Underground Structural Anomalies and Slow Earthquake Activities Around Seismogenic Megathrust Earthquake Zone as Revealed by Inland Seismic Observations

Seismogenic zones of interplate megathrust earthquakes along the Nankai Trough can be subdivided into several segments. At each segment, seismic rupture has occurred at a recurrence interval of about one century. In many cases, some neighboring segments ruptured simultaneously or sequentially after a short interval. One of the factors that controls the properties of such seismic ruptures is the underground structure, including the plate configuration and heterogeneity around the subducting plate (slab) interface. To clarify the mechanism of megathrust earthquakes, detailed surveys and analyses of the underground structures are required. Moreover, the detection of seismic phenomena on the plate interface is important in discussing interplate coupling and interactions between such events and megathrust earthquakes. More and better knowledge of the underground structure around the plate interface has been accumulated by analyses of high-quality data from high-density seismograph networks in inland areas and from joint seismic explorations of the sea and land. Moreover, knowledge regarding a wide variety of newly detected slow earthquakes has contributed toward our understanding of the subducting plate interface. Additional information about underground structures, such as slab segmentations, is expected to provide a better understanding of the occurrence of megathrust earthquakes.