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Detection Method for Pairs of P and S Waves of Deep Low‐Frequency Earthquakes Using a 3‐D Array in the Tokai Area of the Nankai Subduction and Its Application to Hypocenter Determination

We report on a novel method that uses a three‐dimensional (3‐D) array to detect P and S wave pairs of the deep low‐frequency earthquakes (LFEs) occurring along the Nankai subduction zone. In an effort to determine LFE hypocenters precisely we attempt to find their P and S pairs and obtain S‐P times using a 3‐D array (6 km in the horizontal direction) made up 14 seismic stations with deep (600 m at the deepest) borehole seismographs in the Tokai area. During operation we observed remarkable LFE activities on 10–30 November 2010. Using these data, we succeeded in distinguishing between P and S phases in complex LFE seismic waves using an effect of velocity filtering in semblance analysis via the 3‐D array. We also obtained high‐quality propagation parameters (back azimuths and incidence angles) and S‐P times for the LFEs. Referring to these identified P and S phases, after manually reading the arrival times at the 3‐D array and regular network, we could precisely relocate the hypocenters of 15 LFEs. These hypocenters were distributed in a depth range from 26 to 34 km, and approximately followed the plate interface. A comparison of the LFE hypocenter distribution to seismic tomography indicates that the LFEs are distributed in the oceanic crust and mantle wedge with relatively low velocities under an undrained cover with relatively high velocities following the land Moho (land crust bottom). We believe that our detection method will be useful in the automated detection and preliminary hypocenter determination of LFEs.