Earthquake warning system
An earthquake warning system or earthquake early warning system is a system of accelerometers, seismometers, communication, computers, and alarms that is devised for notifying adjoining regions 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 starts at some location and progresses away from the 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 waves generate an abrupt shock. The shear waves generate periodic motion (at about 1 Hz) that is the most destructive to structures, particularly buildings that have a similar resonant period. Typically, these buildings are 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 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 by the train operator in urban rail systems such as BART (Bay Area Rapid Transit) and LA Metro. The appropriate response is dependent on the warning time, the local right-of-way conditions and the current speed of the train. As of 2016, Japan and Taiwan have comprehensive, nationwide earthquake early warning systems. Other countries and regions have limited deployment of earthquake warning systems, including Mexico (the Mexican Seismic Alert System covers areas of central and southern Mexico including Mexico City and Oaxaca), limited regions of Romania (the Basarab bridge in Bucharest), and parts of the United States.
