汶川地震和芦山地震的有助于预报的地震前兆

Abstract

是否存在“有助于预报的地震前兆”？马瑾院士由岩样加载实验发现岩石加载临近强度极值进入亚失稳阶段，发出应变、位移、温度变化特征信号，之后岩样破裂。大地震前是否能观测到这些信号？观测到了，如何识别应力已达峰值强度？姑咱地震台YRY-4分量钻孔应变仪汶川大震前550天中接收到3700多个异常应变脉冲，震后衰减。2014年底发现，脉冲出现初期，表征脉冲数据自洽性的k值大于0.99；大震前夕k值逐渐减小至0.4，数据失洽。地层连续无裂隙是应变数据满足自洽的条件，失洽是震源区地层连续性破坏越来越严重，大震将发生的标志。汶川大地震前五个月时出现5天中一个脉冲事件也没有的平静期，脉冲事件再度增多后大地震发生。岩石加载声发射现象，加载接近峰值强度出现声发射平静期，继续加载声发射再现后岩样破裂。在数千脉冲事件背景下，出现数据失洽及平静期，这两项特征有助于判断地层加载进入亚失稳阶段及何时达到强度峰值，地震必将发生。2013年芦山7级地震前的440天中，姑咱台又记录到1000多个脉冲事件，震前2个月时又出现5天平静期。在适当位置布设精密应变观测网，有可能观测到，并能识别出“有助预报的地震前兆”。汶川、芦山地震观测中得到的认识，具有普遍性还是特例，还要在实践中接受检验。 Whether there exist “earthquake precursors that facilitate earthquake prediction”? From rock experiments Ma Jin, Academician of CAS P, found that when the specimen is loaded near to its strength limit and enters a meta-instable stage, it emits characteristic signals of strain, displacement, and temperature, and then the rock specimen breaks. Before a large earthquake can we observe such signals? If we can, how do we know the stress has reached the peak strength? In 550 days before the Wenchuan earthquake the YRY 4-component strainmeter in Guzan seismic station received more than 3700 abnormal strain pulses, which were then attenuated after the earthquake. In the end of 2014 when the pulses started to appear, it is found that the k value, which characterized the self-consistency of pulse data, was greater than 0.99; whereas the k value decreased gradually till 0.4 immediately before the earthquake, and the data lost self-consistency. For self-consistency of the strain data the Earth strata should be continuous and without cracks, the loss of self-consistency indicates that the continuity of strata in the earthquake source region is increasingly damaged and a large earthquake may occur soon. About 5 months before the Wenchuan earthquake there was a quiet period without a single pulse in 5 days, then after the impulsive events increased again the great earthquake took place. This is similar to the acoustic emission phenomenon during rock loading; a quiet period appears when the load approaches to the peak strength, with continuing loading acoustic emission reappears then the rock specimen breaks. On a background of thousands of impulsive events, the loss of data self-consistency and the appearance of quiet period are two features that help to judge whether the loading of strata has entered the meta-instable stage and when it reaches to the strength limit, thus leading to the inevitable occurrence of an earthquake. In 440 days before the Lushan M7 earthquake of 2013, the Guzan station recorded again more than 1000 pulses, and a quiet period of 5 days appeared 2 months before the earthquake. By setting up a precise strain monitoring network in proper locations, we may be able to observe and identify “earthquake precursors that are useful for earthquake prediction”. Whether the knowledge acquired from Wenchuan and Lushan earthquake observations is general or specific, it remains to be tested in future practice.