Determination of the isotropic component of deep focus earthquakes
by inversion of normal mode data,
Hara, T., K. Kuge, and H. Kawakatsu
Geophys. J. Int., 127, 515-528, 1996.
Abstract
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 non-double-couple
component of the deviatoric moment tensor for
the Japan Sea earthquake (1994 July 21). This
is likely to be caused by source complexity.