Most seismologists agree that predicting
earthquakes days in advance is not going to be possible
anytime soon. But borrowing from a system developed in Japan,
scientists have devised an early warning system that will
alert southern California residents seconds before a temblor
begins.
When an earthquake strikes, the first pulse of energy to
radiate from the point of origin comprises primary (p) waves.
P waves travel about two times as fast as the secondary (s)
waves that follow them and that carry the destructive,
ground-shaking power characteristic of large quakes. The s
waves are traditionally used to assess the magnitude of an
event, but this information is only gleaned after the fact.
The new system measures the first waves to arrive -- the p
waves -- and thus provides warnings up to tens of seconds
before the ground begins to move. That might seem like a
negligible amount of time, but it's enough to take cover
beneath a table or shut off gas lines and water mains.
Richard Allen of the University of
Wisconsin and Hiroo Kanamori of the California Institute of
Technology describe the system, called ElarmS, today in the
journal Science. They developed ElarmS using historical
earthquake data collected by an array of sensors spread
throughout southern California. Measuring the frequency
content of the p waves allowed Allen and Kanamori to figure
out what the damage potential from a given quake might be at
any point in the region. The system works because smaller
earthquakes tend to send out high-frequency p waves whereas
large-magnitude events radiate lower frequency energy.
ElarmS is designed to
detect earthquakes small and large, because "only if it works
for small magnitude events can we be sure the system is
operational," Allen says. "If we're only looking at
magnitude-6 earthquakes we'd only be testing every 30 years or
so, and that's not practical." Now those seismologists that
run southern California's already-established network of
sensors, TriNet, are integrating Allen and Kanamori's computer
program into their system. ElarmS will be tested using
real-time data from small earthquakes over the coming years.
Allen believes it will be most useful in developing safer
automated transportation systems, buildings and city
infrastructure that can be programmed to react to incoming p
waves. "It is perhaps not reasonable to expect people to
respond to an alarm they hear once every 30 years," he says,
"but we have the potential now to build infrastructure that
responds to these warnings and protects occupants."
--Laura Wright
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