Spatio-temporal variation of microseismicity near the focal region of the 2011 off the pacific coast of Tohoku Earthquake detected by ocean-bottom seismic observation

In Miyagi-Oki region, the area of large coseismic slip of the 2011 off the pacific coast of Tohoku Earthquake (M 9.0), it has been thought that interplate earthquakes with magnitude of ~7.5 occur repeatedly, with an interval of about 37 years. We started a seismic observation in the region, using 5 ocean bottom seismometers (OBSs) in 2002, so as to assist in making long-term forecasting of a forthcoming M-7.5 earthquake by monitoring small earthquake activity in the region. This OBS observation was followed by a series of repeated deployment of OBSs, in which OBSs are recovered and deployed at the same locations to allow long-term continuous data acquisition. In the repeated observation, we also increased the number of OBSs: When the 2005 Miyagi-Oki earthquake (M 7.2) occurred, 15 OBSs were in place and the Tohoku earthquake on March 11 was observed by 23 OBSs. In this research, we built a waveform database by integrating the records obtained from OBS with the data from routine observation points on land as well as another database of completely continuous OBS waveform data. The former database has been used for hypocenter relocations by combining use of on- and offshore seismic data to provide an earthquake catalogue for Miyagi-Oki region with reliable hypocenter locations. In this paper, we examine the continuous waveform records of OBSs to understand spatio-temporal variation of the microseismicity. The continuous records contain many small earthquakes, which do not appear in the catalogue due to the limitation of the detectability of the onshore seismic network. By including these small earthquakes, it is expected that we can examine if the seismicity shows any changes before the occurrence of major earthquakes such as that in 2005 and the Tohoku earthquake, much more in detail than the previous attempt using the available catalogue.