1980 Eureka earthquake
   HOME

TheInfoList



OR:

The 1980 Eureka earthquake (also known as the Gorda Basin earthquake) occurred on November 8 at along the northern coastal area of
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
. 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 7.3 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''), this strike-slip earthquake was the largest to occur in California in 28 years. Although damage was considered light, several loss estimates equaled or exceeded $2 million, and six injuries resulted when two vehicles came down with the partial collapse of a highway overpass on
US 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 ...
in Fields Landing. The north coast of California experiences frequent
plate boundary Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
earthquakes near 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 th ...
and intraplate events also occur within the
Gorda Plate The Gorda Plate, located beneath the Pacific Ocean off the coast of northern California, is one of the northern remnants of the Farallon Plate. It is sometimes referred to (by, for example, publications from the USGS Earthquake Hazards Program) a ...
. Due to the regional
seismic risk Seismic risk refers to the risk of damage from earthquake to a building, system, or other entity. Seismic risk has been defined, for most management purposes, as the potential economic, social and environmental consequences of hazardous events th ...
, the nuclear portion of the Humboldt Bay Nuclear Power Plant was shut down in the 1970s. No substantial damage occurred to the two
fossil-fuel A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ma ...
units that were still operational at the facility. Several types of sensors were installed at the site to capture strong motion data in this seismically-active area, but the majority of records from the event were considered unreliable due to faulty equipment or inadequate maintenance. Only one piece of equipment at the facility provided data by which an estimate of the
peak ground acceleration Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a par ...
could be made.


Tectonic setting

Near
Cape Mendocino Cape Mendocino (Spanish: ''Cabo Mendocino'', meaning "Cape of Mendoza"), which is located approximately north of San Francisco, is located on the Lost Coast entirely within Humboldt County, California, United States. At 124° 24' 34" W longitude ...
, the Mendocino Triple Junction is an area of active seismicity where three tectonic plates come together. The
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 ...
(also known as the Mendocino Fault east of the
Gorda Ridge The Gorda Ridge (41°36'19.6"N 127°22'03.1"W), aka ''Gorda Ridges'' tectonic spreading center, is located roughly off the northern coast of California and southern Oregon. Running NE – SW it is roughly in length.Carey, Stein, & Rona. (1990). ...
) is a
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 subduct ...
that separates the
Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...
and
Gorda Plate The Gorda Plate, located beneath the Pacific Ocean off the coast of northern California, is one of the northern remnants of the Farallon Plate. It is sometimes referred to (by, for example, publications from the USGS Earthquake Hazards Program) a ...
s. To the south, the relative motion between the Pacific Plate and
North American Plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacif ...
is accommodated by the
San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...
, and to the north, the Gorda Plate is converging with the North American Plate at the
Cascadia Subduction Zone The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is a very long, sloping subduction zone where the Explorer, Juan de Fuca, a ...
. Earthquakes within the Gorda Plate are the result of north–south compression at the Mendocino Fault.


Earthquake

The left-lateral strike-slip earthquake was the largest to occur in California since the
1952 Kern County earthquake The 1952 Kern County earthquake occurred on July 21 in the southern San Joaquin Valley and measured 7.3 on the moment magnitude scale. The main shock occurred at 4:52 am Pacific Daylight Time (11:52 UTC), killed 12 people, injured hundreds ...
. The mainshock (which was described as a multiple-rupture, with four subevents in the initial 80 seconds) and its aftershocks occurred on a northeast-trending fault that extended from near the Mendocino Fault to a point northwest of
Eureka Eureka (often abbreviated as E!, or Σ!) is an intergovernmental organisation for research and development funding and coordination. Eureka is an open platform for international cooperation in innovation. Organisations and companies applying th ...
. Movement along the fault is due to a north–south compressional regime and the resulting
intraplate deformation Intraplate deformation is the folding, breaking, or flow of the Earth's crust within plates instead of at their margins. This process usually occurs in areas with especially weak crust and upper mantle, such as the Tibetan Plateau (Figure 1). Int ...
of the Gorda Plate. Previous events in this area were the January 1922 7.3 and the January 1923 7.2 shocks. The earthquake occurred offshore (to the northwest of Eureka, about west of Patrick's Point) and had a maximum Mercalli intensity of VII (''Very strong''). Some people reported intense shaking that lasted for 15 to 30 seconds. In the epicentral area, items were knocked off shelves and furniture was displaced, but damage was considered light (the smaller 5.2 event that occurred in June 1975 caused more damage).


Damage

While most of the damage from the Gorda Basin earthquake was considered light, there were some exceptions. In Eureka, effects included broken windows and dishes, fallen chimneys, and merchandise that fell from store shelves. In Fields Landing, King Salmon, Loleta, and Big Lagoon areas, the strongest effects included surface cracks on the ground, soil liquefaction, small landslides and
rockfall A rockfall or rock-fallWhittow, John (1984). ''Dictionary of Physical Geography''. London: Penguin, 1984. . is a quantity/sheets of rock that has fallen freely from a cliff face. The term is also used for collapse of rock from roof or walls of mi ...
s, and numerous slumps along the Eel River. Similar effects occurred along the Old Coast Highway near
Trinidad Trinidad is the larger and more populous of the two major islands of Trinidad and Tobago. The island lies off the northeastern coast of Venezuela and sits on the continental shelf of South America. It is often referred to as the southernmos ...
and Moonstone, where the roadway was reduced to one lane of travel in some areas. Several homes were knocked off their foundations and a highway overpass collapsed in Fields Landing.
Seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy ...
amplification, poor design, or inadequate construction style may have contributed to losses there.


Tompkins Hill Road overpass

The Tompkins Hill Road overpass is situated just south of Fields Landing and was built in the late 1960s. It suffered slight damage during the 1975 earthquake and was due for a retrofit in 1981. The overpass was constructed with cement
abutment An abutment is the substructure at the ends of a bridge span or dam supporting its superstructure. Single-span bridges have abutments at each end which provide vertical and lateral support for the span, as well as acting as retaining walls ...
s on earthen ramps on either end and a series of concrete support columns in the middle. Eight reinforced concrete spans accommodated northbound and southbound lanes of traffic, with no anchors connecting the spans with each other or to the abutments. At the time of the shock, two of the southbound spans came off their support (a ledge) and six people were injured when a
Volkswagen Beetle The Volkswagen Beetle—officially the Volkswagen Type 1, informally in German (meaning "beetle"), in parts of the English-speaking world the Bug, and known by many other nicknames in other languages—is a two-door, rear-engine economy car, ...
and a small pickup truck plummeted off the bridge.


Humboldt Bay Power Plant

The Humboldt Bay Power Plant is located about north of the collapsed highway overpass and operated
fossil-fuel A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ma ...
and nuclear units in the 1960s and 1970s. The nuclear unit was cancelled in 1976 because of seismic safety concerns, but the two fossil-fuel units remained in operation. The Nuclear Regulatory Commission performed a post-event survey of the facility's systems in late 1980, but inspectors found only minor effects to the plant's structure, piping, tanks, and other mechanical equipment. It was found that the two units were automatically shut down at the time of the shock for various protective concerns and that there were minor cracks in masonry and concrete, sheared bolts, pipe leaks, and slight movement of water tanks. The deformation of a reinforced masonry wall that resulted in a variable-width gap was the only structural issue.


Intensity

With an area of perceptibility of more than 97,000 mi2, it was felt from 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 ...
and the Central Valley to western Nevada, and as far as Salem in northern Oregon. Intensity VI (''Strong'') effects were limited to a narrow coastal zone that included
Myers Flat Myers Flat (formerly Myers) is a census-designated place in Humboldt County, California. It is located south-southeast of Weott, at an elevation of 203 feet (62 m). The ZIP Code is 95554. The population was 146 at the 2010 census. Originally c ...
in the south and
Brookings, Oregon Brookings is a city in Curry County, Oregon, United States. It was named after John E. Brookings, president of the Brookings Lumber and Box Company, which founded the city in 1908. As of the 2020 census, the population was 6,744. History F ...
in the north. Damage at this intensity level included broken plate glass windows, cracked walls, foundation and chimney damage, and significant loss of merchandise. In Fields Landing, two homes that came off their foundations, the fallen overpass, and broken gas, water, and sewer lines were associated with intensity VII (''Very strong'') shaking.


Strong motion

Pacific Gas and Electric The Pacific Gas and Electric Company (PG&E) is an American investor-owned utility (IOU). The company is headquartered in the Pacific Gas & Electric Building, in San Francisco, California. PG&E provides natural gas and electricity to 5.2 milli ...
initially reported that
peak ground acceleration Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a par ...
s in the range of 0.16–0.4 '' g'' were recorded on the floor of the refueling building on the plant's strong motion instruments (
accelerometer An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is acc ...
s). Low voltage from a faulty power supply left the instruments in a condition that was functional, but the records were not considered reliable. Three TERA Technology film recorders were also in use as a backup system, but these instruments also did not produce any usable records, because lack of maintenance had allowed dirt and grit to get inside. Only records from one instrument (an Engdhal peak shock recorder) was believed to be operating correctly, and with a close examination of the energy dispersed at various frequencies, an estimate of 0.15–0.25 '' g'' was given for the event at that location.


See also

*
List of earthquakes in 1980 This is a list of earthquakes in 1980. 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


References


External links


M7.2 – offshore 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, ...

When the bridge fell: Father looks back on 30th anniversary of quake that claimed overpass
– ''
Times-Standard The ''Times-Standard'' is the only major local daily newspaper covering the far North Coast of California. Headquartered in Eureka, the paper provides coverage of international, national, state and local news in addition to entertainment, sports, ...
''
A Quake, a Collapse and No Responsibility
– ''
The New York Times ''The New York Times'' (''the Times'', ''NYT'', or the Gray Lady) is a daily newspaper based in New York City with a worldwide readership reported in 2020 to comprise a declining 840,000 paid print subscribers, and a growing 6 million paid d ...
''
IE Circular No. 81-03, Inoperable Seismic Monitoring Instrumentation
U.S. Nuclear Regulatory Commission * {{Good article 1980 earthquakes Earthquakes in California History of Humboldt County, California 1980 in California Eureka, California