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

日本近期的地震勘探是在地质构造复杂的地区进行的。例如，该区是指活动断裂区、火山岩区等活动构造区及其周围的勘探。它不能忽略随机分布的散射体对散射波的影响，这是地震反射中常规方法难以识别的。在这项研究中，我们研究了地壳不均匀引起的散射能量对地震反射实验记录的影响。这项研究主要通过两种方法进行。一是地震反射实验中的散射波检测。另一种是研究非均质性特征，如散射体的空间分布和非均质性强度。在森雅断裂区进行了地震反射和折射实验。1896年的陆库地震(M7.2级)出现了位错逆断层。利用横跨…区块的P波浅层地震反射勘探结合地震断层和地质调查资料，对该区新发现的4条断裂进行了评价。这些断层出现在深达四百米的运移段。这5条断层均为东倾逆断层。地震断层东侧出现褶皱构造。而在地震断层西侧观察到间歇性平面反射体。本区活动断裂仍在向西迁移。在本实验中，地震断层出现了强反射震相。这表明断层面上的非均质性很强，并产生了散射波。反射相的强度取决于反射点。这也说明断层具有很强的非均质性和空间变异性。本文利用地震台阵资料讨论了单次散射模型。通过对观测地震台阵记录的分析，我们发现倾斜叠加记录的尾波能级小于单站记录的尾波能级，这与单次散射模型的预期一致。倾斜堆积波形的能级与堆积方向有关。震源方向的包络与震源方向相反的包络的不同，归因于震源、阵列和时间的几何关系所反映的散射壳的面积大小。不同倾斜堆积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。通过对地震台阵资料进行相似性分析，发现了日本东北部6.1级地震震源区附近散射体分布的时间变化。在6.1级地震前一个月和之后两个月在震源区上方引爆的两次爆炸的倾斜叠加波形中发现了这种变化。根据大地测量结果，该区地下存在一个可能是岩浆储集层的压力源，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。这一差异表明，只有在三个月后，构成尾部的散射体才会向东移动，向更深的深度移动。在两次爆炸的间隔期间，该地区的地震和火山活动非常活跃。山的火山活动。几个月前开始的6.1级地震就在岩手县以北，在此期间仍在继续。可以认为，散射体的变化与研究区的地震和/或火山活动有关。较少

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