日本上地壳S波衰减成像

地球介质的衰减特性可以从地震波中的频谱信息中获取，并且能够提供地球内部结构非均匀性的重要信息.与大量的速度结构研究相比，对衰减结构的研究却较少，且大多仅对日本群岛局部地区做研究.本文运用M_L振幅层析成像技术反演获得了整个日本群岛上地壳的品质因子Q₀值（1 Hz时的Q值）.计算中共利用了日本Hi-net台网971个台站记录到的5559个地震事件，超过60000 条Sg波最大振幅-频率数据，并且满足震中距小于2°、震源深度小于10 km.计算结果表明,低Q₀值区域主要存在于日本群岛的中央火山带，与日本新生代以来的火山分布非常一致；同时，位于从神户到京都东北地区、以及四国岛东部等地的非火山区，断层密集，对应的Q₀值也很低；另外，西太平洋俯冲海沟与日本群岛东海岸之间的区域也是低Q₀值区，这说明高温、岩石强烈破碎、深厚的沉积物是造成地震波强烈衰减的主要原因.高Q₀值主要分布在中央火山带前缘与日本东海岸之间.

S-wave attenuation tomography of upper crust in Japan

Seismic attenuation (Q-factor) can be estimated by extracting the amplitude-frequency information contained in seismic waveforms. We applied M_L amplitude tomography to estimate attenuation of upper crust in Japan. More than 60000 Sg-wave maximum amplitude data from 5559 events recorded by 971 stations were selected from the dense High-Sensitivity Seismographic Network (Hi-net) under the condition that epicentral distance is less than 2°; event depth is less than 10km. The lateral S-wave Q-factor variations of upper crust at 1Hz in Japan were obtained, and the result indicates that the low Q values exist in the central Japan islands, and have almost the same distribution with volcanoes; the low-Q zones also exist in non-volcanic regions at the Kyoto and Kobe prefectures, and eastern Shikoku with dense faults there; in addition, low Q was found between eastern coast and Japan trench. These suggest that the high attenuation (low Q value) results from high temperature, cracked rocks and thick sediment. The high Q values mainly exist between the front of volcanoes and the Japanese east coast.