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, 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 an 8.0 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 volcanoes 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 and an offset of about 9 meters (30 feet). 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 List of earthquakes in California 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 Essay and map concerning the Fort Tejon earthquake,