Insignificant isotropic component in the moment tensor of deep earthquakes,
Kawakatsu, H.
Nature, 351,
50-53, 1991.
Abstract
The mechanism responsible for deep focus earthquakes, which occur at depths
of 300-700 km in subducting slabs, has been a long-standing problem
in geophysics.
Unlike shallow earthquakes, deep earthquakes cannot not be attributed to
frictional instabilities across a fault plane, because of high frictional
resistance to sliding at depth.
A volumetric change associated with a phase transition, expected to occur
at depth, is often invoked as the physical mechanism; if so the resulting
source mechanism should contain a major isotropic component.
Many researchers$"" sup 1-7$ have attempted to observe
such an isotropic component,
nevertheless no one has yet convincingly proved or disproved its presence.
There exists a portion of the seismogram which is well suited to resolve the
isotropic component of deep earthquakes but which has not been analyzed by previous researchers.
A systematic analysis of 19 major deep earthquakes shows that
no significant isotropic component (less than 10% of the seismic moment) exists.
A sudden implosive phase change can be thus ruled
out as the primary physical mechanism for deep earthquakes.