EEW

An earthquake warning system is a system of accelerometers, seismometers, communication, computers, and alarms that is devised for regional notification of a substantial earthquake while it is in progress. This is not the same as earthquake prediction, which is currently incapable of producing decisive event warnings.

An earthquake is caused by the release of stored elastic strain energy during rapid sliding along a fault. The sliding will start at some location and progress away from this hypocenter in each direction along the fault surface. The speed of the progression of this fault tear is slower than and distinct from the speed of the resultant pressure and shear waves, with the pressure wave traveling faster than the shear wave. The pressure wave will generate an abrupt shock while the shear waves can generate a periodic motion (at about 1 Hz) that is the most destructive in its effect upon structures, particularly buildings that have a similar resonant period, typically buildings around eight floors in height. These waves will be strongest at the ends of the slippage, and may project destructive waves well beyond the fault failure. The intensity of such remote effects are highly dependent upon local soils conditions within the region and these effects are considered in constructing a computer model of the region that determines appropriate responses to specific events.

Such systems are currently implemented to determine appropriate real-time response to an event in determining train operator response for urban rail systems such as BART (Bay Area Rapid Transit). The appropriate response will be highly dependent upon the warning time, the local right–of–way conditions, and the current speed of the train.

As of 2016, Japan and Taiwan are two countries with comprehensive nationwide earthquake early warning systems. Other countries and regions have limited deployment of earthquake warning systems, including Mexico (installed to issue alerts to Mexico City primarily), limited regions of Romania (the Basarab bridge in Bucharest) and parts of the United States. The earliest automated earthquake pre-detection systems were installed in the 1990s, for instance in California the Calistoga fire station's system which can automatically trigger a citywide siren to alert the entire area's residents. While many of these efforts are governmental, several private companies also manufacturer earthquake early warning systems to protect infrastructure such as elevators, gas lines and fire stations.

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