地震預警系統建設

After experiencing heavy casualties and extensive damage in powerful earthquakes since 2008, China is developing a way to give at least a few extra seconds' warning for those near quake epicenters.

The system, called the National Earthquake Early Warning and Intensity Quick Reporting System, which will have an estimated cost of about 2 billion yuan ($324 million), is expected to be built before 2017 by the China Earthquake Administration — if the National Development and Reform Commission approves the project this year, the administration said.

The administration said the system will detect earthquake-related shaking that are in the form of P-waves, which move rapidly but are harmless, and which arrive before the more-powerful and destructive shear waves, or S-waves.

"As earthquakes cannot be predicted, getting people to safety quickly before the destructive waves arrive can largely decrease casualties," Li Shanyou, deputy director of the Institute of Engineering Mechanics under the China Earthquake Administration, told China Daily.

He added that the few seconds of warning can also help a train driver slow down to avoid possible derailment, warn a doctor to stop a surgery, give a shutdown signal to chemical factories, prevent potential leaks and fires, and alert officials at critical projects, such as nuclear plants.

Experts estimate that casualties could be reduced by 14 percent with a three-second advance warning of an earthquake, and 39 percent with a 10-second warning.

The program includes the construction of more than 5,000 quake-monitoring stations across the country, guaranteeing every city above county level will have at least one. The system will be in four major areas — northern China, the north-south seismic belt, coastal regions of southeastern China, and northwestern Xinjiang Uygur autonomous region — with a station every 25 km.

Li said if quake stations are about 30 km apart, the early-warning system can issue warning information about 10 seconds after a quake happens below ground.

Detecting P-waves with the system can provide a few precious seconds to people before the powerful S-waves arrive.

"The S-waves generally move at a speed of 3.5 km per second, which means before the warning is issued, S-waves have already spread in a radius of 35 km from the epicenter," he said, adding that the area within the 35-km radius of the epicenter, which is called the "blind area", cannot be warned.

Lushan county in the Ya'an earthquake in Sichuan province was in the blind area, Li said.

The early warning cannot play a role in small quakes, but if it is large with a huge affected area, the system will dramatically reduce damage and lead to fewer deaths, he said.4月20日，四川省雅安市蘆山縣發生7.0級地震，截至週四三已造成196人遇難；2010年4月14日，青海省玉樹縣發生7.1級地震，2,698人遇難；2008年5月12日，四川汶川發生8.0級強震，遇難和失蹤人數達8.7萬人。

面對如此頻繁且震級較高的地震，建立“國家地震烈度速報與預警工程”已迫在眉睫。日前中國地震局工程力學研究所副所長李山有在接受《中國日報》專訪時對該工程進行了詳細的介紹。

“國家地震烈度速報與預警工程”是由覆蓋全國的5000餘個地震臺站組成，工程完成後，不僅全國範圍內每個縣級以上城市都將佈設1個或1個以上的地震臺站，平均臺站間距爲40公里左右；而且華北地區、南北地震帶、東南沿海地區及新疆西北部地區等四個區域將組成平均臺站間距爲25公里左右的地震觀測臺網，具備對破壞性地震的預警能力。

據悉該工程已於2012年12月正式進入國家發改委立項評估階段，計劃投入約20億元，建設時間爲5年。

李山有副所長還介紹了該工程的預警原理是利用地震產生的縱波(縱波傳播速度快，能量小，一般不會造成破壞)和橫波(橫波速度相對較慢，但能量大，是造成地震破壞的主要原因)的傳播時間差，在自動監測到縱波後快速估算地震參數，搶在橫波傳到某地前發出警報信息。這樣可以爲離開地震震中一定距離(預警盲區)的外圍區域提供數秒至數十秒的預警時間。

公衆利用這短暫但寶貴的時間可以因地制宜地採取避震或逃生措施；企業利用這短暫的時間可以提早採取應急處置措施，例如行駛中的高速列車可制動減速避免出軌翻車，輸油氣管線可關閉運行避免油氣泄露引起污染或火災，醫院可中止精密手術(如眼科)避免操作事故，化工企業可緊急終止可能產生次生災害操作等，核電廠等重大工程可提前進入應急狀態避免次生災害。

根據《西北地震學報》的報道統計，3秒的地震預警時間，傷亡減少率將達到14%；10秒，則可減少傷亡39%；20秒，可減少傷亡63%。

不過預警系統也有其自身的侷限性。

李山有說如果地震臺站的間距爲30公里，預警系統進行數據等待和處理的時間大約爲10秒，而破壞性地震橫波以大約每秒3.5公里向外傳播，10秒鐘將覆蓋半徑爲35公里的區域，而這個區域無法提供預警，稱作預警盲區。

例如此次雅安地震，位於震中的蘆山縣就是預警盲區。

如果地震的震級不是很大，可能產生破壞的範圍就處於盲區之內，此時再向預警盲區外的區域發佈預警信息沒有減災意義。

但是，李山有介紹對於大地震來說，其破壞範圍可能很大(如汶川地震有300多公里)，可預警的潛在地震破壞區域也很大，距離震中越遠，預警時間越長，減災效果就越明顯。

除了爲地震外圍區域提供預警，地震烈度速報與預警工程可以在震後數分鐘後給出地震影響的程度和分佈，爲政府部門災情判斷、應急決策和救援力量調配等提供依據，爲生命線工程的震後搶險恢復提供參考。