Earthquake early warning system

An earthquake early warning system detects seismic waves and rapidly alerts people or automated systems before stronger shaking reaches their locations. It provides seconds to tens of seconds of warning, not advance prediction of an earthquake.

P wave: A fast seismic body wave that compresses and expands material as it travels through Earth. Its early arrival can reveal an earthquake before slower, more damaging waves arrive.
ShakeAlert: An earthquake early warning system serving parts of the United States' West Coast. It illustrates how regional sensor networks generate public and automated alerts.
Earthquake prediction: An attempt to specify an earthquake's future time, location, and magnitude before it begins. Early warning detects an ongoing earthquake rather than predicting its occurrence in advance.
Lead time: The interval between an alert and the arrival of the hazardous event at a location. Distance from the source and communication delays determine how much action time remains.
S wave: A seismic body wave that shears material perpendicular to its direction of travel. Its later arrival often marks the onset of stronger shaking that an alert aims to precede.
Earthquake Early Warning (Japan): Japan's nationwide system for issuing warnings of strong shaking before it arrives. Its alerts reach phones, broadcasters, railways, and other systems across a densely populated region.
Earthquake forecast: A probabilistic estimate of earthquake likelihood over a stated area and time period. Forecasts describe future risk over longer periods, unlike seconds-scale event alerts.
Blind zone: An area near an earthquake's source where strong shaking may arrive before an alert. No system can overcome the short travel time between nearby sources and locations.
Seismometer: An instrument that measures ground motion, including motion caused by seismic waves. Networks use its measurements to detect earthquakes and estimate their location and size.
Earthquake warning system in Mexico: A public earthquake alert network that uses sensors near the Mexican Pacific coast. Its coastal detection can provide warning to Mexico City before waves reach the capital.

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