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本文应用调制小震法对松潘地震前的震源过程进行了剖析,初步得到震前六年半(直至发震)这一时段内震源区和源外调整单元地震活动的时空图像,其特点如下: 1.第一阶段(1970—1973.5.7)在此阶段其地震活动的空间图像主要是形成调制小震带的围空区和空段区,但在震源区仍有非调制小震活动表明,该时段震源区和外围区具有不同的介质强度,背景震源区初步形成。2.第二阶段(1973.5.8—1974.12)在此阶段,地震活动的空间图像主要表现为震源区内非调制小震的消失,震源区北端中强震活跃。这说明震源区形成,应力正在加速积累。3.第三阶段(1975.1—1976.8.15)在此阶段未来震源区两端出现调制小震带的交汇区。震源区内非调制小震恢复活动。说明震源区两端的应力调整单元已基本形成,震源区应力水平相当高,震源已具备发震条件。4.在第四阶段的最后半年震中区以及外围(约300×200平方公里)小震调制比出现异常。说明震源区及其邻区应力水平已相当高,这是松潘大震来临的中期指标。5.临震前二个月内震源外围出现与外因时间同步的小震频度峰值,分别出现在6月27日、7月13日和8月15日。根据它们的同步性以及异常时间的倍九特征可进行松潘大震的短临预报。根据松潘震源过程的研究,利用小震在不同阶段的震源过程时空特征,有可能对一次大震作出不同时间尺度的预报。
In this paper, we analyze the source process before the Songpan earthquake using the modulated small earthquake method, and obtain the space-time images of the seismic activity of the source and the source outside the adjustment unit in the period of six and a half years before the earthquake (up to the onset of earthquake). The characteristics are as follows: 1. The first stage (1970-1973.5.7) The spatial image of the seismic activity at this stage mainly formed the enclosed space and empty section of the modulated small seismic zone. However, the non-modulated small earthquakes still showed in the source area, In this period, the source region and the peripheral region have different dielectric strength, and the background source region is initially formed. 2. The second stage (1973.5.8-1974.12) At this stage, the spatial image of seismic activity mainly shows the disappearance of the unmodulated minor earthquakes in the source area, and the strong earthquakes in the northern end of the source area are active. This shows that the source region is formed and the stress is accelerating. 3. The third stage (1975.1-1976.8.15) At this stage, there will be the intersection of modulated small seismic zones at both ends of the future source area. Non-modulated small earthquakes in the focal area resume activity. It shows that the stress adjustment unit at both ends of the source region has been basically formed, the stress level in the source region is quite high, and the source has the condition of earthquake occurrence. 4. During the last half of the fourth phase, the modulation ratio of small earthquakes in the epicenter area and in the periphery (about 300 × 200 km 2) was abnormal. It indicates that the stress level in the source area and its adjacent area has been quite high, which is the mid-term indicator of the Songpan earthquake. In the two months before the quake, the frequency of the small earthquakes occurred in the periphery of the epicenter synchronously with the exogenous time appeared on June 27, July 13 and August 15 respectively. Based on their synchronicity and the times of abnormal times, the short-term prediction of Songpan earthquake can be carried out. Based on the study of the Songpan source process, it is possible to make predictions on different timescales of a major earthquake using the space-time characteristics of the source processes of small earthquakes at different stages.