K. Kuge et al., THE 12 JULY 1993 HOKKAIDO-NANSEI-OKI, JAPAN, EARTHQUAKE - EFFECTS OF SOURCE COMPLEXITY ON SURFACE-WAVE RADIATION, Bulletin of the Seismological Society of America, 86(2), 1996, pp. 505-518
We present an analysis of the data dependency of surface-wave inversio
ns for source parameters of the 12 July 1993 Hokkaido-Nansei-Oki, Japa
n, earthquake. This dependency is demonstrated by differences between
source mechanisms obtained from different data sets, particularly from
Rayleigh waves or Love waves, although inversions of various data set
s yield unanimously thrust-fault mechanisms. For the inversion of Rayl
eigh waves, the mechanism is characterized by a shallow-dipping west-p
lunging nodal plane, which is consistent with P-wave first-motion data
and the distribution of aftershocks close to the hypocenter. However,
for the inversion of both Rayleigh and Love waves, the mechanism is c
haracterized by a shallow-dipping east-plunging nodal plane. Several n
umerical experiments were carried out to test a hypothesis that the da
ta dependency is attributed to the effects of source complexity on the
surface-wave radiation, with the source complexity being characterize
d by tempo-spatial changes in focal mechanism and moment release durin
g the rupture process. The experiments show that source parameters obt
ained from inversions of surface waves with periods as long as a few h
undred seconds can be largely affected by using various models of sour
ce complexity proposed previously for the Hokkaido earthquake, and the
effects on Love waves tend to be systematically different from those
on Rayleigh waves. Our preferred source model is characterized by sign
ificant tempo-spatial changes in focal mechanism. The experiments base
d on this model provide a good explanation for the data dependency, al
though our analysis cannot completely reject the alternative explanati
on that attributes the dependency to the manifestation of errors in mo
deling the surface-wave propagation and/or inversion instability. The
model is consistent with the data of body waves and long-period surfac
e waves for a very broad frequency band, implying the existence of wea
k zones with various fault geometries along the eastern margin of the
Japan Sea.