Ocean-bottom pressure and acceleration data simultaneously recorded by the DONET seafloor network during the 2011 Tohoku earthquake approximately 800 km from the earthquake epicenter are processed and analyzed. The close location of pressure and acceleration sensors together with the high data-sampling rate enable us to quantitatively examine and interpret pressure variations together with ocean-bottom acceleration for the first time to our knowledge. To interpret observed data, we introduce a set of characteristic frequencies that enable us to identify physical processes responsible for water layer behaviour dependent on the frequency of ocean-bottom oscillations. Explicit formulas are given for calculating all of the characteristic frequencies, which are the basis for introducing nonoverlapping frequency bands, i.e., hydroacoustic waves, forced oscillations, and gravity waves. The physical correctness of such a subdivision is confirmed by the high coherence and nearly zero phase difference between in-situ measured pressure and acceleration variations observed in the forced oscillation frequency band – a band neither hydroacoustic nor gravity waves are generated by ocean-bottom oscillation because the water layer simply follows the ocean bottom, generating forced oscillations. The dominant, long-lasting pressure fluctuations recorded by DONET during the 2011 earthquake are associated with the forced oscillation, or, more precisely, with water and sedimentary layer coupling oscillation. DONET clearly observed the 2011 Tohoku tsunami signal during more than 24 hours following the earthquake. In contrast to DART records, phase dispersion was not manifested in the tsunami signals registered by DONET.
Rotation motions of cabled ocean-bottom seismic stations during the 2011 Tohoku earthquake and their effects on magnitude estimation for early warnings