Crustal structure around the eastern end of coseismic rupture zone of the 1944 Tonankai earthquake

The high potential for a major earthquake to occur in the Suruga Trough, "Tokai Earthquake", has been recognized since 1969 and is closely related to how far the coseismic rupture during the 1944 Tonankai earthquake propagated eastward. To understand what kind of structure might have interrupted the rupture of the 1944 event, it is necessary to obtain the crustal structure around the eastern end of its rupture zone. The crustal model deduced from a wide-angle seismic survey shows characteristics of the subducting oceanic crust and the Neogene-Quaternary accretionary sediments bounded by the crustal block of the island arc around the eastern end of the rupture zone of the 1944 event. This model shows that the subduction oceanic crust has a particularly irregular geometry 40 km landward from the deformation front. Comparing our crustal models with a recently published coseismic rupture distribution of the 1944 event deduced from tsunami waveforms, there is no slip in the east of the irregularity of the slab. Moreover, it seems that coseismic rupture propagated northeastward avoiding this irregularity. The spatial relationship between the irregularity of the slab found in this study and the rupture distribution is analogous to that between the subducted seamount found off the Shikoku Island and the rupture distribution of the 1946 Nankai earthquake. Although the existence of a subducted ridge parallel to the Nankai Trough off the Tokai district has been suggested based on published geomagnetic data and seafloor topography, our crustal models may support the existence of a slab irregularity more suggestive of a subducted seamount, at least, at the eastern end of the coseismic rupture zone of the 1944 event. Such irregular geometry of the subducting oceanic crust, in particular subducted seamounts, may be one of the causes that control the rupture process of great earthquake along the Nankai Trough.