The collective sigh of relief from seismologists almost registered on the Richter scale. The magnitude 6.0 earthquake that struck near the central California town of Parkfield (population 37) today was "much anticipated but long delayed," says seismologist Ross Stein of the U.S. Geological Survey (USGS) in Menlo Park, California.
In the 1980s, researchers wired the section of the San Andreas fault in anticipation of a repeat of the moderate quake that had struck there every 20 or 30 years since at least 1857. The hope was that the next Parkfield quake would have some harbinger, which could be used to forecast future temblors. But given that the last Parkfield quake came in 1966, seismologists had expected the next one by the late 1980s. No such luck.
The anticipated quake was long overdue, but "it happened, and we caught it," says seismologist Robert Wesson of USGS in Denver, Colorado. The complex network of instruments installed around Parkfield should make this "the most well-recorded earthquake in history," says seismologist Michael Blanpied of USGS in Reston, Virginia. There were no immediate reports of activity on the fault before the main shock, but future scrutiny of the records of seismicity, strain, magnetism, water-well levels, and strong ground motion should provide unprecedented detail of how fault ruptures get started, propagate, and stop.
At press time, seismologists were looking southeast of Parkfield toward Los Angeles. The great 1857 earthquake there was likely preceded by a Parkfield quake that, 9 hours later, triggered the much bigger quake to the south. So far, there is so sign of anything heading south.
More about Parkfield from the USGS