Pretty much none of the quotes from experts or news reports about Monday night's 4.5M earthquake in the East Bay — or the recent swarm of mini-quakes in other parts of the Bay Area — have been comforting or satisfying. And least of all among these is the news that the rupture that caused the quake occurred nine miles below the earth's surface on a fault that was previously unmapped by seismologists.
Scientists have been intrigued by how widely the quake was felt at 10:33 p.m. Monday, likely because of its depth. The quake occurred on a fault that lies between the Concord and Calavares faults, and while previously unknown it is "one of thousands of small cracks that snake outward from a half-dozen or so major California faults, such as the San Andreas and Hayward," as the Chronicle reports.
The interconnectedness of all these major and minor faults remains incompletely understood. So while seismologists typically say that the minor quakes that happened off the coast of Pacifica last week and the 4.7M quake on Tuesday near Hollister are all unrelated and coincidentally close in time, these claims have a way of sounding hollow as we sit here wondering if a bigger quake is yet to strike.
The Hayward Fault is the big one that we've been told to watch out for for decades now. It's well overdue for its next rupture, though it's impossible to say whether a different fault will catch us unawares first.
"These big faults can be locked, and the stress builds up to the point until the rock can’t hold anymore and it slips. But between these faults there are all these little fractures," says retired U.S. Geological Survey geologist David Schwartz, speaking to the Chronicle. He notes that Monday's quake occurred only about two surface miles from the Concord Fault, which is considered a big one. He refers to this smaller fault as a "blind thrust fault," which breaks when "one side of the Earth’s crust moves up over the other side, creating a dangerous thrust motion." Such a fault was responsible for the damaging 6.7M Northridge earthquake in Los Angeles in 1994, despite being considered minor in size.
"Any earthquake of this magnitude can occur anywhere in the Bay Area at any time," Schwartz says, adding that a big one is definitely coming.
As for aftershocks to come, there's still a 2 percent chance of one coming from the Pleasant Hill-area fault in the coming days, and that could be larger or smaller than the first one.