Source parameters and correlation coefficients of focal mechanisms for Yingjiang earthquake sequences in 2011

Source parameters of the 2011 MS5. 8 Yingjiang earthquake sequence were calculated by using the digital waveform data and the Brune 's source model theory. The correlation coefficients are calculated using the low-frequency spectral amplitude. Combined with the focal mechanism solutions of the MS≥4 earthquakes,the events with similar focal mechanism solutions can be grouped by clustering method. The results show that there are 2 dominant focal mechanic groups with high correlation coefficients in the earthquake sequence: The one is the type of mainshock focal mechanism,they are strike-slip fault,and the principal directions of compressive stress P-axis are NNE with lowplunge. The other is the type related to the local structure,there are normal faulit,and the principal directions of compressive stress Paxis are SSE with high-plunge. Based on the results of the relocated earthquake sequence,it 's found that the stress drop and focal mechanism types show following a certain spatial and temporal variation characteristics:( 1) In the foreshock stage,the focal mechanism solutions converse at a certain type. The type of the main shock focal mechanism solution is dominant in the fore shock period. The stress drop has a tendency to increase gradually 1month before the main shock, but it 's not high overall. The earthquakes with the high stress drops are around the main shock.( 2) In the dense aftershock stage,the focal mechanisms are scattered into 2 groups: The main shock type which distribute along with the Dayingjiang fault and the local stress field type which conjugate to the fault. The earthquakes with high stress drops are along the Dayingjiang fault and are mainly concentrated in epicenter area. There are the low stress drops of earthquakes in the conjugate belt.( 3) In the later aftershock stage,the number of earthquakes with the local stress field type declined,the focal mechanism solutions turn into the recovery of mainshock type. The stress drops of the earthquakes are higher than the two previous stages. The earthquakes with high stress drops distributed along the Dayingjiang fault,but are more dispersed. The earthquakes with small magnitude and high stress drop and with big magnitude and low stress drop may be a sign of the activity of the earthquake sequence. Combining the source parameters and the focal mechanism solution may be an effective method for analyzing the stress field variation.