Abstract

Free oscillations of the Earth provide crucial constraints for seismic source properties of very large earthquakes, especially on the seismic moment. In this work, the scalar moment and mechanism of the 11 March 2011 Tohoku‐Oki earthquake are examined through the analysis of 15 modes. The approach utilizes receiver strips that compress mode excitation information, and are also used to set up an objective data weighting scheme. The moment tensor inversion is performed in two steps. An initial solution is obtained from a linear inversion by fitting the real and imaginary components of the receiver‐strip spectra. In order to take advantage of the difference in the robustness of the amplitude and phase spectra, the second procedure solves a nonlinear problem in an iterative approach. Because the 2011 Tohoku‐Oki earthquake is at a shallow depth, special attention is needed to treat the dip‐slip components of the moment tensor. Low‐frequency modes are practically insensitive to these components, and presents a challenge in estimating the mechanism and seismic moment based purely upon free‐oscillation data. The final result is consistent with the thrust mechanism reported elsewhere, and gives a scalar moment corresponding to Mw 9.1.

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