1979 Coyote Lake earthquake
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The 1979 Coyote Lake earthquake occurred at on August 6 with a
moment magnitude The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...
of 5.7 and a maximum
Mercalli Intensity The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...
of VII (''Very strong''). The shock occurred on the
Calaveras Fault 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 aseismi ...
near Coyote Lake in
Santa Clara County, California Santa Clara County, officially the County of Santa Clara, is the sixth-most populous county in the U.S. state of California, with a population of 1,936,259, as of the 2020 United States Census, 2020 census. Santa Clara County and neighboring Sa ...
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 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 in ...
, and following the event local governments evaluated their response to the incident.


Tectonic setting

The
San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...
system (SAF) is a network of right-lateral
strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
faults that form a portion of a complex and
diffuse Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
plate boundary. The faults span on and off shore along the California portion of the
Pacific Rim The Pacific Rim comprises the lands around the rim of the Pacific Ocean. The ''Pacific Basin'' includes the Pacific Rim and the islands in the Pacific Ocean. The Pacific Rim roughly overlaps with the geologic Pacific Ring of Fire. List of co ...
, and in the area near San Francisco Bay, the extent of the various fault strands are limited to about wide from east to west. This system of faults terminates in the north at the
Mendocino Triple Junction The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in the ...
where the north-northwest trending SAF meets the east trending
Mendocino Fracture Zone The Mendocino Fracture Zone is a fracture zone and transform boundary over 4000 km (2500 miles) long, starting off the coast of Cape Mendocino in far northern California. It runs westward from a triple junction with the San Andreas Fault a ...
. It terminates in the south in a more gradual fashion at the
Salton Sea The Salton Sea is a shallow, landlocked, highly saline body of water in Riverside and Imperial counties at the southern end of the U.S. state of California. It lies on the San Andreas Fault within the Salton Trough that stretches to the Gulf o ...
, where displacement transitions to a series of
transform fault A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subductio ...
s and
spreading centers A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a Diverge ...
along the
Gulf of California Rift Zone The Gulf of California Rift Zone (GCRZ) is the northernmost extension of the East Pacific Rise which extends some from the mouth of the Gulf of California to the southern terminus of the San Andreas Fault at the Salton Sink. The GCRZ is an incipi ...
. Several strands of the SAF in the eastern region of the San Francisco Bay Area are the Hayward–Rodgers Creek and
Calaveras Fault 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 aseismi ...
s. The Hayward Fault exhibits
fault creep 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 in ...
, but it also has potential for large earthquakes, like the
1868 Hayward earthquake 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 ear ...
that occurred on its southern segment. The northern Calaveras Fault meets the Hayward Fault near the
Calaveras Reservoir Calaveras Reservoir is located primarily in Santa Clara County, California, with a small portion and its dam in Alameda County, California. In Spanish, Calaveras means "skulls". The reservoir is fed mainly by Arroyo Hondo and Calaveras Cree ...
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.


Earthquake

The earthquake occurred on the Calaveras Fault near Coyote Lake in
Santa Clara County Santa Clara County, officially the County of Santa Clara, is the sixth-most populous county in the U.S. state of California, with a population of 1,936,259, as of the 2020 United States Census, 2020 census. Santa Clara County and neighboring Sa ...
. Although the
Hayward Fault Zone The Hayward Fault Zone is a right-lateral strike-slip geologic fault zone capable of generating destructive earthquakes. This fault is about long, situated mainly along the western base of the hills on the east side of San Francisco Bay. It runs ...
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 The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area Go ...
, 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 Berkeley Seismological Laboratory (BSL) is a research lab at the Department of Geology at the University of California, Berkeley. It was created from the Berkeley Seismographic Stations, a site on the Berkeley campus where Worldwide Standard S ...
. 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 In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
motion occurred over on a vertical fault, and that the total amount of slip varied with depth, with more slip occurring in the shallower regions.


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 Penney OpCo LLC, doing business as JCPenney and often abbreviated JCP, is a midscale American department store chain operating 667 stores across 49 U.S. states and Puerto Rico. Departments inside JCPenney stores include Mens, Womens, Boys, Girl ...
had a hole and cracks in its ceiling and a
parapet A parapet is a barrier that is an extension of the wall at the edge of a roof, terrace, balcony, walkway or other structure. The word comes ultimately from the Italian ''parapetto'' (''parare'' 'to cover/defend' and ''petto'' 'chest/breast'). Whe ...
collapsed at a law office. At
Casa de Fruta Casa de Fruta (Spanish: literally "House of Fruit") is a large roadside attraction located in an unincorporated area of Santa Clara County, just west of the Merced County line, in the Pacheco Valley of Northern California, along State Route 15 ...
, a service station sustained structural damage, as did a
fire station __NOTOC__ A fire station (also called a fire house, fire hall, firemen's hall, or engine house) is a structure or other area for storing firefighting apparatuses such as fire engines and related vehicles, personal protective equipment, fire h ...
at
Pacheco Pass Pacheco Pass, elevation , is a low mountain pass located in the Diablo Range in southeastern Santa Clara County, California. It is the main route through the hills separating the Santa Clara Valley and the Central Valley (California), Central Val ...
. An early estimate by the
Small Business Administration The United States Small Business Administration (SBA) is an independent agency of the United States government that provides support to entrepreneurs and small businesses. The mission of the Small Business Administration is "to maintain and stren ...
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 An ambulance is a medically equipped vehicle which transports patients to treatment facilities, such as hospitals. Typically, out-of-hospital medical care is provided to the patient during the transport. Ambulances are used to respond to medi ...
) were treated and released, but six were admitted.


Intensity

At the most extreme points of its perceptibility, it was felt with intensity II or III (''Weak'') at
Healdsburg Healdsburg is a city located in Sonoma County, in California's Wine Country. At the 2010 census, the city had a population of 11,254. Owing to its three most important wine-producing regions (the Russian River, Dry Creek, and Alexander Valley ...
in the north and Santa Barbara in the south,
Visalia Visalia ( ) is a city in the agricultural San Joaquin Valley of California. The population was 141,384 as per the 2020 census. Visalia is the fifth-largest city in the San Joaquin Valley, the 42nd most populous in California, and 192nd in ...
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 Reno ( ) is a city in the northwest section of the U.S. state of Nevada, along the Nevada-California border, about north from Lake Tahoe, known as "The Biggest Little City in the World". Known for its casino and tourism industry, Reno is the ...
. 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 Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection of light energy **Laser beam *Particle beam, a stream of charged or neutral particles **Charged particle beam, a spatially localized grou ...
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.


Strong motion

The event was captured on seismographs at distances of up to , including
Richmond Richmond most often refers to: * Richmond, Virginia, the capital of Virginia, United States * Richmond, London, a part of London * Richmond, North Yorkshire, a town in England * Richmond, British Columbia, a city in Canada * Richmond, California, ...
, with the unit "down hole" in
bay mud Bay mud consists of thick deposits of soft, unconsolidated silty clay, which is saturated with water; these soil layers are situated at the bottom of certain estuaries, which are normally in temperate regions that have experienced cyclical glacia ...
. Other underground instruments on the
BART Bart is a masculine given name, usually a diminutive of Bartholomew, sometimes of Barton, Bartolomeo, etc. Bart is a Dutch and Ashkenazi Jewish surname, and derives from the name ''Bartholomäus'', a German form of the biblical name ''Barthol ...
Transbay Tube The Transbay Tube is an underwater rail tunnel that carries Bay Area Rapid Transit's four transbay lines under San Francisco Bay between the cities of San Francisco and Oakland in California. The tube is long, and attaches to twin bored tunnels ...
, as well as the Richmond site, showed accelerations that were very low. In
San Juan Bautista San Juan Bautista is the Spanish-language name of Saint John the Baptist. It may refer to: Places Bolivia *San Juan Bautista, Bolivia, Jesuit mission ruins near the village of San Juan de Taperas Chile *San Juan Bautista, Chile, Juan Fernández ...
, the
U.S. Route 101 U.S. Route 101, or U.S. Highway 101 (US 101), is a north–south United States Numbered Highway that runs through the states of California, Oregon, and Washington, on the West Coast of the United States. It is also known as (The Royal Roa ...
/ 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 Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
at the campus of
West Valley College West Valley College is a public community college in Saratoga, California. It is part of the California Community College system. History The college was founded as West Valley Junior College in 1963. It was to serve the footprint of the Campbell ...
in Saratoga provided records of interest. Of a number of dams that had instruments installed, the
San Luis Dam San Luis Dam is a major earth-filled dam in Merced County, California, which forms San Luis Reservoir, the largest off-stream reservoir in the United States. The dam and reservoir are located in the Diablo Range to the east of Pacheco Pass and ab ...
at
San Luis Reservoir The San Luis Reservoir is an artificial lake on San Luis Creek in the eastern slopes of the Diablo Range of Merced County, California, approximately west of Los Banos on State Route 152, which crosses Pacheco Pass and runs along its north ...
had the strongest response. The highest acceleration of 0.44 ''g'' was seen at the San Ysidro School in Gilroy.


Aftershocks

The
United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...
operated a network of
seismograph A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The output ...
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 Strike and dip is a measurement convention used to describe the orientation, or attitude, of a planar geologic feature. A feature's strike is the azimuth of an imagined horizontal line across the plane, and its dip is the angle of inclination m ...
of the Calaveras Fault in that area and were classified into three distinct groups. The east-dipping northeastern group, a
diffuse Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
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.


Response

While none of the affected counties or cities declared a
state of emergency A state of emergency is a situation in which a government is empowered to be able to put through policies that it would normally not be permitted to do, for the safety and protection of its citizens. A government can declare such a state du ...
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 Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
problems,
emergency power system An emergency power system is an independent source of electrical power that supports important electrical systems on loss of normal power supply. A standby power system may include a standby generator, batteries and other apparatus. Emergency p ...
s, and seismic safety.


See also

*
List of earthquakes in 1979 This is a list of earthquakes in 1979. Only earthquakes of magnitude 6 or above are included, unless they result in damage and/or casualties, or are notable for some other reason. Events in remote areas will not be listed but included in statist ...
*
List of earthquakes in California The earliest known California earthquake was documented in 1769 by the Spanish explorers and Catholic missionaries of the Portolá expedition as they traveled northward from San Diego along the Santa Ana River near the present site of Los Angeles ...
*
List of earthquakes in the United States The following is a list of notable earthquakes and tsunamis which had their epicenter in areas that are now part of the United States with the latter affecting areas of the United States. Those in ''italics'' were not part of the United States whe ...


References


External links


M5.7 – Northern California
United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...
* {{Earthquakes in the United States 1979 earthquakes Earthquakes in California History of Santa Clara County, California History of the San Francisco Bay Area Coyote Lake August 1979 events in the United States 1979 natural disasters in the United States