Imperial County Quake Swarm a Reminder of California's Fate

It's been a busy last few days in the Brawley Seismic Zone at the south end of the Salton Sea. There have been nearly 500 quakes since Saturday in the few square miles between the towns of Brawley, Westmoreland, and Calipatria, the largest two on Sunday afternoon with magnitudes of 5.3 and 5.5, and Imperial Valley residents are -- understandably -- a bit nervous. So are people outside the Valley. Across southern California people are asking whether this swarm of earthquakes indicate that the Big One is on its way.

And they do... but not in the way you might think.

The Big One is definitely coming, make no mistake about that. It could happen in a hundred years, or before you finish reading this article. Seismologists are lately entertaining the possibility that the entire southern San Andreas Fault, from Brawley to Monterey County, could rupture in a single magnitude 8.1 quake, stronger than any ever recorded in California. And if that happens, it's only a matter of time until the next Big One, and the next, until coastal California has slid northward to the vicinity of British Columbia. So bolt your bookshelves.

But do this weekend's quakes in Imperial County bring the Big One any closer? Probably not, according to the U.S.Geological Survey (USGS).

The Brawley Seismic Zone, which extends from Bombay Beach to El Centro, runs between the southern end of the San Andreas Fault and the Imperial Fault to the west. The zone has been the site of quite a few earthquake swarms like the one now seemingly dying down. There was one in 2005, whose strongest quake had a magnitude of 5.1, and one in 1981, and several in the years from 1973-1979, and almost certainly at intervals of every few years for millennia before that.

What causes earthquake swarms? Volcanic activity can, for one thing, as magma moves below the surface, and a few recent swarms in the Mammoth Mountain area are likely the result of subsurface volcanism. Distant earthquakes can trigger swarms as well.

But in the case of the Brawley Seismic Zone, the swarms are likely the result of the earth's crust slowly, inexorably tearing itself apart.

The Brawley Seismic Zone is generally considered to be a "spreading center" -- a place where tectonic plates are forced apart by the movements of magma deep beneath the surface. The zone is at the very north end of the East Pacific Rise, which for most of its 5,600-mile length is a submarine chain of volcanic mountains running almost to Antarctica. Along the spine of the East Pacific Rise magma comes up to the surface and forms new oceanic crust, which pushes the crust on either side away from the ridge. The Rise runs right up the middle of the Sea of Cortez to the Brawley Seismic Zone, where it ends -- and the San Andreas Fault begins.

The East Pacific Rise is thought to be the fastest-spreading divergent plate boundary on the planet, splitting the crust at an average of six inches a year. At the mouth of the Sea of Cortez, given the resistance of continental crust on either side, that slackens to about an inch a year, still a remarkable rate of movement. In essence, the northern section of the East Pacific Rise is a giant wedge splitting the southwestern portion of California away from North America. The wedge point is near Brawley, and the split has spread as far north as Desert Hot Springs. If not for the Colorado River's sediment, the Imperial and Coachella Valleys would be part of the Sea of Cortez, the rift valley that has opened up where the East Pacific Rise has split North America. The split contributes to forces on the San Andreas Fault -- though other movements of the Pacific and North American plates make the fault's dynamics a bit more complex than that -- and will eventually shear off a section of the continent altogether. In 40 million years time, the coast will have moved far enough north that Los Angeles will be west of Seattle -- and just imagine the resentment that'll cause among Seattleites.

This week's earthquake swarm takes place in that context. Though the exact cause of the swarm isn't known, the earth is stretching at Brawley. Magma is rising to the surface: the Salton Buttes a few miles to the north of this week's swarm are remains of magma that surfaced somewhere around 10,000 years ago and earlier. The split is only going to get wider, and the north end of the East Pacific Rise move farther north. The 2012 Brawley quake swarm may be nothing more than a small rumble, not causing more that a little damage, but it's part of a gigantic process that will inevitably bring catastrophic change to California.

Chris Clarke is an environmental writer of two decades standing. Director of Desert Biodiversity, he writes from Joshua Tree regularly at his acclaimed blog Coyote Crossing and comments on desert issues on KCET weekly. Read his recent posts here.