Estimation of in-situ stresses in and around the Kitakami area in Northeast Honshu, Japan, by means of deformation rate analysis

在日本本州东北部的北上地区，地震活动很低。通过将该地区的应力场与地震活动区进行对比，有望揭示该地区应力场发生地震的条件。本州东北部被认为位于北美板块之下，太平洋板块向西俯冲。在北上地区，板块边界或板块边界附近发生大地震时，地壳应变的同震变化主要表现在东西分量上。然而，大地测量表明，在大约75年的长时间内，南北或东北-西南方向的扩展占主导地位。这些似乎彼此不一致。这种不一致的状态仍然是未知的。应力场与地壳应变的关系有望解决这一问题，并提供关于板块边界对板内地壳应力场作用的认识。根据上述期望，采用变形率分析（DRA）方法在Kitakami地区的四个地点测量了地应力。这些地点最多位于距离太平洋海岸约40公里的范围内。结果表明：1)各测点最大水平应力的量级均接近或略小于垂直应力。2)各站点的最小水平应力轴沿南北或东北-西南方向分布。3)最大剪切面上最大剪应力与正应力之比r为0.17 ~ 0.35。(1)和(2)的结果与长期水平应变变化一致。这表明北上地区的应力场以及日本东北部的应力场并不是由太平洋板块与北美板块的碰撞直接机械形成的。将结果(3)与1984年长野县西部M=6.8地震震源区的应力进行比较，两者的地震活动似乎都在r大于0.3左右的位置附近较为活跃。通过对比可知，北上地区最大剪应力的平均值不小于震源区，剪应力的变化也小于震源区。这可能说明应力场的不均匀性对地震的发生起着重要的作用。少

In the Kitakami area in northeast Honshu, Japan, seismic activity is known tobe low. The comparison of the stress field in this area with the area of high seismic activity is expected to reveal the conditions on the stress field to generate earthquakes. The northeast Honshu is considered to be placed on the north American plate under which the Pacific ocean plate subducts toward the west. In the Kitakami area, the coseismic change in crustal strain with a large earthquake at or near the plate boundary is predominantly observed in the east-west component. Nevertheeless, geodetic measurements have shown that the extension in the north-south or the northeast-southwest direction dominates during a long term of about 75 years. These appear to be inconsistent each other. What this inconsistency states is still unknown. The relation of the stress field to the crustal strain is expected to yield a solution about this problem and to present a knowledge about the role of the plate boundary on th … More e stress field in the intraplate crust.For the above expectation, in-situ stresses are measured at four sites in the Kitakami area by deformation rate analysis (DRA). The sites are located within about 40 km at most apart from the coast facing the Pacific ocean. This measurement reveals as follows : 1) The largest horizontal stress is nearly equal to or slightly smaller than the vertical stress in magnitudes at all sites. 2) The smallest horizontal stress axis lays along the north-south or the northeast-southwest direction at every site. 3) The ratio r of the maximum shear stress to the normal stress on the maximum shear plane ranges from 0.17 to 0.35. The results of (1) and (2) are consistent with the horizontal strain change of long term. This suggests that the stress field in the Kitakami area and thus that in the northeast Japan is not mechanically formed directly by the collision of the Pacific ocean plate to the north American plate. Comparing the result (3) with the stresses in the focal area of the 1984 Western Nagano Prefecture earthquake of M=6.8, for both the areas seismic activity seems to be active around the sites where r is larger than about 0.3. Further, the comparison shows that the maximum shear stress in the Kitakami area is not smaller on an average and the variation of the tress is smaller than the focal area. This may suggest that the non-uniformity of stress field plays an important role on generating earthquakes. Less