Adamant

    我们以日本的地震早期报警系统（Earthquake Early Warning system）为例，简单介绍一下地震早期报警的概念和应用，帮助大家了解并正确认识地震预测的难度和相关知识。

    日本在2006-08-01正式发布了其地震早期报警系统，能够提供地震发生前10-20秒的提前报警，以减少地震造成的损失。众所周知，地震是通过地震波传递的，其实地震波和声波一样都有一定的传递速度。如果在震中位置设置一台地震仪，一旦发生地震就可以在极短的时间内发现分析并将分析结果传递到报警终端。那么理论上讲，如果信息分析和传递所需的时间小于地震波传递的时间，这个“时间差”就是能够提供的预警时间。日本的地震早期报警系统正是运用了这一原理，从而达到抗震减灾的目的。当然，既然原理清楚了，那么这一系统的局限性也很明显：

1. 地震仪必须离震中很近，这就需要大量的相关设备按照一定的密度设置，密度越大，报警能力越强；

2. 预警时间取决于离震中的距离，只有离震中一定距离（比如100公里）的地方才能体现“时间差”，对于震中附近没有报警能力；

3. 预警时间存在上限，地震波的传递速度决定了只可能以秒为单位预警，不可能再有提高；

什么是地震早期预警系统

  The Earthquake Early Warning system provides advance announcement of the estimated seismic intensities and expected arrival time of principal motion. These estimations are based on prompt analysis of the focus and magnitude of the earthquake using wave form data observed by seismographs near the epicenter.
  The Earthquake Early Warning is aimed at mitigating earthquake-related damage by allowing countermeasures such as promptly slowing down trains, controlling elevators to avoid danger and enabling people to quickly protect themselves in various environments such as factories, offices, houses and near cliffs.

    地震早期报警系统可以对地震的预计强度和预期到达时间提供报警。这套系统能快速分析震中附近的地震仪观察到的波形数据从而得出地震中心和强度的报警信息。地震早期报警系统可以通过紧急制动列车，控制电梯等从而避免危险，使工厂，办公室以及家中的人们躲避以保护自己，从而达到抗震减灾的目的。

来源：日本气象厅/Source: Japan Meteorological Agency

http://www.jma.go.jp/jma/en/Activities/eew1.html

地震早期预警系统的局限性 

Timing
  The window of time from the announcement of an Earthquake Early Warning until the arrival of the main tremors is very short, i.e. a matter of seconds (or between several seconds and a few tens of seconds).
  In areas that are close to the focus of the earthquake, the warning may not be transmitted before strong tremors hit.

时间窗口

    地震早期报警系统所能报告的时间窗口非常有限，只能提供以“秒”为单位的预警（通常为地震到达前的几秒到几十秒）。离震中越近的地方，报警时间越短，甚至会在地震发生后才能报警。

False alarms
  When using data from only one seismograph, false Earthquake Early Warnings may occur as a result of noise from accidents, lightning or device failure.

错误报警 

    当从一台地震仪获得数据时，由于事故产生的噪音、闪电或者设备故障等原因都会使地震早期报警系统发生错误报警。

Magnitude estimation
  There are limits to the accuracy of estimating magnitude, especially for large earthquakes.
  It is difficult to separate earthquakes and provide accurate warnings when multiple earthquakes occur almost simultaneously or in close proximity to each other.

震级的预测

    特别是对于大型地震，震级预测的精确性有很大的局限。当震中附件多个地震几乎同时发生时，很难精确区分每个地震并报警。

Seismic intensity estimation
 There are limits to the accuracy of estimating seismic intensity by statistical attenuation formula, as well as limits to the prediction of land surface amplification.

地震强度预测

    根据统计学上的衰减公式预测得到的地震强度和地面放大作用是不够精确的。（注：也就是说会出现过高或过低估计地震强度的情况。）

来源：日本气象厅/Source: Japan Meteorological Agency

http://www.jma.go.jp/jma/en/Activities/eew3.html