Variations in shear wave splitting during aftershocks of the Luquan earthquake in Yunnan Province

Abstract

Shear wave splitting has been measured from analyzing the three-component digital seismograms recorded at Guiquan station after the 1985M _s6. 1 Luquan earthquake in Yunnan Province. The variations in parameters of shear wave splitting with time for over 100 aftershocks have two periods, the local stress period and the regional stress period. In the local stress period, there exist two vertical, paralell crack sets intersecting at about (50–60°), both affect on the propagation of S-waves, and the local stress is slightly stronger than the regional stress. With the activity of aftershock going down and the local stress dying away, it is returned to the state of the regional stress in the focal area. The polarizations of the fast split S-wave and their period variations are identical with the azimuths and changes of the principal compressive stress axis of focal stress field inferred independently from earthquake mechanisms, hense, it is interpreted that the shear wave splitting is the effects of anisotropy of EDA cracks controlled by stress field. The time delay of the slow split S-wave, except the difference between the two periods shows in some examples that it increases in a few hours before an event and decreases in a few days after an event on the individual background of period.