Calif. Quake Location Scenic and Seismic

PASADENA, Calif. – The earthquake that struck Central California on Monday occurred in a region of scenic beauty and seismic activity, where the nearby clash of the Pacific and North American plates is actively pushing up coastal mountain ranges.

The magnitude-6.5 quake is believed to have ruptured along an estimated 15-mile segment of the Oceanic fault in San Luis Obispo County (search). The fault zone frequently experiences small and moderate temblors, as well as infrequent large quakes, seismologists with the U.S. Geological Survey (search) said.

"We know over the long term this kind of earthquake can be expected to recur many times -- at least over geologic time," Bill Ellsworth, chief scientist of the USGS's earthquake hazards team in Menlo Park, told reporters.

At least 50 aftershocks, of magnitudes 3.0 and larger, hit in the hours after the late morning quake. Dozens more were likely, said Lucy Jones, scientist in charge of the Pasadena office of the USGS.

"We'll be having aftershocks for months in that region and people should expect it," Jones said.

The yet-to-be pinpointed fault in Monday's quake is what's called a thrust fault. In thrust faults, one block is pushed up and over another, as if being moved up a ramp.

The faults are formed by compression or squeezing of the Earth's crust. That action pushes up mountains like the Santa Lucia Range (search), which plunges down to the ocean along California's scenic Highway 1, about midway between Los Angeles and San Francisco.

"These are the faults associated with mountain building," said David Schwartz, a USGS geologist in Menlo Park. The quake was only slightly smaller than 1994's magnitude-6.7 Northridge quake, which also hit on a thrust fault.

That earthquake hit a densely populated area and killed 72 people, injured 9,000 and caused an estimated $15.3 billion in insured losses. Monday's quake hit a relatively sparsely populated region.

The northwestward motion of the Pacific plate relative to the North American plate dominates the tectonics of central California. The fault that ruptured Monday can be considered a secondary component of that movement, Schwartz said.

Most of the movement is accommodated by movement on a second kind of fault, called a strike-slip fault. The San Andreas, located about 35 miles inland from the epicenter of Monday's quake, is the most famous of California's strike-slip faults.

Monday's quake was too small to trigger quakes on the notorious San Andreas, nor was it expected to increase the likelihood of quakes there. If anything, it may have had an "inhibiting" effect on the fault, Jones said.

Previously, the largest temblor in the area was a magnitude-6.2 quake that struck in 1952, according to the USGS.

Monday's quake was the largest in California since a magnitude-7.1 earthquake in the remote desert east of Los Angeles in 1999.