An improved double-difference earthquake location algorithm using s P phases: application to the foreshock and aftershock sequences of the 2004 earthquake offshore of the Kii peninsula, Japan (Mw=7.5)

We have used s P phases to improve the hypocentral locations of the earthquakes that occurred offshore southeast of the Kii peninsula in association with the M w =7.5 mainshock on September 5, 2004. The earthquakes were more than 100 km from the onshore seismic network and, thus, their focal depths were poorly constrained. The s P phases were recorded about 7–11 s after the initial P phases, with both phases having almost the same apparent velocities. The computation of ray-paths revealed that the arrival times of the later s P phases are sensitive to focal depths. We have recalculated the hypocenters and origin times for 36 events with more than six s P phase identifications on seismograms recorded at the High Sensitivity Seismograph Network by including s P phases in a double-difference earthquake location algorithm, which eliminates the errors introduced by crustal heterogeneity. The relocation results were then compared with those from the Japan Meteorological Agency based on traditional absolute location techniques.