Observability of the isotropic component of a moment tensor,
H. Kawakatsu
Geophys. J. Int., 126, 525-544, 1996.
Abstract
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 starting from the first P-wave arrival
to just before the arrival of the first surface wave-train, 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 of deep earthquakes.
This conclusion is contradictory to some earlier studies,
e.g. 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 kind of normal mode data
above 2mHz that previous researchers used.
At the same time, however, the possibility of resolving the isotropic
component using normal mode data below 2mHz is suggested.