Seismic attenuation in a nonvolcanic swarm region beneath Wakayama, southwest Japan

Abstract

Seismic attenuation in the Wakayama swarm area of southwest Japan is estimated by optimally fitting the theoretical S coda model to an average S coda envelope in the frequency range of 1–48 Hz. The average S coda envelope is confirmed to have a common decay curve independent of the source‐station distance. Intrinsic attenuation is found to peak at around 4 Hz, suggesting thermal diffusion in grain size or crack length domains with dimensions of the order of 0.7–1 mm, which is reasonable for crustal rock. Scattering attenuation is well approximated by the relation 0.054f−1 in this frequency range, with an inferred peak at 0.5 Hz, suggesting that the statistical model is characterized by an exponential autocorrelation function with a correlation length of about 1 km and a fractional velocity fluctuation of 21%. Scattering is the dominant cause of attenuation at frequencies below 2 Hz, while intrinsic attenuation becomes predominant above 4 Hz. Although values of coda attenuation (Qc−1) generally lie between the total and intrinsic attenuation, the differences between these three values becomes very small above 4 Hz, indicating that the coda attenuation measurements provide a reasonably good estimate of the total attenuation. The total attenuation was found to coincide well with the total apparent attenuation estimated by a linear inversion using direct S waves from swarm earthquakes, confirming the validity of the present results.