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.