Observation of Earth Free Oscillation Modes Using Cross Least Squares Wavelet Method

Throughout  the past three decades, precise relative gravimetric data contributed additional knowledge about the Earth’s interior through careful analyses of superconducting gravimeter (SG) records particularly after strong earthquake events. When an earthquake hits, the Earth generates vibrations similar to those produced of a drum when it is hit. The vibrations of the Earth after an earthquake event are called the Earth’s Free Oscillations with infinite number of modes. In the past ten years (2004 – 2013), the Earth was hit by three major strong earthquakes with a magnitude higher than or equal to 8.8 Mw. The SG data from more than 40 stations recorded after earthquakes in 2004, 2010 and 2011 are used to investigate the properties of the Earth free oscillation modes: their frequencies, amplitudes, and quality factors.

In this study, the SG data from the Global Geodynamics Project (GGP) stations are used where all the unwanted signals/noises such as solid earth tides, ocean loading and polar motion are removed first from gravity data. Then, the least squares wavelet analysis method is used to estimate the atmospheric admittance as a function of time and frequency in the long period seismic band. After pressure correction, gravity residual of each station is used to estimate each station's  time and frequency representation. Finally, the multi-station cross least squares wavelet analysis is applied to define and determine the spheroidal and toroidal modes of the Earth free oscillations. Many spheroidal and toroidal modes associated with their singlets are detected precisely in the frequency band from 300 uHz to 1500 uHz.