华北北部场源前兆观测资料分析(一)──场源前兆特征与机制

ANALYSIS OF OBSERVED DATA OF FIELD AND SOURCE PRECURSORS IN NORTHERN PART OF NORTH CHINA──THE CHARACTERISTICS AND MECHANISM OF FIELD AND SOURCE PRECURSORS

1976年唐山地震前，在距震源很远的地区内都观测到一些前兆趋势变化，如重力、重力位二次徽商W△、水氧、地电阻率、水位和油井出油量等变化。作者认为这些变化不是由震源体直接引起的，而是在区域应力场的作用下，在某些活动断层附近，浅层岩，尤其是含水砂岩层和含油层受挤压出现的一些与地震有关的异常现象。其特征是：（1）异常范围大，可能在距震源很远的地方发生，但就同一种方法的多个观测点来说却又是局部的，即只有其中部分测点才能观测到异常，不少测点观测不到异常。（2）异常发生的时间大致相同。有些异常有同步变化的特征，如同时上升或同时下降。（3）临震前多数异常有恢复的趋势。这些特征与引起异常的机理有关。作者还从理论上计算了这种趋势异常量级，重力变化100×10-6cm／s2左右，重力位二次微商变化（1～2）×10－9／s2，地电阻率变化2％～3％，Rn变化7．4～11．1Bq／L，这样的变化量在活动断层附近的一些台站可能观测到。作者还研究了干旱降雨对某些前兆的影响，其影响量级可以被一些方法观测到，因此在确定是否是地震异常时，必须注意利用综合分析的方法排除干旱降雨的影响，减少异常的多解性。

The tendency changes of precursors were observed by several methods in the large area far from the seismic focus before 1976 Tangshan earthquake, for example, gravlty,secondary derivative of gravitational potential W△, geoelectricity resistivity, radon, water level and output of oil well, etc. Authors point out that these changes are not direct effects of earthquake source. Under the action of strain field, and near some activity faults the shallow rocks especially the water-bearing stratum or oil-bearing rocks are compressed, resulting in occurrence of precursors related to the earthquakes. The characteristics of those precursors are that (1) the area of anomaly is large. These anomalies may occur in the region far from the seismic source, but they are local for the more observed points by using the same method, namely, they can be observed in part of those points only，but for others, they can be not. (2) The starting time of anomaly occurrence is approximate. It shows the feature of synchronous change. (3) There is recovery tendency for many precursors before earthquake occurrence. These characteristics are related to the mechanism resulting in precursory anomaly. The order of magnitude of some precursors is calculated by authors. The change of gravity is about 100×10-6 cm/s2, the secondary derivative change of gravitational potential W△, is about (1~2)×10-9/s2. The change of geoelectricity resistivity is about 2% ~ 3%, for radon is about 7. 4 ~ 11. 1 Bq/L. These anomalous volumes could be observed by using this method at the stations near the active fault. Authors studied the effects of drought and precipitation on some precursors by some methods. Therefore, when we try to determine the earthquake precursors, we have to eliminate the influence of drought and precipitation and reduce the multiple solutions by analysing method.