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進入《TAIPEI TIMES》 Earthquake alerts to be sent faster: CWB
A screen grab shows a presidential earthquake alert sent on Feb. 6 following an earthquake in Hualien County.
UPGRADES: Shear waves created by earthquakes cause more damage, but are slower, traveling 100km in 25 seconds, making their detection crucial to early warning
By Shelley Shan / Staff reporter
The nation’s earthquake early warning system would send alert messages 10 seconds after an earthquake is detected when the construction of submarine electric cables is completed next year, down from 15 to 16 seconds, the Central Weather Bureau （CWB） said yesterday.
The system is designed to warn people that an earthquake has been detected and this would give people a few more seconds to seek safety or protect themselves, the bureau said.
The bureau has land-based stations and undersea cables to detect earthquakes.
For earthquakes whose epicenters are on land, alerts can be sent within 15 to 16 seconds of a temblor being detected.
However, if an earthquake occurs offshore, it can take 25 to 30 seconds for the system to transmit an alert.
Seismological Center Director Chen Kuo-chang （陳國昌） said that the bureau would finish installing 580km of undersea cables next year, which are to run from Toucheng Township （頭城） in Yilan County, through the Nanao （南澳） and Heping （和平） sea basins, and the Luzon Volcanic Arc to Hengchun Peninsula and end in Fangshan Township （坊山） in Pingtung County.
The bureau is improving the facilities used in earthquake detection stations and updating the software they use, which would shorten the time needed to transmit alerts to 10 seconds, Chen said.
The transmission time for offshore earthquake alerts would be reduced to 10 to 15 seconds, he said.
The early warning system would still not provide timely alerts for so-called “earthquake blind spots,” which are areas that are too close to an earthquake’s epicenter, Chen said.
Nevertheless, the bureau’s new facilities would reduce the blind spots from 50km from the epicenter to 35km, he said.
According the bureau, when an earthquake occurs, seismographs are able to record primary （P） waves and shear （S） waves. P waves travel faster than S waves.
The former cause very little damage, whereas S waves shake the ground perpendicular to the direction that the wave is moving.
The bureau said that if an earthquake were to occur 100km from a city, P waves would arrive at the city 15 seconds after the earthquake, whereas S waves would arrive in 25 seconds.
If a device that detects seismic waves is installed in a city, it would issue an alert when it detects P waves, giving residents 10 seconds to prepare themselves before the S waves arrive.
The time that a person has to react to an earthquake depends on their distance from its epicenter, the bureau said, adding that if an earthquake happened in Hualien, the early warning system might not be of use to them, but it could help those in Yilan County or Taipei.
新聞來源：TAIPEI TIMES
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