Estimation of Seismic Centroid Moment Tensor Using Ocean Bottom Pressure Gauges as Seismometers

We examined the dynamic pressure change at the seafloor to estimate the centroid moment tensor solutions of the largest and second largest foreshocks (M_w 7.2 and 6.5) of the 2011 Tohoku-Oki earthquake. Combination of onshore broadband seismograms and high-frequency (~20–200 s) seafloor pressure records provided the resolution of the horizontal locations of the centroids, consistent with the results of tsunami inversion using the long-period (≳10 min) seafloor pressure records although the depth was not constrained well, whereas the source locations were poorly constrained by the onshore seismic data alone. Also, the waveforms synthesized from the estimated CMT solution demonstrated the validity of the theoretical relationship between pressure change and vertical acceleration at the seafloor. The results of this study suggest that offshore pressure records can be utilized as offshore seismograms, which would be greatly useful for revealing the source process of offshore earthquakes.