EVALUATION OF EFFECTS OF SCATTERED WAVE BY CRUSTAL INHOMOGENEITY ON SEISMIC EXPLORATION RECORDS

日本近年来的地震勘探多在地质构造复杂的地区进行。例如，在活动断裂带、火山岩等活动构造区内及其周围的勘探。它不能忽略随机分布的散射体对散射波的影响，这是地震反射中常用技术难以识别的。本文研究了地壳不均匀性对地震反射实验记录的影响。这项研究以两种主要方式进行。一是地震反射实验中散射波的检测。二是研究散射体的空间分布和非均匀性强度等非均匀性特征。在仙亚断裂带进行了地震反射和折射实验。1896年里kuu地震（M7.2）发生了逆断层位移。通过横贯露头的p波浅层地震反射测量和地质调查，估计了本区新出现的4条断层。这些断层出现在深达400米的迁移段。这5条断裂为东倾逆断层。地震断裂带东部出现褶皱构造。然而，在地震断裂带的西部观测到间歇性的平面反射体。研究区内活动断裂仍在向西移动。在本次实验中，地震断层处出现了强反射相。这说明断面上的非均匀性较强，会产生散射波。反射相位的强度取决于反射点。这也说明断层具有较强的非均质性和非均质性的空间差异性。利用地震阵列数据讨论了单散射模型。通过对观测到的地震台阵记录的分析，发现斜叠记录的尾波能级小于单台站记录的尾波能级，与单散射模型预测的结果一致。斜叠加波形的能级与叠加方向有关。在震源方向与相反方向叠加的包络层之间存在差异，主要是由震源、阵列和运行时间的几何关系所反映的散射壳面积所决定的。不同倾斜叠加方向的能级变化表明水平排列的散射体比垂直排列的散射体占优势。本文的分析可能对探测散射各向异性有用。对日本东北部6.1级地震震源周围的散射体分布进行了似然分析。这种变化是在6.1级地震发生前一个月和发生后两个月在震源区域上方引爆的两次爆炸的倾斜叠加波形中发现的。根据大地测量资料，该地区下方存在可能为岩浆储层的压力源，其位置在观测期间发生了变化。两次爆炸的倾斜叠加波形略有不同。这种差异表明，尾部的散射体在三个月后才向东移动，并向更深的深度移动。在这一地区，地震和火山活动在两次爆炸间隔期间非常活跃。几个月前开始的6.1级地震以北的岩手山火山活动在此期间仍在继续。可以认为，散射体变化与研究区域的地震和/或火山活动有关。少

Recent seismic exploration in Japan is carried out in complex geological structure area. For instance the area is the exploration in and around active tectonic area such as active fault area and volcanic area. It can not neglect an effect of scattered wave by randomly distributed scatterers, which is difficult to identify by usual techniques in seismic reflection. In this study, we investigated effects of scattered energy by crustal inhomogeneity on records obtained in seismic reflection experiments. The study has carried out in two major approaches. One is detection of scattered waves in seismic reflection experiment. Another is investigation for characteristics of inhomogeneity as spatial distribution of scatterer and strength of inhomogeneity.An seismic reflection and refraction experiment has carried out in the Senya faults area. Displacement had appeared with reverse faulting on Rikuu Earthquake in 1896 (M7.2). By means of P-wave shallow seismic reflection survey across the outcro … More p of the earthquake fault and geological survey, New four faults are estimated in this area. These faults appear in the migration section down to four hundred meters depth. These five faults are reverse faults with east dip. Folded structure appear in the east of the earthquake fault. However, intermittend plane reflectors are observed in the west of the earthquake fault. The active fault is still migrating westward in this study area. In this experiment, strong reflected phases appear by earthquake faults. This suggests inhomogeneity is strong on the fault plane and generate scattered wave. The strength of reflected phase is depend on reflection point. This also suggests that the fault has strong inhomogeneity and spatial variation of inhomogeneity.We discuss the single scattering model by using seismic array data. By analyzing observed seismic array records, we found that coda energy level for slant-stacked record is smaller than that for single station record as expected from the single scattering model. The energy level of slant-stacked waveform is dependent on stacking direction. The difference between envelopes stacked in the direction of hypocenter and in the opposite one attributes to the area of scattering shell reflected by geometrical relationship among source, array, and lapse time. The energy level variation for different slant-stacking directions suggests that horizontally aligned scatterers predominate over vertical ones. The present analysis is probably useful to detect scattering anisotropy.Temporal variation of the scatterer distribution has been detected around the focal area of M6.1 earthquake, northeastern Japan, by applying semblance analysis to seismic array data. The variation was found in slant-stacked waveforms of two explosions detonated just above the focal area one month before and two months after the M6.1 earthquake. According to geodetic observations, a pressure source that might be magma reservoir exists beneath this region and its location changed during the observation period. Slant-stacked waveforms are slightly different between the two explosions. The difference suggests that a scatterer contributing to the coda part is shifted toward the east and to the deeper depth only after three months. In this region, seismic and volcanic activity was very active during the interval of the two explosions. Volcanic activity of Mt. Iwate just north of the M6.1 earthquake, which started several months before, continued during the period. It can be considered that scatterer change relates to seismic and/or volcanic activity in the studied area. Less