Determination of the complex frequencies for the normal modes below 1mHz after the 2010 Maule and 2011 Tohoku earthquakes

Abstract

<p>Based upon SG (superconducting gravimeter) records, the autoregressive method proposed by Chao and Gilbert [1980] is used to determine the frequencies of the singlets of seven spheroidal modes (₀S₂, ₂S₁, ₀S₃, ₀S₄, ₁S₂, ₀S₀, and ₃S₁) and the degenerate frequencies of three toroidal modes (₀T₂, ₀T₃, and ₀T₄) below 1 mHz after two recent huge earthquakes, the 2010 Mw8.8 Maule earthquake and the 2011 Mw9.1 Tohoku earthquake. The corresponding quality factor <em>Q</em>s are also determined for those modes, of which the <em>Q</em>s of the five singlets of ₁S₂ and the five singlets (<em>m</em>=0, <em>m</em>=±2, and <em>m</em>=±3) of ₀S₄ are estimated for the first time using the SG observations. The singlet <em>m</em>=0 of ₃S₁ is clearly observed from the power spectra of the SG time series without using other special spectral analysis methods or special time series from pole station records. In addition, the splitting width ratio <em>R</em> of ₃S₁ is 0.99, and consequently we conclude that ₃S₁ is normally split. The frequencies and <em>Q</em>s of the modes below 1mHz may contribute to refining the 3D density and attenuation models of the Earth.</p>