Free-oscillations of 2011 Tohoku earthquake: source models and data

The new instrumentation at the Geodetic Observatory Pecny, Czech Republic, (GOPE) consisting of superconducting gravimeter and a broadband seismometer allows to study a number of geodynamic phenomena in a broad frequency range. We focus on the employment of GOPE data to test synthetic free-oscillation calculations. High-accuracy pseudospectral method, converting the fundamental PDEs for free-oscillations to a matrix-eigenvalue problem, is applied to compute synthetic seismograms up to 2.5 mHz for the 1-D PREM parameters. The effects of the Coriolis and centrifugal forces as well as hydrostatic ellipticity are included. We use several point- and finite-source solutions of the 2011 Tohoku earthquake obtained from long-period seismic and GPS records. We point to the possibility of calculating large unobserved multiplet amplitudes due to singlet coherences. We demonstrate that these coherences are not present if finite-source models are applied but they are obtained for a point-source model. In the studied spectral range the 2011 Tohoku earthquake fast source models are in a good agreement with the observed data from the GOPE observatory. Whereas analysis of longest-period normal-mode data of the 2004 Sumatra-Andaman earthquake resulted in an increase of the early Mw values, the early Tohoku Mw estimates seem to be satisfactory. In this aspect, the 2011 Tohoku earthquake is similar to the 2010 Maule earthquake. We also deal with the amplitude estimates of degree-one overtones over the globe to be able to estimate whether they are, in principle, observable in the IAG Global Geodynamics Project network.