九重火山地域における地震波減衰構造

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The Kuju active volcanic region locates in the central Kyushu graben area. To investigate the upper crustal structure beneath this volcanic region, the seismic network (7 stations) was newly established and the earthquakes which occurred in and around Kyushu Island were observed. Since the observational condition of each seismic station was not same, the corrections of site-effects and other seismic recording circumstances were performed to be enabled to compare and examine the seimic waves recorded at each station. As a result, the amplitudes of seismic waves which passed through the Kuju volcanic region were recognized to be obviously attenuated. The attenuating region located in the area of 5-15 km northward from Mt. Kuju and in the depth of 5-20 km. The average Q-values between 2 seismic stations could be obtained from the spectral ratios of seismic waves. The Q-values were estimated 50-100 level at the attenuating region. At the non-attenuating region the values were above 200-500. Because these Q-values are average at the ray paths between the seismic stations, the Q-values at the attenuating region are anticipated to become locally more little value. The location of the attenuating region coincides with the negative Bouguer gravity anomaly area and the low seismic active area. It is indicated that, beneath the northern part of the Kuju active volcanic region, there are the presence of seismic wave attenating, low density and brittle materials.
1996-03-05

九重火山地域における地震波減衰構造

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