Abstract
Small-scale seismic velocity heterogeneity has been studied through the calculation of peak amplitude ratio as a means to quantify the strength of seismic wave scattering at volcanoes in Japan. This ratio is defined as the ratio of the maximum (peak) P wave energy in the transverse component seismogram envelope over that of the three-component sum seismogram envelope (transverse + radial + vertical). According to the previous study using Japan’s Hi-net seismometer network, the peak ratio is observed to be larger near the (active) quaternary volcanoes. However, these Hi-net stations are not positioned on the volcanoes themselves. This study systematically examines the peak ratios at 47 active volcanoes across Japan, using seismometers operated by the Japan Meteorological Agency (JMA). Analyses were performed at four frequency bands: 0.5–1, 1–2, 2–4, and 4–8 Hz. We found that the JMA stations yield higher peak ratios than the Hi-net stations. Their differences are statistically significant at the 99.9% confidence level in all frequency bands. We also examined the differences between the ground surface and borehole stations of the JMA network. The former shows larger peak ratios, and for most frequency bands, the differences are also statistically significant at the 99.9% confidence level. This suggests an intensification of small-scale medium heterogeneities especially at shallow depths at active volcanoes, and that scattering might have been enhanced at the very shallow parts.Graphical
Highlights
IntroductionSmall-scale heterogeneities in seismic velocity exist in the crust and lithosphere, and they can invoke the scattering of seismic waves (Sato et al 2012)
According to Sato (2006), assuming the Gaussian type random medium, peak ratio can be approximated as 1.81ε2h/a where ε describes the root-mean-squared fractional fluctuation in seismic velocity which controls the amplitude of velocity heterogeneity within the medium, a is the correlation distance which controls the size and shape of patches of velocity heterogeneity, and h is the thickness of the heterogeneous medium
Nishimura (2012) analysed the envelopes of teleseismic P waves recorded by the high sensitivity seismograph network (Hi-net) borehole stations in Japan, and found that tectonically active regions, for example, those in the proximity of quaternary volcanoes or active faults, showed higher peak ratios
Summary
IntroductionSmall-scale heterogeneities in seismic velocity exist in the crust and lithosphere, and they can invoke the scattering of seismic waves (Sato et al 2012). Nishimura (2012) analysed the envelopes of teleseismic P waves recorded by the Hi-net borehole stations in Japan, and found that tectonically active regions, for example, those in the proximity of quaternary volcanoes or active faults, showed higher peak ratios. Results From 204 JMA seismic stations deployed around active volcanoes in Japan, peak ratios were computed at 134, 140, 136, and 109 stations under the frequency bands of 0.5–1, 1–2, 2–4, and 4–8 Hz, respectively.
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