STUDY OF THE HF SEISMIC ATTENUATION IN KINKI REGION, JAPAN, USING THE RAY THEORY ELASTIC ATTENUATION EFFECT IN 3-D VELOCITY MODEL AND THE 3-D STRUCTURE OF Q-VALUE

摘要

We studied seismic waves attenuation in Kinki region, Japan, in high frequency range 1-10Hz. To do this it was assumed that observed amplitude Fourier spectrum is a product of source, elastic path attenuation, inelastic path attenuation and site effects. Inelastic path attenuation is described by the Q-value. Frequently, to estimate path attenuation effect, Q-value is inverted under simplified assumption that geometrical spreading is spherical. Actually, elastic attenuation is complex effect that includes geometrical spreading in non-uniform velocity model, reflection and conversion on major velocity discontinuities, free-surface effect. In this study, elastic path attenuation was calculated using the raytheory approach in 3-D velocity model. Source and Site effects were eliminated using the double-spectral ratio scheme. Travel times inside blocks were calculated by the 3-D ray tracing. After this, Q-values in blocked media were inverted using a tomography approach. For this study, we developed 3-D velocity model for Kinki area. This model includes: low-velocity layer (LV), upper crust (UC), lower crust (LC), subducted Philippine Sea plate (SP) and mantle wedge (MW). The whole media of the wave propagation were divided into blocks with constant Q-value according to the tectonic structure. For inversion we used borehole data of the Hi-net and Denkyoken networks and hard rock/stiff soil data of CEORKA network. Analysis of the results of inversion shows that largest Q～150f 0.8 was in UC block, smallest Q=70f 0.57 was in LC+MW block; for SP Q=175 f 0.3. In UC, Q-value is larger in central Kinki region: Q=220 f 0.86, than in the south: Q=120 f 0.55 or in northern part of Kinki region: Q=125 f 0.68. Based on results of inversion we calculated examples of attenuation correction factor: the difference between path attenuation calculated by the developed model and by the uniform model, for a set of sources.