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1976年7月28日唐山7.8级大地震没有临震前震,但是有中期前震。1975年4月1日至12月1日出现的一个由唐山向北东40—45°延伸的小地震条带就是该大地震的中期前震活动表现。它与后来唐山大地震的震源断层一致,只是延伸的更远些。前震条带上地震系列的能量方Σ2 1/E根之和随时间t的变化是:1975年11月9日地震之前Σ2 1/E随t线性增加,之后转为非线性的快速增加,这意味着介质性质发生了明显变化,震源断层迅速扩展。唐山大地震的这种前震活动特征归纳如下:1.呈条带状,小地震迁移方向指向震中;2.在其共轭方向上有相应的小地震条带,大地震发生在其与前震条带的交汇处;3.前震条带上地震系列的能量方根之和随时间t由线性增加转成非线性的快速增加。 1969年7月18日渤海7.4级大地震也具有符合上述特征的前震条带。如果在地震活动区布设了微震台网,并能及时分析研究小地震的时、空变化以及小地震条带上Σ2 1/E(t)变化情况,那么就有可能对唐山、渤海型大地震作出中期预报。前震特征的第一、二条可以用来预报未来大地震的地点;第三条用未作时间预报,根据异常持续时间可以估计未来大地震的震级。
On July 28, 1976, there was no pre-quake on Tangshan earthquake of magnitude 7.8, but there was a medium-term pre-earthquake. A small seismic strike extending from Tangshan to the east 40-45 ° northeast from April 1 to December 1, 1975, is the mid-term foreshock performance of the earthquake. It coincides with the focal fault of the later Tangshan earthquake and only extends further. The variation of the sum of Σ2 1 / E roots of the series of energies for the seismic series in the pre-seismic belt with time t is: Σ2 1 / E increases linearly with t before the earthquake on November 9, 1975, and then changes to a nonlinear and rapid increase Means that the nature of the media has undergone significant changes, focal fault rapidly expanded. The features of such foreshock activities of the Tangshan earthquake are summarized as follows: 1. In the form of ribbons, the migration direction of small earthquakes points to the epicenter; 2. There is a corresponding small seismic band in the conjugate direction, 3. The sum of the square root of the energy series of the seismic series on the front shock strip increases from linear to non-linear and rapid increase with time t. On July 18, 1969, the 7.4 magnitude earthquake in the Bohai Sea also possessed the foreshock stripe conforming to the above characteristics. If the microseismic network is set up in the seismic activity area and the changes of Σ2 1 / E (t) on the time and space changes of small earthquakes and the small seismic bands can be timely analyzed and analyzed, it is possible to analyze the changes of Σ2 1 / E (t) Make mid-term forecast. The first and second of the pre-earthquake features can be used to predict the location of future earthquakes. Third, the unpredicted time series can be used to estimate the magnitude of future large earthquakes based on the duration of anomalies.