松潘大震震源过程研究

STUDY ON THE SOURCE PROCESS OF THE SONGPAN GREAT EARTHQUAKE

摘要:

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。

Abstract:

Based on the viewpoint that there must be adjustment elements of stress at the ends of earthquake source region and medium of the source region is stronger than that in the neighbour.This paper,by means of the modulated small earthquake method,analyses the source process of the Songpan earthquake (M=7.2).The temporal-spatial pattern of small earthquakes indicates that the source process has three stages.
1.The first-stage (1970-May 7,1973)
In this stage,a gap appeared in the seismic pettern by the modulated small earthquakes,but no modulated small earthquakes still took place in the gap.It indicates that the homogeneity and the strength of source region are greater than those in its outside part.The background of source region is formed in this stage.
2.The second stage (May 8,1973-Dec.1974)
During this time,in the seismic pattern,non-modulated small earthquakes disappear in the source region,hut more active for strong and moderate earthquakes in the northern end of the source,which shows that the hypocentral area has been formed and stress is being accumulated.
3.The third stage (Jan.1975-Aug.15,1976)
In this stage,the junction regions of the modulated small earthquakes belts are formed at the ends of the Songpan source region.The non-modulated small earthquakes began to be active again in the source,which shows that the adjustment elements at the ends of the source have been formed.In this time,the source region has provided the conditions for the occurrence of earthquakes.
4.During a few months (about half a year) before the 1976 Songpan earthquake,in the epicentral area and its periphery (about 300×200sq.km),there an anomalous ratio of modulated small earthquakes,which means that there is a very high stress in the source and its periphery.By this we can judge the strong earthquake is coming.
5.There appeared peaks of small earthquakes on June 27,July 13 and Aug.15 in a large scale of the middle part of North-South earthquake belt before this earthquake,the first two of which are synchronized with solid tide,the last of which happened on the day before eanthquake.Such a synchronization means that a large earthquake is impending to occur.