腾冲火山区剪切波和尾波衰减初步研究

PRIMARY STUDY ON ATTENUATION OF SHEARS WAVES AND CODA IN TENGCHONG VOLCANO AREA

用云南腾冲火山地区观测的地震数字化记录进行了尾波和剪切波的衰减测量。用AkiandChouet和Sato单次散射模型获得 5 2秒和 18秒流逝时间尾波的品质因子分别为Qc (f) =5 7×f- 0 94和Qc (f) =2 6×f- 0 95 ,表明地壳上部和地壳上地幔的地震波衰减频率变化率一致。两者均未显示出活火山地区尾波衰减的异常特征。尾波掠过空间椭球的平均半轴 ,前者约为 80km ,大大超出了腾冲火山分布的地理范围 ,后者约为 30km ,在腾冲火山分布的地理范围之内。用尾波规一双台S波振幅比法进一步缩小所用地震波掠过的空间 ,最后获得了S波随频率的衰减有类似于前人报导的活火山地区尾波频率衰减的特征 :相对于低频 (1 5～ 6Hz) ,高频 (6～2 0Hz)的Q- 1s 变化很小。从地表位置看 ,腾冲最年青的全新世的马鞍山和老龟坡火山就在S波掠过范围内 ,打鹰山火山也在近旁。这一结果表明在这三座火山体之下存在低速高衰减体。火山或处于休眠状态 ,或在消亡过程之中。

Attenuation measurements of coda and shear waves have been made by digital data recorded in Tengchong volcano area. The quality factors of coda for lapse times 52s and 18s are Qc(f)=57xf 0 94 and Qc(f)=26xf 0 95 respectively, which are obtained by the single scattering model developed by Aki, Chouet(1975) and Sato(1977). It is shown that both of attenuation changes with frequencies in upper crust and the crust and the upper mantle are coincided. Both of them do not indicate anomaly features of coda attenuation in active volcano areas. The averaging half axis of coda ellipsoid is 80km for the former, which is greatly bigger than range of Tengchong volcano distribution; 30km for the later, which is within the range of the volcanoes. The space that the seismic wave sampled is further reduce by the method of two station amplitude ratio for S waves with coda mormalization. Finally, the attenuation variation with frequencies of S waver is obtained, which has similar feature of attenuation of coda with frequency in active volcano areas that was reported by previous reaearchers. The Q S -1 variation of high frequency(6-20Hz) is little comparing with low frequency(1 5-6Hz). From the view of position, the youngest Maanshan and Laoguipo volcanoes in Holocene Epoch are just located in the place that S wave passes, and Dayingshan volcano is nearby. The results show that there exists a body with low velocity and high attenuation. Therefore, the volcanoes are probably in dormant state or dying.