Abstract
The geodetic observation on the seafloor will have more important role to investigate a disastrous large earthquake at plate boundaries. We developed a new system to extend the observation period toward long periods by adding a precise absolute pressure gauge （APG） to the broadband ocean bottom seismometer （BBOBS） that have already in several use, namely as the BBOBS+APG. By counting the APG's signals based on the highly stable frequency source inside the OBS recorder, this system could draw the maximum performance of the APG. The resolution of pressure data can be smaller than 1 Pa after the low-pass filtering. The BBOBS+APG system has been applied in practical observation since 2009. One of three APG used in the first long-term deployment showed relatively large drift rate of -2 hPa/month, which was caused by this APG itself. During the aftershock observation of the 2011 Tohoku-Oki earthquake, two BBOBS+APG recorded pressure changes close to the epicenter of the M7 earthquake occurred on 10 July 2011. Vertical displacements observed near the epicenter are difficult to be modeled with the fault plane already studied by other data. It might be explained if two conjugate fault planes made slips nearly same time or successively, because two linear trends are clearly recognized in the epicenter distribution of the aftershock observation of this M7 earthquake by the OBS array. The extension of the observation period covering the slowslip event （SSE） has been achieved by this BBOBS+APG system, which was an aim at the beginning of this development. We will start fine observations of the SSE to elucidate the mechanism of inter-plate earthquakes with other ocean floor instruments. And, we continue to make advance in the pressure measurement especially for resolving the drift in the current APG.
