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

在日本本州岛东北部的北上地区，地震活动是众所周知的低。通过与高地震活动区的应力场对比，可望揭示应力场的孕震条件。本州东北部被认为位于太平洋板块向西俯冲的北美板块之下。在北上地区，与板块边界或附近的大地震的地壳应变的同震变化主要在东西分量中观察到。尽管如此，大地测量表明，在大约75年的长期内，南北或东北-西南方向的延伸占主导地位。这些似乎是相互矛盾的。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可望解决这一问题，并提供关于板块边界在地壳运动中的作用的知识。 ...更多信息 基于上述认识，利用形变速率分析法对北上地区4个测点进行了地应力测量。这些遗址位于太平洋沿岸最多40公里的范围内。结果表明：1）各测点最大水平应力的大小与垂直应力的大小相近或稍小于垂直应力; 2)各测点最小水平应力轴均沿沿着或东北西南方向分布。3)最大剪应力与最大剪切面上正应力的比值r在0.17 ~ 0.35之间。（1）和（2）的结果与长期水平应变变化一致。这表明，北上地区乃至日本东北部的应力场不是由太平洋板块与北美板块碰撞直接机械地形成的。将结果（3）与1984年长野县西部M=6.8地震的震源区应力进行比较，对于这两个地区，地震活动似乎在r大于约0.3的场地周围活跃。此外，比较表明，北上区域的最大剪应力平均而言并不小，应力的变化小于焦点区域。这可能表明，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