Determination of the isotropic component of deep focus earthquakes by inversion of normal-mode data

We determine the isotropic component of large deep earthquakes by inversion for their full six-component moment tensors using the normal-mode data. We show that it is possible to reduce the correlation between the isotropic component and other components and to determine the isotropic component independently by analyses of normal-mode data at periods longer than 500 s. We find no significant isotropic component for the earthquakes we studied; the magnitudes of the estimated isotropic components are comparable to the uncertainty due to the mislocation of the centroid. The magnitude of the isotropic component is at most 5 per cent of the deviatoric seismic moment if it exists. This suggests that, if a rapid phase change of mantle minerals occurs during the rupture of deep earthquakes, the mantle material that transformed would be confined within an extremely thin layer. We also conduct moment tensor inversion using body waves and surface waves in the period band between 20 and 500 s. We find that there is a relatively consistent nondouble-couple component of the deviatoric moment tensor for the Japan Sea earthquake (1994 July 21). This is likely to be caused by source complexity.