Subduction off Miyagi and Fukushima prefectures, northern Honshu, has been recognized as having a triple-planed structure of seismicity at the deep thrust zone in the 40-60 km depth range. This triple seismic zone is composed of thrust-type earthquakes, down-dip compressional and down-dip tensional earthquakes from top to bottom. At the time of the 2011 Tohoku-oki earthquake, peculiar phenomena such as radiation of short-period seismic waves and pre- and after- slow slips within the asperities of M7 class earthquakes were observed over this thrust zone west of the 2011 main rupture zone. Further to the south, where the Philippine Sea plate is subducting beneath Kanto, a triple seismic zone has also been recognized particularly under southwest Ibaraki prefecture. The thrust-type earthquakes at the top of the triple seismic zone off northern Honshu and beneath Kanto have been believed to be interplate events representing the relative motion between the overriding and subducting plates. I conclude that the thrust-type earthquakes beneath southwest Ibaraki prefecture are in fact within the crust of the subducting Philippine Sea slab, not at the surface because their slip vectors are different from the relative motion between the subducting and overriding plates. Therefore, there would be an aseismic plate boundary above the seismicity. I also show that the dip angles of the westerly dipping fault planes of the thrust-type earthquakes off Miyagi prefecture are smaller by 6° in average than the dip of the slab surface in this region, except for the six years prior to the Tohoku-oki earthquake, i.e. prior to 2006. Furthermore, the slip vectors coincide with the relative motion between the overriding and subducting plates only during this period. I infer that the topmost earthquakes of the triple seismic zone off Miyagi prefecture prior to 2006 are thus likely to be within the crust of the subducting plate. The slow slips before and after the Tohoku-oki earthquake would have occurred not within the asperities but along the aseismic plate interface, and the short-period seismic waves would have been radiated due to fractures within the crust associated with the overshooting rupture at the time of the Tohoku-oki earthquake. Many of the so-called repeating earthquakes at the topmost surface of the subducting plate would be in fact intra-crustal events within the slab. M9 earthquakes would interact with the triple seismic zone, not only mechanically, but also through fluid migration, because earthquakes in the triple seismic zone involve dehydration reaction. The irregularity of the occurrence of M9 earthquakes might be due to the inhomogeneous distribution of hydrated minerals in the incoming plate. The subduction zones having M9 earthquakes or under Kanto have a collisional character. I propose to term subduction having both a collisional character and a triple seismic zone as “super-subduction”. The relative motion between the plates is accommodated by the deformations of the crust of the subducting slab as a “plate boundary zone”. The viewpoint of “super-subduction” is necessary to understand earthquakes in subduction zones with a collisional character and dehydration reactions in the slab.
On the basis of statistical (soft) clustering of a horizontal displacement field from Global Navigation Satellite System, we conduct tectonic classification around the Izu Peninsula, a collisional zone in Japan. We determine the best number of the clusters by a statistical index, and classify 44 observation points into “Fuji cluster”, “Southern Izu cluster”, “Sagami cluster”, or “Oshima cluster”. The boundaries between the clusters almost correspond to locations of recent large earthquakes. Although several points differ from a recent concept of the Izu microplate, we obtain the following perspectives: (1) Northern part of the Izu Peninsula is integrated with Honshu, the main island of Japan. (2) Multiple cluster boundaries merge around Izu Tobu Volcano Group. (3) Fault zones of landward extension of subduction trenches (the Suruga Trough and the Sagami Trough) do not correspond to the cluster boundaries.
Broadband ocean bottom seismometers (BBOBSs) have been developed since 1990s and put into practice to explore the structure of the Earth’s interior beneath oceanic regions, e.g., mid-oceanic ridges, subduction zones, hotspots, and oceanic lithosphere-asthenosphere boundary. The BBOBSs have been used to investigate earthquake source process including slow earthquakes and oceanographical phenomena. The best way for the broadband seismic observation in the oceanic region seems that the borehole seismic observatory connected with the ocean floor cable, which is realized in several networks deployed close to the coast but it is rare even now. For scientific targets far away from the coast, we still need to develop and operate the autonomous system of BBOBS with better S/N for a temporary observation for focused scientific targets. In this review article, focusing mainly on achievements made in Japan, we describe a history of the development of BBOBS and some related instruments, scientific results based on observations with the BBOBSs, and future direction of broadband ocean bottom seismology.