Blackwell Publishing Journal Backfiles 1879-2005
Previous studies have suggested a low-Q anomaly beneath the Hida Mountain Range (the Japan Alps). However, data have been too sparse to determine propagating characteristics of seismic waves beneath the Japan Alps, or to argue strongly for the existence of the low-Q anomaly.In order to study the low-Q anomaly in detail, 11 portable seismograph systems were deployed on a line across the northern part of the Japan Alps, Central Honshu, Japan, from July 26 to October 17 in 1989. The seismographs record ground-velocity amplitude. After corrections for the focal mechanisms and site effects by an empirical method, the normalized rms amplitude of P-wave coda was used to study effects of attenuations. It becomes clear in the present study that:(1) the shallow crust (z 〈 3-5 km) beneath the profile causes normal attenuation of P waves,(2) strong P-wave attenuation was observed from an intermediate-depth earthquake at some stations close to the crest of the Hida Range, and(3) a marked low-Q anomaly can be outlined in the crust at 5-15 km depth in the central part of the range, i.e. around Mts Norikuradake, Yakedake and Yarigatake.The crust at 5-15 km depth beneath these mountains is characterized not only by low Q but also by low velocity, low density and low seismicity. This coincidence suggests the presence of a porous region saturated with a partial melt beneath the Hida Mountain Range.
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