- Parkfield-San Bernardino earthquake
Parkfield Earthquake is a name given to various large
earthquake s that occurred in the vicinity of the little town ofParkfield, California , in theUnited States . The San Andreas fault runs through this town, and six successive magnitude 6 earthquakes occurred on the fault at unusually regular intervals, between 21 and 24 years apart (with an average of every 22 years), between1857 and1966 .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 1883) and a "creeping" segment to the north where two
tectonic plate s are continuously moving without major earthquakes.Research
Geologist 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 s and other instruments in and around Parkfield starting in 1985. This was known as the Parkfield Earthquake Experiment.In June 2004, the United States Geological Survey|USGS 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 new San Andreas Fault Observatory at Depth SAFOD program.Analysis
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.Aftershocks
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 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.
Recent developments
In
December 2004 ,seismologist s at theUniversity of California, Berkeley announced the discovery of subtle tremors near Cholame, 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 a 7.8 magnitude quake at Fort Tejon in 1857. These tremors were discovered using deep boreholeseismometer s that avoid surface noise. Thespectral signature s of these motions are more similar to those ofmagma movement nearvolcano s 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 inSouthern California , a distance of about 360 km (225 miles) and an offset of about 9 meters (30 feet) [http://www.data.scec.org/chrono_index/forttejo.html] . Theepicenter 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 6.0foreshock that was centered at Parkfield.ee also
For information about the studies and instrumentation associated with this region see
Parkfield, California#Geology External links
* [http://quake.wr.usgs.gov/research/parkfield/ The Parkfield, California, Earthquake Experiment]
* [http://www.icdp-online.de/sites/sanandreas/news/news.html San Andreas Fault Observatory at Depth]
* [http://quake.wr.usgs.gov/recenteqs/FaultMaps/120-36.htm Parkfield area current and recent activity map]
* [http://www.agu.org/pubs/crossref/1993/92JB02154.shtml Interaction of the San Andreas fault creeping segment with adjacent great rupture zones and earthquake recurrence at Parkfield]
* [http://www.data.scec.org/chrono_index/forttejo.html Essay and map concerning the Fort Tejon earthquake, showing epicenter near Cholame]
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