大震前震源区的确定与大空区形成的物理机制

DETERMINATION OF EARTHQNAKE PREPARATION AREA AND PHYSICAL MECHANISM OF FORMING BIG SEISMIC GAP

摘要:

震源组合模式表明^[1],一个震源的形成必须具备应力积累条件和应力释放条件,因此,震源端部必须存在岩石强度小或摩擦阻力小的弱介质区段。在大区域构造应力场的作用下,弱介质区段由于不能承担很大的应力而把应力转移到邻近岩石强度高的地方去,在那里积累应力形成震源。

Abstract:

In this paper we investigate the method of finding seismic gap and determining earthquake preparation area and discuss the physical mechanism of forming big seismic gap from dynamic view point.
1. Determination of earthquake preparation area
The occurrence of a great earthquake has a dynamic action on the neighbouring crustal medium. This action is different for various media with different strength. In the regions with low rock strength, moderate or small earthquakes occur, but seismicity is low in the regions with high strength. Therefore we regard the dynamic effect of great earthquake occurrence as a detective measure for finding the seismic gap and deciding the earthquake preparing region located in some distance from the former earth-quake. According to the above-mentioned method and the combination model of source formation (i. e. the earthquake source consists of the stress accumulating element and two stress adjustment elements located at both ends of the stress accumulating element), we ha used the dynamic detective effect to check the seismic gap and the earthquake preparing region of four great earthquakes located in sourth western China. The estimated intensities are nearly the same as the actual earthquake intensities.
2. Physical mechanism of the formation of big seismic gap
In general, there are two kinds of seismic gap. The first is a smaller gap with its areas approaching the dimension of the source volume. Such gaps can be explained with the combination model. The second is a big gap with its dimension much bigger than that of the source volume. A new explanation for the formation of big gap is proposed. A big gap may be formed when the vertical movement of the upper manlet acts on the overlaying crust in a very big area, in which the preparing source is involved. As the vertical movement can affect the medium strength and stress distribution, the gap, as mentioned in the combination model, will be enlarged. In this paper physical discussion and theoretical calculation for the formation of the big gap are given.