When Mexico City radio stations sounded a warning 50 seconds before a magnitude 7.2 earthquake struck on Sept. 14, queries poured in to the Pasadena field office of the U.S. Geological Survey.
"People wanted to know why we don't have a warning system here," said Jim Mori, the Geological Survey's scientist-in-charge in Pasadena. "Even a member of the [Los Angeles County] Board of Supervisors was curious."
And the answer?
"We had to tell them that here, it's more difficult."
But not impossible. The federal Geological Survey, Caltech and two dozen local companies and utilities are collaborating to develop an automated system they hope will soon flash a warning when a damaging earthquake begins in Southern California.
Such a system would rely on the physics of quakes to give a warning. The most damaging energy waves unleashed by a quake surge through the ground at about 2 1/2 miles a second. The epicenter of the quake two weeks ago was 190 miles from Mexico City, so the ground didn't begin to shake in the capital for about 76 seconds.
The warning system uses sensors to detect the initial ground shaking, then feeds that information back to a computer ahead of the approaching seismic waves. Radio stations in Mexico City received an automated alert that triggered a broadcast urging listeners to take cover.
While quakes felt in Mexico City often begin near the distant Pacific coast, Los Angeles faces a different situation. Shock waves from the nearest point on the mighty San Andreas Fault are only about 16 seconds from Downtown Los Angeles (and waves travel from Northridge to Downtown in eight seconds)--hardly enough time for a signal to be picked up, processed and broadcast so that people can react.
Better if a major quake should occur on the San Andreas Fault near Indio. The shaking would take about 45 seconds to reach Los Angeles, allowing more warning time.
According to Caltech seismologist Egill Hauksson, it will be at least five years before an early warning system comes on line in Southern California for utilities and law enforcement.
It will be even longer before the public hears sirens or broadcasts that relay the warnings.
But eventually, according to Hauksson and other scientists on the project, the Southern California system will be more elaborate and reliable than the one in Mexico.
Although plans in Mexico call for relaying alarms directly to schools and hospitals and airing warnings on television as well as radio, false alarms have been an occasional problem. Relatively few sensors are used to detect the start of an earthquake.
To avoid false alarms, the California scientists have decided that at least three seismographic sensors will have to record shaking before any warning is disseminated. And because earthquakes here can come from almost any direction at any time, "we're going to need 200 to 250 sensors [compared to 16 now]," Mori said.
In a briefing last week, Hauksson said that when the $18-million Southern California system is fully operational, it should be able to display a computerized map of the shaking intensities of a magnitude 6 earthquake centered 175 miles from Los Angeles even before any shaking begins in the city.
A first warning that a powerful quake is occurring, and where it is, will arrive before the shaking for any magnitude 6 quake more than 35 miles away, he said. Ground motion estimates will arrive before the shaking for such a quake more than 50 miles away.
(Already, Caltech has a computer that flashes news that a big quake is occurring to the school's Seismological Laboratory before the shaking arrives, but it doesn't say where the quake is located).
For larger quakes, the first warnings will come as quickly. But in magnitude 7 and 8 temblors, because of the greater duration of the shaking and larger size of the rupture zone, the ground motion readings and maps of the shaking will take longer.
Hauksson said that shaking maps are a higher priority than early warnings because having quick information about the magnitude, location and distribution of a quake's power will help police and fire departments, electric and gas utilities and railroads respond to emergencies.
Hiroo Kanamori, director of the Caltech Seismological Laboratory, expressed some doubt about the value of an early warning system for the general public. "It's not too useful for ordinary people--at least ones who are not well educated on what to do," he said.
Hauksson recalled that the late Charles Richter, a famed Caltech seismologist, always said that the safest thing to do in an earthquake was to remain inside for at least 15 seconds. Then, if the shaking was intensifying, one should run outside--with a wary eye peeled for falling objects--because the building might collapse.