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Parkfield is located at 35.899N, 120.431W 1. Its elevation is 1,530 feet.
The town itself is located in the central coastal range in Central California. Mining used to be a prosperous activity in this community, but the mines were exhausted below economic recovery levels and the industry moved elsewhere. Today, it is a small town of about 900 people who are mostly ranchers and farmers or merchants supporting these activities. There is a small tourism industry in the town that basis itself on the prospect of earthquakes (see the geology section below). The Parkfield slogan is, "Eat here when it happens, Sleep here when it happens."
Parkfield lies along the San Andreas Fault, one of the most geologically active faults. Parkfield traditionally has an earthquake of 6 or greater magnitude every 22 years. In 1985, the US Geological Survey predicted that there would be a major earthquake in this community in 1993, but no such earthquake came until September 28, 2004 when a magnitude 6.0 earthquake struck at 10:15 am Pacific Daylight Time.
Parkfield is the most closely observed earthquake zone in the world. Scientists measure the strain in rocks, heat flow, and geomagnetism constantly around Parkfield. The observance of the San Andreas fault in Parkfield will hopefully help scientists understand earthquakes and maybe some day predict major earthquakes along the San Andreas fault and across the world.
Since 1985, the US Geological Survey (USGS) has been working on The Parkfield Experiment, a long-term research project on the San Andreas fault. "The experiment's purpose is to better understand the physics of earthquakes - what actually happens on the fault and in the surrounding region before, during and after an earthquake"
The reason that earthquakes occur so regularly here is that it lies 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. As the earthquake was eleven years overdue it lends some credence to the folk saying "a watched pot never boils". The additional time did offer the opportunity to add improvements in instrumentation as the technology was further developed.
In 2004, work began just north of Parkfield on the San Andreas Fault Observatory at Depth (SAFOD). The goal of SAFOD is to drill a hole nearly 2.5 miles (4 kilometers) into the Earth's crust and into the San Andreas Fault. An array of sensors will be installed to capture and record earthquakes that happen near this area. This is expected to be be completed midyear 2005 will be located at the source of numerous microquakes — these have a magnitude of around 1.0.
Taken from USGS site for 120W, 36N (click for current and recent activity)
The 6.0 magnitude primary shock was the result of a fault movement of about 18 inches (.5 meter). At the time of this writing, Oct 8, 2004, 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 (5.0) aftershocks continued for more than a week on a northwesterly progression, each further from the principle shock. 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
Excerpt of USGS earthquake list - only magnitude 3.0 and greater (reverse chronological order) — numerous small earthquakes below 2.0 magnitude continue with a few stronger than 3.0 (list is no longer updated)
Note that the range of earthquakes between 3.0 and 5.0 encompass a range of energy release in the ratio of 1:100, as the magnitude scale is logarithmic. The ground motion associated with earthquakes in this range is highly dependent upon the distance from the epicenter and the particular soil conditions at the observation point.