Source and scatering effects on the spectra of small local earthquakes

Abstract

Abstract In order to separate the scattering effect from the source effect on high-frequency seismic waves, the spectra of S and coda waves are studied simultaneously for 900 small earthquakes (magnitude 3 to 4) in the Kanto region, Japan. The observed S-wave spectrum is the source spectrum modified by the attenuation and forward-scattering effect, while the coda spectrum is the source spectrum modified by the attenuation and back-scattering effect. From previous studies (Aki, 1980a, b) on the same data set, we know about the attenuation effect. Comparing the S-wave spectrum with the coda spectrum, we find that our data are consistent with the source spectrum with corner frequencies 3 to 6 Hz, and high-frequency decay ω−1 to ω−2, and support the hypothesis that scattering is the major cause of attenuation of S waves in the lithosphere, in which the small-scale Rayleigh scattering (Q−1 ∝ f3) applies to frequencies lower than 3 Hz and the large-scale scattering (Q−1 ∝ f0∼−1) to higher frequencies. The high-frequency decay of the source spectrum appears to be somewhat steeper for shallower earthquakes. We found that two areas with significantly different tectonic features shared similar attenuation and source effects, but showed very different envelopes of seismograms due to different forward-scattering effects. The forward-scattering effect may be the most sensitive to the structural difference.