Parkfield earthquake is a name given to various large

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

s that occurred in the vicinity of the town of Parkfield, California,

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 Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

. 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 (horizontal) ...

runs through this town, and six successive magnitude 6 earthquakes occurred on the fault at unusually regular intervals, between 12 and 32 years apart (with an average of every 22 years), between 1857 and 1966. The most recent significant earthquake to occur here happened on September 28, 2004. Earthquakes may occur regularly here because the location is about midway on a fault segment between a ''locked'' segment to the south (last major earthquake 1857) and a ''creeping'' segment to the north where two

tectonic plate 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 te ...

s are continuously moving without major earthquakes.

Research

Geologists who hoped to study what happens before a quake, and in particular any signs that might enable them to predict future earthquakes, installed an elaborate array of

seismometer 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 outpu ...

s, creepmeters,

strainmeter A strainmeter is an instrument used by geophysicists to measure the deformation of the Earth. Linear strainmeters measure the changes in the distance between two points, using either a solid piece of material (over a short distance) or a laser inter ...

s, and other instruments in and around Parkfield starting in 1985. Scientists with the USGS and UC Berkeley had predicted, with a 90 to 95% confidence level, that an earthquake would strike the Parkfield area between 1985 and 1993. This was known as the Parkfield Earthquake Prediction and the Parkfield Earthquake Experiment, conducted by the USGS. Attempts at predicting the quake continued until January 2001, but an earthquake of 5.5 magnitude or greater did not occur from 1985 until the 2004 quake. In June 2004, the

USGS 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, ...

in partnership with the National Science Foundation began drilling a deep hole to house instruments to monitor the fault at depth. This action was a part of the San Andreas Fault Observatory at Depth (SAFOD) program. Because of the regularity of large events (>5.5) at the Parkfield location (events in 1857, 1881, 1901, 1922, 1934, and 1966), and the fact that the waveforms from many of these events were almost identical, it was believed that the same segment of fault ruptured each time. This led to the prediction in 1984 of a similar event in 1993.

2004 event

The 6.0 magnitude primary shock in 2004 was the result of a fault movement of about 18 inches (.5 meter). There have been no indications found that could have been used to predict this earthquake. Although well overdue, the probability of this quake occurring in 2004 has been estimated at about ten percent. The magnitude of the event was consistent with previous earthquakes in this region. Substantial aftershocks continued for more than a week after the initial event, moving in a northwesterly progression. In early October, there was a cluster of small earthquakes near

Paso Robles Paso Robles ( ), officially El Paso de Robles (Spanish for "The Pass of Oaks"), is a city in San Luis Obispo County, California, United States. Located on the Salinas River approximately north of San Luis Obispo, the city is known for its ho ...

near a parallel fault to the west. These may be in response to the transfer of stress to these faults after the release of stress at Parkfield. Past earthquakes have also occurred to the east of Parkfield at about the same distance from the San Andreas fault near Coalinga and

Avenal Avenal (Spanish for "Oat field") is a city in Kings County, California, United States. Avenal is located southwest of Hanford, at an elevation of . It is part of the Hanford– Corcoran Metropolitan Statistical Area (MSA Code 25260), which e ...

Recent developments

In December 2004,

seismologist Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

s at the

University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant un ...

announced the discovery of subtle tremors near

Cholame Cholame (; Salinan: ''Tco'alam'') is an unincorporated community in San Luis Obispo County, California, United States. It sits within a mile of the San Andreas Fault at an elevation of above sea level and is located at . Cholame is reached v ...

, a hamlet near the San Andreas fault directly south of Coalinga. This is in a region of the locked fault below the Parkfield episodes, last creating an 8.0 magnitude quake at Fort Tejon in 1857. These tremors were discovered using deep borehole

s that avoid surface noise. The spectral signatures of these motions are more similar to those of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

movement near

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates ...

es than of typical earthquakes, but it is believed that the motions are not due to magma or fluid motion. It is hoped that this new discovery may sometime inform scientists as to the degree of danger presented by known locked faults. It is not currently expected that this knowledge will be refined into a precise predictive tool.

Fort Tejon earthquake

What is possibly the largest earthquake on the San Andreas fault in the last several hundred years is the 1857 Fort Tejon earthquake, with a fault rupture from the general vicinity of Parkfield to San Bernardino in

Southern California Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban ...

, a distance of about and an offset of about 9 meters (30 feet). The

epicenter The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Surface damage Before the instrumental pe ...

of this earthquake is (by various sources) believed to be somewhere in the region from Cholame to Parkfield, a location at the extreme northern end of the locked portion of the fault and at the southern end of the rapidly periodic segment. It is believed that this earthquake was preceded by a magnitude 6.0 foreshock that was centered at Parkfield.

References

External links

The Parkfield, California, Earthquake Experiment
– United States Geological Survey
San Andreas Fault Observatory at Depth
– United States Geological Survey
Interaction of the San Andreas fault creeping segment with adjacent great rupture zones and earthquake recurrence at Parkfield
– ''

Journal of Geophysical Research The ''Journal of Geophysical Research'' is a peer-reviewed scientific journal. It is the flagship journal of the American Geophysical Union. It contains original research on the physical, chemical, and biological processes that contribute to the un ...