A long-term seismic array observation was conducted around the North Mariana island arc using 10 long-term ocean bottom seismometer (LTOBS), which gathered data for one year starting in 2001 October. The purpose of the observation was to research the deep seismic activity and the structure of the Mariana subduction system. By using data from nine of the LTOBS, which covered about 11 months of elapsed time, more than 3000 local events were found through the hypocentre determination with a one-dimensional velocity model. Only 59 of these events were listed in the same period on the US National Earthquake Information Centre's Preliminary Determination of Epicentres (PDE) publication. The hypocentres with small determination errors clearly show a double seismic zone (DSZ) reaching 150 km in depth with two seismic planes separated by about 30 km. The extension of the DSZ's seismic activity remains high until a depth of about 270 km. In the backarc basin, some shallow seismic activities were found as linear foci around the current ridge axis. High seismic activity was also recorded to the north and the west of a large serpentine seamount in the forearc. In addition, a zone almost devoid of earthquakes exists in the upper plane of the DSZ with the low velocity of the P phase (VP) region modelled by an inversion. This region looks like a narrow channel running east to west. A shear wave splitting study of the area indicates a change of splitting direction near the southern edge of this channel. A result of a gravity study also shows negative gravity anomalies in the same area. This region could be a serpentinized forearc mantle with water supplied to it from a seamount that was in the past carried downwards with the subducting Pacific Plate.