September 2, 2007

Sciencedude: 4.7 quake jostles Orange County

Gary Robbins

There was a 2.2 quake at 4:46 p.m. today beneath the sea floor, 16 miles west of Newport Beach.

A magnitude 4.7 earthquake erupted beneath the Santa Ana Mountains at 10:29 a.m. today, producing shaking that was widely felt across Orange County.

“This was a wake-up call that the Santa Ana Mountain system, combined with the Elsinore fault system, represent a seismic threat to Orange County,” said Eldon Gath, the geologist who runs Earth Consutlants International of Tustin.

When the quake hit, Gath was sitting at his desk in Tustin, finishing his disseration. The subject: The seismic origins and potential of the Santa Ana Mountains.

Gath says it is unlikely that the region would experience a significant aftershock from today’s temblor. But he said it’s currently unclear whether the quake was produced by a thrust fault system that pushes the mountain up, or by a strike-slip system, which moves the mountains northward into the Los Angeles basin. The nearby Elsinore fault, which could produce a 7.0 or bigger quake, is a strike-slip fault.

The Santa Ana Mountains have produced quakes in the 4.5 range in recent years. But Gath said that it has largely been a “quiet mountain.”

“The problem,” says Gath, “is whether the mountain is building up to something bigger.”

Dave Bowman, chair of geological sciences at Cal State Fullerton, said, “At first it looked like the epicenter of the quake was on top of the Elsinore fault. But that’s become clear as more data has come in. The quake could have started on the Santa Ana Mountain fault system.”

Among those who felt today’s quake was DeeDee Gollwitzer of Laguna Hills, who told us by e-mail, “I was sitting at my computer editing some images and I felt the walls of my home moving … It was like a couple of small jerks back and forth is the best way to describe it.”