Quake Jolts Central Orange County : Moderate Quake Made a Big Boom to Those Nearby

The earthquake that jolted Orange County on Friday afternoon was described as moderate by seismic experts, but it seemed like The Big One to people riding the epicenter directly under them in Newport Beach.

The quake, which was a magnitude 4.6, was a “strike-slip” quake, in which two segments of the earth’s Pacific plate are believed to have slid horizontally into each other, according to Steve Bryant, a seismic analyst at Caltech in Pasadena. Earth noises--in this case a loud boom rather than a rumbling--accompanied the underground action.

“People were startled--it shook pretty good and in a wide area,” he said. “Very near the center of the earthquake, people felt the very high frequency, rapid vibrations and a big boom. As it got farther out, the quake felt more like a rolling motion.”

The temblor was felt most strongly in central Orange County, but it rattled people and buildings over hundreds of miles, from San Diego to downtown Los Angeles, Santa Monica and Riverside. At the outer reaches, the intensity of the quake depended on ground composition, with buildings on softer soil shaking with stronger force.

Orange County’s Santa Ana River basin, extending to the coast in an area called an alluvial fan, is composed of sandy soil with a high water table and is prone to extreme shock. Only relatively minor damage, however, was reported in the area Friday.

“On hard rock, a quake is felt a lot less than on certain types of soil,” Bryant said. “And people on upper floors will feel a quake a lot more than people on lower floors.”

Another expert, UC Irvine civil engineering department chairman Medhat Haroun, described the quake as generating a strong upward motion in the Newport Beach-Irvine area. However, he said that in Santa Ana, a horizontal movement a was reported.

“I thought it was an explosion, some kind of underground explosion,” he said. “The motion I felt was sharp and vertical, not a swaying at all. I felt two significant shocks and nothing after that.

“I think that is typical if you are close to the area of the earthquake. As you get farther away, the motion is usually less severe.”

Haroun, whose specialty is measuring how buildings and other structures weather such underground blasts, estimated that Friday’s quake measured 6 to 7 points on the 12-point intensity scale.

Sensitive devices that measure how buildings react to quakes, called Strong Motion Accelerographs, recorded the movement of the Engineering Building on film from the roof and in the basement. No data was available Friday from the film, will be analyzed over the next several days.

The quake is believed to have struck along the Newport-Inglewood Fault, the geological fracture that caused the deadly and devastating Long Beach earthquake in 1933. That quake, which had a magnitude of 6.3, killed 127 people and collapsed buildings and schools, causing $40 million in damages. The epicenter of the 1933 quake was near the intersections of Brookhurst Street and Pacific Coast Highway in Huntington Beach.

Friday’s quake was consistent with 100 years of activity along the fault line and not likely a precursor to “something big,” Bryant said. Earthquake experts have predicted that a major quake in the 7.5 to 8.3 range will strike Southern California in the next 30 to 50 years, killing up to 50,000 people.

“This does not portend anything,” according to Bryant. “Aftershocks are always a possibility, but with an earthquake of this size, an aftershock would probably not be very big.” Still, he said, there is a small chance that Friday’s quake could be a foreshock of a larger quake to come.

The sharp vertical movement of the quake, particularly in the area close to the epicenter, was very different from the motion felt in Orange County on Oct. 1, 1987, when a quake measuring 5.9 killed eight people in Whittier and heavily damaged many of the buildings in the city’s old downtown.

The Whittier quake was a “thrust” or “dip-slip” action, in which one side of a fault moves vertically against another side. Although the Whittier Fault extends along the northeastern border of Orange County, there was little damage outside of La Habra and Buena Park, and county residents described the quake as a forceful rolling motion.

Southern California has 10,000 to 13,000 earthquakes a year, with a higher than normal number of quakes recorded in the Los Angeles basin in the last year. Such increases have sometimes preceded a major earthquake but, more often, the cycle has faded uneventfully.

“Ninety-nine percent of those quakes are so mild you don’t feel them,” Bryant said.