K. Kuge et T. Lay, SYSTEMATIC NON-DOUBLE-COUPLE COMPONENTS OF EARTHQUAKE MECHANISMS - THE ROLE OF FAULT ZONE IRREGULARITY, J GEO R-SOL, 99(B8), 1994, pp. 15457-15467
Several recent studies have revealed statistical correlations between
earthquake mechanism type and associated non-double-couple components
in catalogs of seismic moment tensors. This systematic behavior may re
sult either from biases in the solutions due to Earth structure effect
s in different tectonic regimes, or from source radiation effects. For
certain large non-double-couple events, detailed analyses have shown
that multiple subevents with different fault orientations produce the
non-double-couple radiation. We generalize this idea and simulate non-
double-couple components (NDCC) resulting from subfaults with variable
geometry, showing that the statistical behavior of the NDCC in moment
tensor catalogs can be generally accounted for by such source complex
ity. Assuming that an earthquake fault consists of many subfaults with
random fluctuations about some mean geometry, the total moment tensor
for failure of the system under a regional stress state can be repres
ented by the sum of moment tensors of the subfaults. The sign of the N
DCC in the composite moment tensor directly reflects the stress state
applied to the fault system, and the NDCC amplitude is expected to be
systematic over a wide range of cumulative seismic moment (M(o)) given
the basic fractal nature of fault systems. These simulation results a
re consistent with the global behavior of the NDCC reported in the Har
vard centroid moment tensor catalog for 1977-1991. Parameters such as
the applied stress state, parameterization and randomness of the subfa
ult geometry, and seismic moment distribution among the subfaults affe
ct the predicted NDCC amplitude. Change in the parameters as a functio
n of subfault seismic moment controls the shape of the NDCC - M(o) rel
ationship. Regional variations of the NDCC - M(o) relationship may thu
s reflect regional variations in the fault zone parameters, raising th
e possibility that the regional NDCC behavior may be used to infer str
ess state and subfault distribution in various source regions.