The earthquake that rattled Berkeley early Thursday morning, the ninth tremor in a little over a week, could signal that more quakes, and maybe even the “Big One,” are on their way.
The string of quakes in Berkeley on the Hayward Fault began with a magnitude 4.0 quake last Thursday afternoon. Although the “swarm” — multiple earthquakes of similar magnitudes that occur in a short time span — may be an isolated incident, it could foretell the coming of a larger quake, according to Jack Boatwright, a seismologist with the U.S. Geological Survey.
Typically, earthquakes occur in a main shock-aftershock sequence, with the first quake — the main shock — about a magnitude unit greater than the aftershocks. But the earthquakes over the past eight days have had relatively similar magnitudes and no clear decline in energy. The quake Thursday was stronger than many that preceded it at magnitude 3.6.
Scientists have said a quake of 6.7 magnitude or greater is imminent on the Hayward Fault, which runs directly under the UC Berkeley campus. The past five large quakes have occurred 140 years apart on average, and since the last was in 1868, the next large quake is overdue.
Smaller quakes usually function as “foreshocks” and precede a large quake, according to Boatwright.
“Earthquakes don’t occur out of nowhere,” he said.
However, unless the frequency of these quakes increases, it is difficult to determine whether this month’s series of earthquakes is an isolated “swarm” or a foreshock, he said.
“You don’t know you have a foreshock until you have a mainshock that follows,” he said. “Some foreshock sequences start to accelerate. They start to have more and more small events and that starts to be a key that something is happening,” he said.
This month’s quakes are “a little more energetic,” than the last “swarms” in Berkeley, which were in December 2006 and September 2003. The location of the three sets are similar, with this month’s starting where the 2006 quakes were and then progressing a little to the north.
Whether or not these smaller quakes will end in the “Big One,” they will not reduce the magnitude of a large quake when it hits, Boatwright said.
“That the smaller earthquakes are releasing stress that would otherwise show up in a large earthquake is a pretty common fallacy and, for the most part, is entirely wrong,” he said.
Soumya Karlamangla is the assistant city news editor.