Temporal changes in shear wave splitting after theM=5.2 shimian earthquake of June 9, 1989

Just after the occurrence of anM=5.2 earthquake in Shimian, Sichuan, two three-component seismometers were set up near the epicenter of the earthquake. Field observations were made for the four-year period from 1989 to 1992. Analysis of the data recorded from either aftershocks following theM=5.2 earthquake or from small earthquakes scattered in this region indicates the presence of shear wave splitting. And shear wave splitting varies with time. The mean direction of polarization of the faster shear waves is N18°W during the period of aftershock activity, which is consistent with the strike of the faulting plane of the mainshock; but has turned to N46°W from 1990 to 1992, which is consistent with the regional maximum compressive stress. The time delays between split shear waves measured on records from 1990 to 1992 are about half of that in 1989. The results obtained from observations at two temporary stations are similar. This indicates that the temporal changes may be related to the occurrence of theM=5.2 mainshock.