Multichannel seismic profiles crossing source regions of megathrust earthquakes and slow slip events off-Boso, central Japan

Off-Boso is a southeastern coastal region of Boso peninsula neighboring Tokyo metropolitan, locating central Japan. This region is one of the most complex plate configuration regions on the earth because the Pacific Plate (PAP) and the Philippine Sea Plate (PSP) are subducting beneath off-Boso reigon, which is a part of North American Plate (NAP) (or Okhotsk Plate). Associated with the subduction, magnitude eight-class large earthquakes have occurred historically (e.g. 1703 Genroku and 1923 Taisho earthquakes) bringing serious damages to Tokyo metropolitan. Moreover, slow-slip events have been observed with 5-7 year interval off Boso region. To reveal the structural characteristics associated with the various seismogenesis off-Boso region, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has conducted multichannel seismic (MCS) surveys. The MCS data is important information for the drilling of Kanto Asperity Project (KAP), which is one of IODP proposal. A 7800 cu. in. (140 liter) tuned airgun array was shot with 50-m interval, detected by a 444-ch hydrophone streamer with 12.5 m group interval. In 2008, three NE-SW lines were conducted for revealing the subduction images of the PSP crossing the regions of stable sliding, slow-slip and slip deficit. In 2009, a NW-SE long line and a NE-SW short line were carried out to obtain the structural images of PSP and PAP perpendicular to trench axis and the 1703 Genroku asperity in the vicinity of Boso peninsula, respectively. From NE-SW lines of 2008, the morphology of the top of PSP is not smooth but rugged below the NAP. The northeastern end of PSP is confirmed by the MCS data combined with previous MCS profiles. Moreover, deep events interpreted as the top of PAP are recognized below the reflections of the PSP. The shallow part above the top of PSP can be divided into four blocks based on the MCS images and bathymetries: 1) trough deposits block, 2) thrust block, 3) thick sediments covered block, and 4) thin sediments covered block. In the second block, remarkable reflection events are observed from the top of PSP to near seafloor, which is resemble to the spray fault in the Nankai Trough (e.g. Park et al., 2002). Comparing with the results of geodetic studies (e.g. Sagiya, 2004), the third block overlaps the 1996 slow-slip event area. The first and second blocks are slip-deficit area of the 1996 slow-slip event.