strike and dip of the Calaveras Fault in that area and were classified into three distinct groups. The east-dipping northeastern group, a diffuse middle group, and a shallow and nearly vertical southwestern group showed variations of slip, especially in the 18 months after the mainshock, when fault creep was significantly higher in the northeastern and middle groups. [10]

Response

While none of the affected counties or cities declared a state of emergency following the event, the Small Business Administration approved a request by the Office of Emergency Services for a disaster declaration in late September. This formality paved the way for low interest loans for commercial or residential properties that suffered damage, but only about 50 claims were expected. Local authorities had trained for disasters on a regular basis, and the response to the light damage was considered smooth, though some officials sought room for improvement. Discussions followed the event, with local governments focusing on telecommunication problems, emergency power systems, and seismic safety. [8]

See also

Related Research Articles

<span class="mw-page-title-main">Aseismic creep</span> Surface displacement along a geological fault without earthquakes occurring

In geology, aseismic creep or fault creep is measurable surface displacement along a fault in the absence of notable earthquakes. Aseismic creep may also occur as "after-slip" days to years after an earthquake. Notable examples of aseismic slip include faults in California.

<span class="mw-page-title-main">1987 Whittier Narrows earthquake</span> Earthquake in southern California

The 1987 Whittier Narrows earthquake occurred in the southern San Gabriel Valley and surrounding communities of Southern California, United States, at 7:42 a.m. PDT on October 1. The moderate magnitude 5.9 blind thrust earthquake was centered several miles north of Whittier in the town of Rosemead, had a relatively shallow depth, and was felt throughout southern California and southern Nevada. Many homes and businesses were affected, along with roadway disruptions, mainly in Los Angeles and Orange counties. Damage estimates ranged from $213–358 million, with 200 injuries, three directly related deaths, and five additional fatalities that were associated with the event.

<span class="mw-page-title-main">Calaveras Fault</span> Geological fault in northern California

The Calaveras Fault is a major branch of the San Andreas Fault System that is located in northern California in the San Francisco Bay Area. Activity on the different segments of the fault includes moderate and large earthquakes as well as aseismic creep. The last large event was the magnitude 6.2 1984 Morgan Hill event. The most recent moderate earthquakes were the magnitude 5.1 event on 25 October 2022, and the magnitude 5.6 2007 Alum Rock event.

<span class="mw-page-title-main">San Jacinto Fault Zone</span> Southern Californian 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.

<span class="mw-page-title-main">1868 Hayward earthquake</span> 1868 earthquake in the San Francisco Bay Area, California, United States

The 1868 Hayward earthquake occurred in the San Francisco Bay Area, California, United States on October 21. With an estimated moment magnitude of 6.3–6.7 and a maximum Mercalli intensity of IX (Violent), it was the most recent large earthquake to occur on the Hayward Fault Zone. It caused significant damage and a number of deaths throughout the region, and was known as the "Great San Francisco earthquake" prior to the 1906 San Francisco earthquake and fire.

The 1984 Morgan Hill earthquake occurred on April 24 at 1:15 p.m. local time in the Santa Clara Valley of Northern California. The shock had a moment magnitude of 6.2 and a maximum Mercalli intensity of VIII (Severe). The epicenter was located near Mount Hamilton in the Diablo Range of the California Coast Ranges. Nearby communities sustained serious damage with financial losses of at least US$7.5 million.

The 1986 Chalfant Valley earthquake struck southern Mono County near Bishop and Chalfant, California at 07:42:28 Pacific Daylight Time on July 21. With a moment magnitude of 6.2 and a maximum Mercalli intensity of VI (Strong), the shock injured two people and caused property damage estimated at $2.7 million in the affected areas. There was a significant foreshock and aftershock sequence that included a few moderate events, and was the last in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986.

The 1948 Desert Hot Springs earthquake occurred on December 4 at 3:43 p.m. Pacific Standard Time with a moment magnitude of 6.4 and a maximum Mercalli intensity of VII. The shock was felt from the central coast of California in the north, and to Baja California in the south, and came at a time when earthquake research in southern California resumed following the Second World War. It was one of two events in the 20th century that have occurred near a complex region of the southern San Andreas Fault system where it traverses the San Gorgonio Pass and the northern Coachella Valley. Damage was not severe, but some serious injuries occurred, and aftershocks continued until 1957.

The 1979 Imperial Valley earthquake occurred at 16:16 Pacific Daylight Time on 15 October just south of the Mexico–United States border. It affected Imperial Valley in Southern California and Mexicali Valley in northern Baja California. The earthquake had a relatively shallow hypocenter and caused property damage in the United States estimated at US$30 million. The irrigation systems in the Imperial Valley were badly affected, but no deaths occurred. It was the largest earthquake to occur in the contiguous United States since the 1971 San Fernando earthquake eight years earlier.

The 1957 San Francisco earthquake occurred on March 22 at 11:44:22 local time with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII. It was located just off the San Francisco Peninsula near the San Andreas Fault and was felt in a limited portion of Northern and Central California. There was a non-destructive foreshock and aftershock sequence that lasted for several months. With financial losses of around US$1 million, damage was considered minimal, with one death and forty injuries.

The 1986 North Palm Springs earthquake occurred on July 8 at 02:20:44 local time with a moment magnitude of 6.0 and a maximum Mercalli Intensity of VII. The shock occurred in a complex setting along the San Andreas Fault Zone where it bisects San Gorgonio Mountain and San Jacinto Peak at the San Gorgonio Pass and was the first in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986. Numerous strong motion instruments recorded the event, one of which showed relatively high accelerations. Between 29 and 40 people were injured, and financial losses were estimated to be in the range of $4.5–6 million.

The 1838 San Andreas earthquake is believed to be a rupture along the northern part of the San Andreas Fault in June 1838. It affected approximately 100 km of the fault, from the San Francisco Peninsula to the Santa Cruz Mountains. It was a strong earthquake, with an estimated moment magnitude of 6.8 to 7.2, making it one of the largest known earthquakes in California. The region was lightly populated at the time, although structural damage was reported in San Francisco, Oakland, and Monterey. It is unknown whether there were fatalities. Based on geological sampling, the fault created approximately 1.5 meters of slip.

The 1898 Mare Island earthquake occurred in Northern California on March 30 at 23:43 local time with a moment magnitude of 5.8–6.4 and a maximum Mercalli intensity of VIII–IX (Severe–Violent). Its area of perceptibility included much of northern and central California and western Nevada. Damage amounted to $350,000 and was most pronounced on Mare Island, a peninsula in northern San Francisco Bay. While relatively strong effects there were attributed to vulnerable buildings, moderate effects elsewhere in the San Francisco Bay Area consisted of damaged or partially collapsed structures, and there were media reports of a small tsunami and mostly mild aftershocks that followed.

The 2019 Ridgecrest earthquakes of July 4 and 5 occurred north and northeast of the town of Ridgecrest, California located in Kern County and west of Searles Valley. They included three initial main shocks of Mw magnitudes 6.4, 5.4, and 7.1, and many perceptible aftershocks, mainly within the area of the Naval Air Weapons Station China Lake. Eleven months later, a Mw  5.5 aftershock took place to the east of Ridgecrest. The first main shock occurred on Thursday, July 4 at 10:33 a.m. PDT, approximately 18 km (11.2 mi) ENE of Ridgecrest, and 13 km (8.1 mi) WSW of Trona, on a previously unnoticed NE-SW trending fault where it intersects the NW-SE trending Little Lake Fault Zone. This quake was preceded by several smaller earthquakes, and was followed by more than 1,400 detected aftershocks. The M 5.4 and M 7.1 quakes struck on Friday, July 5 at 4:08 a.m. and 8:19 p.m. PDT approximately 10 km (6 miles) to the northwest. The latter, now considered the mainshock, was the most powerful earthquake to occur in the state in 20 years. Subsequent aftershocks extended approximately 50 km (~30 miles) along the Little Lake Fault Zone.

The 1934 Hansel Valley earthquake occurred on March 12 at approximately 8:05 a.m. MST with a moment magnitude of 6.6 and a maximum Mercalli intensity of VIII (Severe). The shock originated in the Hansel Valley at the north end of the Great Salt Lake in Utah in the United States. Damage was mostly confined to vulnerable buildings, and two people died. The dip-slip (normal) fault that generated the shock ruptured the surface of the ground and other geologic features were documented. A large aftershock occurred three hours after the initial event and may have caused additional damage.

The 1987 Superstition Hills earthquake affected the Imperial Valley of California and Baja California on November 24, nearly 12 hours after a Mw 6.0 foreshock, called the Elmore Ranch earthquake. The mainshock, measuring Mw 6.5, struck at 17:54 PST, along the Superstition Hills segment of the San Jacinto Fault Zone. The earthquake generated a 27 km (17 mi) surface rupture with additional postseismic slip continuing for a year.

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.

The 1995 Menglian earthquake or 1995 Myanmar–China earthquake occurred on 12 July at 05:46:43 local time in the Myanmar–China border region. The earthquake had an epicenter on the Myanmar side of the border, located in the mountainous region of Shan State. It registered 7.3 on the Chinese surface wave magnitude scale (Ms ) and 6.8 on the moment magnitude scale (Mw ). With a maximum Mercalli intensity assigned at VIII, it killed 11 people and left another 136 injured. Over 100,000 homes in both countries were destroyed and 42,000 seriously damaged. Some damage to structures were also reported in Chiang Mai and Chiang Rai, Thailand. The low death toll from this earthquake was attributed to an early warning issued prior to it happening. Precursor events including foreshocks and some seismic anomalies led to an evacuation of the area before the mainshock struck. It is thought to be one of the few successfully predicted earthquakes in history.

The 1915 Imperial Valley earthquakes were two destructive shocks centered near El Centro, California on June 22. The earthquakes measured Ms 6.25 and occurred nearly one hour apart at 19:59 and 20:57 PST. Both shocks were assigned VIII (Severe) on the Modified Mercalli intensity scale. Heavy damage occurred in the areas of Mexicali and El Centro, amounting to $900,000. At least six people were killed in the earthquakes.

References

  1. 1 2 3 ISC (2015), ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009), Version 2.0, International Seismological Centre
  2. 1 2 Bouchon, M. (1982), "The rupture mechanism of the Coyote Lake earthquake of 6 August 1979 inferred from near-field data", Bulletin of the Seismological Society of America, 72 (3): 745, 748, 749, 756, Bibcode:1982BuSSA..72..745B, doi:10.1785/BSSA0720030745
  3. 1 2 3 4 Stover, C. W.; Coffman, J. L. (1993), Seismicity of the United States, 1568–1989 (Revised) – U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, pp. 94, 165, 166
  4. 1 2 3 Stover, C. W.; von Hake, C. A. (1984), "United States earthquakes, 1979", Open-File Report, Open-File report 84-979, United States Government Printing Office, pp. 32–38, Bibcode:1981use..rept.....S, doi: 10.3133/ofr84979
  5. ↑ Wallace, R. E. (1990), "General features", The San Andreas Fault System, California – USGS Professional Paper 1515, United States Geological Survey, pp. 3–6, ISBN   978-0607716269
  6. ↑ Yeats, R. (2012), Active Faults of the World, Cambridge University Press, pp. 92–94, ISBN   978-0-521-19085-5
  7. 1 2 Uhrhammer, R. A. (1980), "Observations of the Coyotoe Lake, California earthquake sequence of August 6, 1979", Bulletin of the Seismological Society of America, 70 (2): 559, 561, 565, Bibcode:1980BuSSA..70..559U, doi:10.1785/BSSA0700020559
  8. 1 2 Stromberg, P. A.; Baird, B. P. (September 28, 1979), Staff report on the Hollister-Gilroy earthquake, August 6, 1979, California Seismic Safety Commission, pp. 3–8, 12–15
  9. ↑ Porcella, R. L.; Matthiesen, R. B.; McJunkin, R. D.; Ragsdale, J. T. (1979), Compilation of strong-motion records recovered from the Coyote Lake earthquake of 6 August 1979, Open File Report 79-385, California Division of Mines and Geology, pp. 4–10
  10. ↑ Reasenberg, P.; Ellsworth, W. L. (1982), "Aftershocks of the Coyote Lake, California Earthquake of August 6, 1979: A detailed study", Journal of Geophysical Research , 87 (B13): 10, 637, 10, 638, 10, 641, 10, 654, Bibcode:1982JGR....8710637R, doi:10.1029/jb087ib13p10637, S2CID   974822
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.