Constrain large earthquake source mechanism by using low frequency normal mode data

    Normal modes are the vibrating patterns of the Earth in response to the large earthquakes, they are standing waves resulting from the interface of surface waves travelling in opposite directions. Normal mode spectra are split into singlets due to Earth's rotation, ellipticity, and heterogeneity. We first use MINEOS software to forward modeling the Tohoku(2011/03/11) earthquake synthetic seismograms of 1D model (PREM), then extract the spectra of target radial modes from 18 superconducting gravimeter(SG) station records provided by Global Geodynamics Projects (GGP) and use neighborhood algorithm (NA) to invert the source mechanism (magnitude, fault rake, strike, dip ). The NA is an efficient model space search technique and works in two steps: In the first step, the algorithm finds all the models compatible with given data while the posterior probability density of the model parameters are obtained in the second step. The NA can address the problem of non-uniqueness by taking advantage of random sampling of the full model space. The parameter trade-offs are conveniently visualized using joint marginal distributions. In addition, source mechanism uncertainties can be extracted from the posterior probability distribution. Then, we compute the synthetic seismograms taking rotation, ellipticity, and heterogeneity into consideration, extract the spectra of target fundamental spherical modes singlets from 18 SG records and use NA to invert source mechanism and rupture parameters (magnitude, fault rake, strike, dip, rupture length and duration). In the end, we compare the results with others constrained by body waves. Realistic synthetic tests show the importance of accurately modelling lateral heterogeneity, notably for rake angle, rupture length and duration determinations. Overall, we have confirm the low-frequency Earth’s normal modes excited by large magnitude earthquakes can be a robust, independent tool for the determination of earthquake source models and their bulk rupture characteristics.