THE 12 JULY 1993 HOKKAIDO-NANSEI-OKI, JAPAN, EARTHQUAKE - EFFECTS OF SOURCE COMPLEXITY ON SURFACE-WAVE RADIATION

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.