Аннотация
Abstract The rupture boundary of the Tonankai and Nankai earthquakes is located at the south of the Kii Peninsula. Previous researches revealed along-strike segmentation of hypocenters, P-wave anisotropy, low-frequency earthquake (LFE) distribution and subduction depth of the Philippine Sea (PHS) Plate beneath the Kii Peninsula. In addition, it was found that hypocenters at offshore region are also segmented along the Nankai Trough by using ocean bottom seismometers (OBSs) deployed from 2003 to 2007. To investigate such segmentation, in our previous work we determined 3-D velocity structure and hypocenters by using the OBSs and on-land stations. As a result, it was discovered that Vp/Vs ratio is also segmented into three parts within the oceanic crust and at the bottom of the overriding plate, which coincides with the LFE distribution. We interpreted this segmentation as variation of pore fluid pressure. Similar segmentation can be seen in hypocenters. To derive characteristic patterns of the hypocenters for each segment, we conducted a cluster analysis of earthquakes based on waveform cross correlation coefficients (CCs) and hypocenter intervals, where we took varying structural site effects among the OBS stations into account in determining thresholds for CC values. At the western segments, most earthquakes occur within the oceanic mantle, forming NNE-trending linearly aligned clusters, which are considered to occur along preexisting faults. At the middle segment, most earthquakes occur within the oceanic crust and inter-plate earthquake clusters are located at the southern tip of the Kii Peninsula. We interpret these clusters of inter-plate earthquakes represent the heterogeneous state of stress caused by local structural heterogeneity of dense rock body reported in previous studies. The eastern segment is characterized by a paucity of seismicity. From the comparison of magnetic anomaly pattern, it can be considered that this segmentation of Vp/Vs ratio and hypocenters may be related to the opening history of the Shikoku Basin.