EVIDENCE FOR A SEISMIC ATTENUATION ANOMALY BENEATH THE HIDA MOUNTAIN-RANGE, CENTRAL HONSHU, JAPAN

Previous studies have suggested a low-Q anomaly beneath the Hida Mount ain Range (the Japan Alps). However, data have been too sparse to dete rmine propagating characteristics of seismic waves beneath the Japan A lps, or to argue strongly for the existence of the low-Q anomaly. In o rder to study the low-Q anomaly in detail, 11 portable seismograph sys tems were deployed on a line across the northern part of the Japan Alp s, Central Honshu, Japan, from July 26 to October 17 in 1989. The seis mographs record ground-velocity amplitude. After corrections for the f ocal mechanisms and site effects by an empirical method, the normalize d rms amplitude of P-wave coda was used to study effects of attenuatio ns. 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 de pth in the central part of the range, i.e. around Mts Norikuradake, Ya kedake and Yarigatake. The crust at 5-15 km depth beneath these mounta ins is characterized not only by low Q but also by low velocity, low d ensity and low seismicity. This coincidence suggests the presence of a porous region saturated with a partial melt beneath the Hida Mountain Range.