Near-trench Multibeam bathymetry measurements
The multibeam data near the Japan trech were collected by the four different vessels of JAMSTEC (Kaiyo, Yokosuka, Mirai, and Kairei) between 2001 and 2015. Ten overlapped tracks (see Supporting information) can be identified, after examining the historical multibeam datasets near the Japan Trench. The distribution area of the tracks spans more than 260 km in the north–south direction along the trench, with depths ranging approximately 5500–7500 m. For the majority of the survey tracks, the sound velocity profile (SVP) correction is conducted by using the real-time XBT data acquired during the cruise. However, two tracks (Nos. 1 and 8) have multibeam data without real-time XBT measurements, but were corrected with historical XBT data (assisted with Argo float data) instead, which may lead to large uncertainties in the resultant differential bathymetry.