Abstract
Abstract Two large earthquakes (Mw7.3 and Mw7.5) occurred off-shore of the Kii peninsula (Japan) on 5th September 2004. The insufficient station coverage in the area where the large events and their aftershocks occurred led to significant uncertainty regarding their depth location. By using a double-difference approach, we were able to determine more accurate hypocenter locations and better define the main characteristics of the aftershock distribution. The data we used in this study comprise of about 700 earthquakes recorded by the Japanese Meteorological Agency (JMA) from 5 to 12 September 2004. We assume several depth ranges for the initial double difference earthquake locations and select the solution with the smallest rms residual. The resulting hypocenter distribution has two distinctive features: 1) on average the depths are about 20 km shallower than the initial JMA location and 2) the hypocenters are divided into two groups: one shallow (around 12 km depth) and another deep (around 27 km depth). The epicenters of the relocated events define a finer, more clustered structure compared with the epicentral distribution of JMA. The splitting of the earthquake locations into two depth groups is still observed when using a significantly different velocity structure. Our results of the two depth groups for the hypocenters resemble the independently determined hypocentral distribution from the OBS survey.
Highlights
Due to the dense distribution of seismic stations for the onshore areas of Japan, the earthquakes that occur in these regions have relatively accurate hypocenter locations (e.g. Ueno et al, 2002)
Data and Method of Analysis The data used in this study consist of about 700 aftershocks that occurred from 5 to 12 Sept. 2004 off-shore of the Kii Peninsula
While the earthquakes occurred in a rather complex 3D structure, we consider a 1D velocity model to be sufficient since the subducting slab in this region of the Nankai Trough, where the Kii Peninsula events have occurred, dips at a shallow angle
Summary
Due to the dense distribution of seismic stations for the onshore areas of Japan, the earthquakes that occur in these regions have relatively accurate hypocenter locations (e.g. Ueno et al, 2002). For each relocated event we selected the solution with the smallest rms residual among the five trial depths. While the earthquakes occurred in a rather complex 3D structure, we consider a 1D velocity model to be sufficient since the subducting slab in this region of the Nankai Trough, where the Kii Peninsula events have occurred, dips at a shallow angle (see Nakanishi et al, 2002).
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