Data and Methods
To process the multibeam bathymetric data and shipboard geomagnetic data and to
make maps of the data, Generic Mapping Tools (GMT; Wessel and Smith 1998 ),
Marine Geophysics Basic Tools (MAGBAT; Tamaki et al. 1992 ), and MB-System
(Caress and Chayes 1996 ) are used.
After Kobayashi et al. ( 1998 ), seven cruises were conducted by the author in the study
area with cruise identifications, KR98-07, KR99-11 (Ogawa 1999 ), KH-03-1,
KR04-08 (Nakanishi et al. 2004 ), KR06-03 (Nakanishi 2007 ), KH-06-1 and YK08-09
(Nakanishi and Noguchi 2008 ). We also obtained multibeam bathymetric data in six
cruises by R/V MIRAI from 2008 to 2009 for the study of the tectonics of the Pacific
plate, MR08-02 (Yoneyama 2008 ), MR08-03 (Kashino 2008 ), MR08-04 (Shimada
2008 ), MR08-05 (Honda 2008 ), MR08-06, and MR09-03 (Kikuchi and Nishino 2009 ).
Multibeam bathymetric data obtained in the cruises of R/V KAIREI , KR08-10
(Yamano 2008 ), were offered for this study by the shipboard scientists. Multibeam
data obtained in the four cruises of R/V Thomas Washington , R/V Melville , and R/V
Roger Reville , RNDB10WT, MGLN03MV, MGLN04MV, and ZHNG07RR, were
personally supplied by Geological Data Center, Scripps Institution of Oceanography,
University of California, San Diego. Other multibeam data were obtained from the
databases of Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
for research cruises ( http://www.godac.jamstec.go.jp/cruisedata/e/index.html ) and
National Oceanic and Atmospheric Administration/National Geophysical Data
Center (NOAA/NGDC; http://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.
html ). The unpublished bathymetric data collected in eight cruises by R/V Hakuho-
maru were processed for this study. Finally, bathymetric data compiled for this study
are from the 99 cruises listed (Table 1 ). The bathymetric data made from multibeam
data collected by S/V Takuyo and S/V Meiyo of Hydrographic and Oceanographic
Department, Japan Coast Guard (JHOD; e.g., Fujisawa 2009 ) are additionally used to
fill in the data blank areas of the multibeam bathymetric data. Fig. 2 shows ship tracks
of cruises that collected multibeam bathymetric data. The areas surveyed by JHOD
are shown as shaded areas in Fig. 2 .
Most bathymetric data used in this study were well navigated by GPS or other
satellite navigation systems. The accuracy of the GPS navigation is generally tens
of meters. R/V KAIREI , R/V YOKOSUKA , and R/V MIRAI are better navigated by
the differential GPS, whose accuracy is 2-3 m. Kobayashi et al. ( 1998 ) and Cadet
et al. ( 1987 ) used the bathymetric data collected in the KAIKO Project to expose
the topographic expression of several portions of the trenches. The bathymetric data
from the KAIKO project were excluded for this study because ship positions in the
project were determined by Loran-C navigation, whose accuracy is at most several
hundred meters. The inaccurate positioning makes artifacts near swath boundaries
in bathymetric maps.