THE CHARACTERISTICS OF REGIONAL STRESS FIELD IN ANHUI AREA AND THE COHERENCE CHARACTERISTICS OF FOCAL MECHANISM SOLUTIONS BEFORE MID-STRONG EARTHQUAKES

246 focal mechanism solutions of M_L≥2.0 earthquakes in Anhui and adjacent areas since 1974 are calculated using maximal vertical amplitude ratio of Pg and Sg.By using system clustering method, preponderant composite focal mechanism solutions of each subarea in different period is calculated to analyze the characteristic of regional stress field in each subarea.Statistic analysis of stress axes parameters shows that the predominant direction is approximate to the stress field direction in East China.The coherence parameter of focal mechanism solutions in Anhui Area drops obviously before mid-strong earthquakes.