Timing Is the Key in Quake Calculations

For seismologist Lucille Jones, Wednesday night’s 5.0 magnitude earthquake happened at exactly the wrong time.

For one thing, Caltech’s sophisticated new computer had gone on the blink, making it impossible to get instant readings of earthquake magnitudes and locations. Also, the seismologist in charge of the regional earthquake monitoring network--a woman who is reluctant to take vacations for fear of missing an earthquake--had just left for Hawaii . . . on vacation.

And Jones, who had been trying since 9 o’clock to get her 2-year-old son to sleep, finally succeeded a little before 11 p.m.

That’s when a sharp jolt brought Jones scurrying to Caltech’s earthquake lab, where she and a crew of eight or nine other scientists began the hasty task of determining the size and location of the quake and notifying government agencies that might have to handle injuries or damage.

The team, representing Caltech and the U.S. Geological Survey, runs a network of 256 earthquake reporting stations scattered from Big Sur and Mammoth Lakes to the California border. Several stations are located on Santa Catalina and other offshore islands.

The far-flung network automatically feeds 25,000 bits of information each second into two banks of computers at Caltech’s Pasadena campus. But any time the instruments detect a quake of 4.0 magnitude or greater, the scientists go to work. If it happens at night, an alarm sounds on campus and night-shift school employees put out a call--by telephone or beeper--to the seismologists.

Respond Like Firefighters

They respond much like firefighters, racing the clock to reach their computer keyboards.

“It’s not so critical that we’ll run red lights or speed, but we’ll drop whatever we’re doing and come,” said seismic analyst Steve Bryant, who lives a few blocks from the lab. Like many quake experts, Bryant often can tell much about a temblor’s size and location as soon as he feels it.

Nearby quakes produce sharp, short vibrations. Far-away quakes create slower motions. Many times, if he is sitting or lying very still, he can roughly calculate that distance and the quake’s general size in his head. But not on Wednesday. This one caught Bryant off guard.

“I was in the shower,” he said. “They do hit us at some of the worst times.”

Raced to Lab

Although Wednesday’s quake caused a few minor injuries, seismologists could tell immediately that the temblor was one they would need to analyze. Jones did not wait for a phone call. She raced to the lab in about 10 minutes, encountering several other staff members who had beat her there.

The earliest arrivals confirmed the quake by looking through computer printouts of activity at the monitoring stations. All 256 stations picked up the quake, the seventh 5.0-or-greater temblor to strike Southern California since 1987. (Typically, about four such quakes are reported each year statewide.)

Scientists then turned to their computer terminals and seismographs to undertake more detailed studies. Comparing the times when the shock waves arrived at various stations, they placed the epicenter south of Malibu in Santa Monica Bay. They looked at the amplitude of the chart readings. On many of the seismographs, the needles had swung off the charts. But enough data was available to gauge the magnitude.

News Crews Informed

The calculations took about 20 minutes, far longer than they might have with a new, high-powered computer that is inexplicably on the fritz, Jones said. But the findings were announced just in time for television news crews waiting outside the lab door shortly before 11:30, she said.

Bryant described the quake as moderate, not quite large enough to be damaging. Its location, believed to be on the Torrance-Wilmington fault, was several miles offshore, and it was relatively deep, about 7 miles down. Those two factors helped keep down injuries and property damage, the seismic analyst said.

The typical Los Angeles-area quake is about 4 miles down, he said.

Bryant said the quake was felt over a relatively large geographic area in part because it occurred at such a late hour of the evening. “At 11 o’clock, a lot of people are in bed, and they’re fairly still,” he said. “That helps.”