 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 plates are continuously moving without major earthquakes.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 seismometers, creepmeters, strainmeters, and other instruments in and around Parkfield starting in 1985.