近畿中北部におけるコーダ波の減衰特性

抄録

Attenuation property of lithosphere has been studied in various regions by analyzing decay rate of coda amplitude. In this paper, Q_c values were determined for intra-plate earthquakes recorded by the telemetered network in the middle and northern Kinki district, southwest Japan. All events are shallower than 20km having magnitudes of 1.8-4.0. Q_c was determined by fitting a back scattering model to band-pass filtered coda decay curves in five frequency bands from 2 to 32Hz, with 8 lapse times ranging 20 to 85 s. By fitting Q_c to the power law, Q_c=Q_ofⁿ, average values of Q_c and n in this district were obtained as Q_o=74 and n=0.77. These are typical values in the active tectonic region such as island arcs. Although the Q_c value depends on the lapse time used for the analysis, n is almost independent of the lapse time. The radius (r) of scattering area contributing coda wave is roughly estimated as r=V_st/2, where V_s is S-wave velocity and t is lapse time. Therefore, Q_c values can be derived as a function of depth from the relationship between Q_c and lapse time. Q_c increases rapidly with increasing lapse time and becomes nearly constant at large lapse times. This shows that Q_c increases rapidly in the shallow crust and becomes nearly constant in the upper mantle. The increasing of Q_c with depth is larger in the northern part than in the middle part of Kinki district. This suggests that the depth dependence of Q_c is related to the structure of the lower crust and the upper mantle.