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

Abstract

We have used sP phases to improve the hypocentral locations of the earthquakes that occurred offshore southeast of the Kii peninsula in association with the Mw=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 sP 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 sP phases are sensitive to focal depths. We have recalculated the hypocenters and origin times for 36 events with more than six sP phase identifications on seismograms recorded at the High Sensitivity Seismograph Network by including sP 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. © 2006, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.