On correlation between seismic activities and focal mechanisms of subsequent strong earthquakes

On the basis of the results from spatial-temporal scanning of moderate seismic activities prior to 63 strong earthquakes occurred since 1966, we investigate in this paper the characteristics of anomalous evolution of moderate seismic activities prior to strong earthquakes with different focal mechanisms in different tectonic blocks, as well as the correlation between principal seismic activities and nodal planes of focal mechanisms. The research results indicate that the moderate seismic activities prior to strong earthquakes display two significant stages of stress concentration and stress weakening, which are consistent to the evolvement of local tectonic stress field. The first stage, defined as medium-term stage, is characterized by seismic belts, seismic gaps and centralized activities; The second stage, defined as short-term stage, is predominated by continuous quiescence. For the earthquakes occurred on the strike-slip and normal faults, the two stages are relatively evident; while for the earthquakes occurred on the thrust faults, there are not characteristic images and seismic quiescence in both stages. The spatial distribution of moderate shock belts before strong earthquakes show a good coherence with that of the two nodal planes of focal mechanisms, while the spatial distribution of seismic gaps enclosed by moderate shocks display a poor coherence with the nodal planes.