Study of the 2011 off the Pacific Coast of Tohoku Earthquake based on Seafloor and Terrestrial Geodetic Observation

Abstract

The geodetic research group of Tohoku University has been carrying out geodetic observations on shore and off shore of Miyagi Prefecture where earthquakes around M7.5 repeatedly occurred at intervals of about 40 years. We have been measuring horizontal movements of seafloor geodetic stations since 2003, using GPS-Acoustic （GPSA） seafloor positioning systems. We have also been continuously observing vertical crustal movements since 2008 using free-fall/pop-up ocean bottom pressure recorders （OBPRs）. Owing to these geodetic observations that were made in the region above the rupture area of the 2011 M9.0 Tohoku-oki earthquake, we succeeded in detecting unexpectedly large coseismic crustal displacements that had never been measured with geodetic systems on the seafloor. We retrieved data from the OBPRs that had been deployed above the rupture area, and to continue monitoring gradual postseismic slips, we installed replacements for these OBPRs at the same observation sites. We presented a model for the coseismic slip distribution on the plate boundary, and it is widely referenced because it is a model that is constrained by the OBPR data as well as the GPSA and the terrestrial geodetic data. We have also developed a method of real-time kinematic analyses of the data from a GNSS array to estimate the approximate magnitude of large earthquakes within several minutes. We developed a procedure to reduce tidal and non-tidal oceanic components in OBPR data. The pressure data analyzed jointly with onshore strain data revealed that there was a slow slip event near the focal zone about one month before the main event. The observed pressure variations also contain information about the coseismic and postseismic crustal movements associated with the M7.3 earthquake that occurred on March 9 and was estimated to be the largest foreshock. These results are important for clarifying geophysical processes that preceded the giant earthquake.