Observability of the isotropic component of a moment tensor

The observability of the isotropic component of a general moment tensor is investigated by carefully examining its resolvability from the other components. It is shown that the portion of seismograms between the first P-wave arrival and just before the arrival of the first surface wavetrain, which contains many different body-wave phases, can be used to constrain the isotropic component of deep earthquakes. CMT inversion using this portion of the seismogram for major deep earthquakes has revealed that there is no significant isotropic component for deep earthquakes. This conclusion contradicts some earlier studies, for example Dziewonski & Gilbert (1974), who found significant isotropic components of deep earthquakes by analysing normal-mode data. It is shown that the isotropic component of deep earthquakes cannot be resolved independently from the vertical CLVD component of the moment tensor by analysing the type of normal-mode data above 2 mHz that previous researchers used. At the same time, however, the possibility of resolving the isotropic component using normal-mode data below 2 mHz is suggested.