Study on the attenuation relationship of acceleration envelope parameters of Wenchuan earthquake aftershocks: Considering the difference of aftershock focal mechanism and seismogenic region

Spatiotemporal distribution of the aftershock sequence of the Wenchuan earthquake shows obvious segmented characteristics, and the aftershock seismogenic region can be divided into two regions, southwest and northeast. Considering that the southwest segment of the Wenchuan earthquake fault plane is dominated by thrust slip, the northeast side is transformed into thrust and strike slip, and strike slip is dominant at the northeast end, aftershock records consistent with the slip type of the Wenchuan earthquake are selected. Thrust type aftershocks are selected in the southwest region and thrust and strike slip and strike slip aftershocks are selected in the northeast region. Attenuation relationships of three acceleration envelope models for the aftershocks (M w 4.0–6.0) in two regions are established, including the three-segment envelope model, the three-segment envelope model with P and S wave bands, and a single-peak envelope model with P and S wave bands. Results show that there are significant differences in the attenuation characteristics of envelope parameters for different types of aftershocks. (1) For the three-segment envelope model, the durations of the rising stage and stable stage increase as the magnitude and epicentral distance increase. The attenuation coefficient decreases with increasing magnitude and epicentral distance. For the three-segment envelope model with P and S wave bands, the attenuation characteristics of the parameters are consistent with the three-segment envelope model. For the single-peak envelope model with P and S wave bands, the peak times of the P band and S band increase with increasing magnitude and epicentral distance. (2) The envelope parameters related to duration and amplitude in the northeast region of thrust and strike slip and strike slip aftershocks are generally larger than those of the southwest region with thrust type aftershocks. Shallower depths and larger magnitudes of aftershocks in the northeast region may cause this situation.