Observation of Seismic Surface Waves and Earth's Free Oscillations Using a High-Precision Laser Interferometer

对地震面波和地球自由振荡的观测提供了有关地球内部结构的宝贵信息。然而，主要被分析的地震数据是垂直分量，因为水平分量通常包含来自地面倾斜的非常大的低频噪声。在另一项研究中，我们研究了水平分量噪声的来源，发现它与大气压力变化有很强的相关性。压力荷载似乎会引起地面倾斜。因此，只要我们使用摆式地震仪，水平分量的噪声是不可避免的。本研究的目的是用高精度的激光干涉仪观测地震面波和地球的自由振荡，该干涉仪没有地震仪记录中观测到的噪声。主要研究结果如下：1.研制了用于激光干涉仪的碘稳定的Nd：YAG激光系统。2.研制了包含干涉仪系统的双腔管真空系统。为了避免大气压力变化的影响，采用了4个波纹管的补偿机制。为了保持10^-6托的真空度，我们使用了涡轮分子盛大系统。干涉仪基线长度约15米。3.激光干涉仪系统安装在名古屋大学犬山观测站，现已准备好观测大地震激发的地震面波和地球自由振荡。

Observations of seismic surface waves and Earth's free oscillations give precious informationon internal structure of the Earth. However, seismic data that have mainly been analyzed are vertical component since horizontal components usually contain very large low-frequency noise originated from ground tilting. We have investigated the origin of horizontal component noise in another research and have found that it has a strong correlation with atmospheric pressure variation. It seems that pressure loading gives rise to ground tilting. Therefore the horizontal component noise is inevitable as long as we use seismometers with pendulum.This research was aimed to observe seismic surface waves and Earth's free oscillations using a high-precision laser interferometer, which is free from the noise observed in seismometer records. The results are as follows :1.Iodine-stabilized Nd : YAG laser system was developed to use for a laser interferometer. Frequency stability of 1O^<13> were achieved.2.Vacuum system with two chambers and pipes connecting them was developed to contain interferometer system. Compensation mechanism using 4 bellows was adopted to avoid the effect of atmospheric pressure variations. To maintain a vacuum of 1O^<-6> Torr we used a turbo-molecular pomp system. Length of interferometer base line is about 15m length.3.The laser interferometer system was installed at Inuyama observation station of Nagoya University, It is now ready to observe seismic surface waves and Earth's free oscillations excited by large earthquakes.