Abstract
AbstractWe have useds Pphases 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. Thes Pphases were recorded about 7–11 s after the initialPphases, with both phases having almost the same apparent velocities. The computation of ray-paths revealed that the arrival times of the laters Pphases are sensitive to focal depths. We have recalculated the hypocenters and origin times for 36 events with more than sixs Pphase identifications on seismograms recorded at the High Sensitivity Seismograph Network by includings Pphases 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.
Highlights
Earthquake focal depth is an important parameter for assessing seismic hazard, discriminating natural earthquakes from nuclear explosions, and understanding the tectonic process
Most of the hypocenters determined by the Japan Meteorological Agency (JMA) (Fig. 2(a)) were deeper than 30 km, whereas most of the hypocenters relocated by ocean bottom seismometers (OBSs) data (Fig. 2(b)) or by double-difference analyses with direct P- and S- arrival times (Enescu et al, 2005) were shallower than 30 km
The accuracy of absolute hypocenter locations is generally controlled by several factors, including the accuracy of velocity structure, available phases, arrival-time reading accuracy, and the network geometry (Pavlis, 1986)
Summary
Earthquake focal depth is an important parameter for assessing seismic hazard, discriminating natural earthquakes from nuclear explosions, and understanding the tectonic process. The 2004 earthquake offshore of the Kii peninsula occurred close to the deformation front of the Nankai Trough (Fig. 1). Most of the hypocenters determined by the Japan Meteorological Agency (JMA) (Fig. 2(a)) were deeper than 30 km, whereas most of the hypocenters relocated by OBS data (Fig. 2(b)) or by double-difference analyses with direct P- and S- arrival times (Enescu et al, 2005) were shallower than 30 km.
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