A study of aftershocks of the 17 July 1998 Ruey-Li, Chiayi earthquake

On 17 July 1998, an earthquake of magnitude 6.2 (M-L) occurred near Chiayi, southwestern Taiwan. Two days after the occurrence of the mainshock, a tem porary digital seismic network was deployed near its epicenter for 24 days to monitor the aftershock activity. The events within and near the network were first located using a traditional single-event location and then reloc ated using a joint hypocentral determination (JHD) technique to determine t he significance of lateral velocity variations and to improve earthquake lo cations. The station corrections obtained from the JHD analysis vary from 0 .30 to -0.18 sec for the P-waves and from 0.51 to -0.58 sec for the S-waves . The patterns of station corrections suggest that upper crustal velocities on the eastern side of the study area are relatively higher than those on the western side. The depth cross-section of the relocated aftershocks show s a clear pattern of southeast-dipping distribution of the hypocenters. The focal mechanisms and hypocentral distribution of the relocated aftershocks reveal a close relationship between seismicity and the known faults in the study area.