A new detection method for P and S waves of deep low-frequency earthquakes using a 3D array in the Tokai area and its application to hypocenter determination

Abstract:

We have developed a novel method that uses a 3D array to detect the P and S waves of deep low-frequency earthquakes (LFEs) that occur along the subduction zone of the Philippine Sea plate in southwest Japan. Obtaining accurate hypocenters of LFEs is very difficult because their seismic waves are characterized by low amplitude and the absence of sharp pulses. In particular, identifying P phase arrivals is not readily possible using conventional methods and seismic networks. To determine their hypocenters accurately—not only their epicenters but also their depths—we tried to find their P and S wave pairs and obtain S-P times. We constructed a 3D array (6 km in the horizontal direction) using 14 seismic stations in the Tokai area with three component seismographs, including ones with deep (600 m at the deepest) borehole seismographs. We observed remarkable LFE activity occurring in the Tokai area over November 10-30, 2010. We successfully detected not only S waves but also very weak P waves of LFEs using the 3D array data and the semblance method. Assuming a homogeneous half-space model with P wave velocity=4.5 km/s and S wave velocity=2.2 km/s in the 3D array, we calculated the semblance distributions for more than 20 LFEs by utilizing the three factors of: time, back azimuth, and the incident angle of seismic waves. Using the time of the maximum value of the semblance in each component, we detected the direct P wave in the vertical component and the S wave in the horizontal component. Choosing about 10 LFEs with reliable results obtained from the semblance analysis, we applied those factors and S-P times to locate their hypocenters and found their hypocenters to be distributed in the depth range from 28 km to 35 km approximately along the plate interface inclining in depth from 30 km to 32 km.