Earthquakes, Early and Strong Motion Warning
Richard M Allen
Seismological Laboratory, University of California, Berkeley
in Encyclopedia of Solid Earth Geophysics, Harsh Gupta (ed) Springer, p226-233, 2011.
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Strong motion earthquake warning (also known as earthquake early warning) is the rapid detection of an earthquake underway, the estimation of the ground shaking likely to result, and the issuance of an alert to people in the region where the ground shaking is likely to be hazardous. The last decade has seen rapid development of methodologies for early warning which include the use of strong motion observations close to the epicenter to provide warnings at greater distances, the use of P-wave observations across a network to locate the event and map the distribution of likely strong ground motion, and the use of a P-wave detection at a single station to estimate the likely shaking intensity at the same station. These approaches have been listed in order of increasing warning time, but the additional warning time comes at the expense of certainty. The earlier a warning is provided the greater the likelihood of false and missed alarms and the greater the uncertainty in the estimated shaking intensity. Typical warning times range from a few seconds to tens of seconds; the upper limit is about one minute. Identified applications include personal protective measures where individuals move to a safe zone within a few feet; automated mechanical response including stopping trains and isolating sensitive and hazardous machinery and chemicals; and situation awareness by large organizations which can help prevent cascading failures and be available before shaking disrupts communications. Large-scale public warning systems are currently operational in Mexico and Japan, and smaller systems are used in Romania, Turkey and Taiwan. Testing is underway by seismic networks in many other countries, however, implementation will require significant financial, political and sociological hurdles to be crossed.