Activity snapshot 35 hours after Sept 28, 2004 large earthquake. The Parkfield earthquake is a name given to any large earthquake that occurs in the vicinity of the town of Parkfield, California. The San Andreas Fault runs through this town and has consistently produced a magnitude 6.0 or greater earthquake roughly every 22 years since 1857.
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 plates are continuously moving without major earthquakes.
The most recent significant earthquake to occur here happened on September 28, 2004.
Analysis The 6.0 magnitude primary shock 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 Aftershocks (October 11, 2004) note Paso Robles sympathetic events. ( Cholame is located near the southernmost yellow square on the eastern side of the San Andreas fault extending south-southeast from Parkfield) Substantial aftershocks continued for more than a week after the intial 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, seismologists at the University 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 borehole seismometers that avoid surface noise. The spectral signatures of these motions are more similar to those of magma movement near volcanos 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, a distance of about 360 km (225 miles) and an offset of about 9 meters (30 feet)[1] (http://www.data.scec.org/chrono_index/forttejo.html). The epicenter 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.
See also For information about the studies and instrumentation associated with this region see Parkfield, California#Geology
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