Tham khảo tài liệu 'earth sciences - notable research and discoveries part 9', khoa học tự nhiên, địa lý phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 172 EARTH SCIENCES A technician at the Geotechnic Research Center in San Salvador El Salvador monitors seismometer recordings. Yuri Cortez AFP Getty Images tem needs enough sensors to cover a broad area so that there is a good chance an earthquake will be detected early an earthquake arising between detectors situated too sparsely would travel a long time before being noticed and the warning would come too late for people who lived nearby. Japan s system employs about 1 000 sensors buried underground. The sensors transmit information to a computer network that monitors and analyzes the data. A few seconds after an earthquake the computer estimates the location and intensity and sends a warning to the affected region if the earthquake is serious enough to pose a threat. The alarm goes out on the major television and radio channels. Instructions are also provided to prevent a panic such as a massive rush for the exits in crowded buildings. Some of the response can be auto- mated for example switches to shut off heavy machinery and elevators can be linked to the warning system. People may also have time to turn off gas lines reducing the chance of a fire in their home or apartment. Fires are especially dangerous because the rubble and broken water lines make fighting fires exceptionally difficult in the aftermath of an earthquake. Much of the destruction of the 1906 San Francisco earthquake resulted from fires that burned out of control. Seismologists and engineers in California are studying plans to build a similar warning system but Japan s system is not yet perfect. Its first alarm issued in April 2008 for an earthquake on the island of Okinawa having an approximate magnitude of came a few seconds too late to provide adequate warning. The earthquake was minor though and did little damage. Warning systems are also necessary for tsunamis. No alarms went out during the Indian Ocean tsunami of 2004 which caught everyone off guard. Undersea earthquakes cause these .
