Dependence of Coda Q on Frequency and Lapse Time in the Western Nagano Region, Central Japan

Abstract

We have investigated the dependence of coda Q (Q_C) on frequency and lapse time using the data observed at short hypocentral distances (1-23 km) over a wide frequency range (1-64 Hz). Q_C was determined by applying a single isotropic scattering model to the bandpass-filtered seismograms from aftershocks of the 1984 Western Nagano Prefecture, Earthquake. Q_C depends on frequency, f in Hz, according to a power law, Q_C=Q₀fⁿ. We also found a strong dependence of Q_C on lapse time, in particular, for short lapse times less than 10 s. This lapse time dependence is characterized by an increase in Q₀ and a decrease in n with lapse time. We interpreted this observation as indication of depth variation of Q_S and estimated a crustal model of one-dimensional Q_S structure from Q_C measured at various lapse times. The model exposes low Q_S in a surface layer and a significant increase in Q_S with depth in the upper 2 km of the crust. Q_S in the uppermost crust is compatible with that measured at a deep borehole in the Kanto Plain, central Japan. However, we cannot attribute the lapse time dependence of Q_C to the sole effect of depth, variation of Q_S, because Q_C calculated from the Q_S model shows more gradual increase than the observation. The scattering coefficient (g₀) estimated from the amplitude ratio of S- and coda waves, as well as Q_S calculated from the energy flux model, suggest a contribution of multiple scattering to the lapse time dependence for long lapse times. For short lapse times, on the other hand, depth variation of g₀, as indicated by the change in n with lapse time, is possibly responsible for the sharp increase in Q_C with lapse time. Thus the apparent lapse time dependence of Q_C is caused by a combination of depth variation of Q_S and g₀, and multiple scattering effect.