Conductivity structure around the upper boundary of a seismogenic layr of inland type earthquakes.

微震观测最密集的地区之一是京都北部地区和滋贺县西北部地区。该地区适合研究发震地层的构造和条件。选择了18个点作为该地区观测点的候选点。由于人工电磁噪声的存在，因此本研究需要远距离参照法。因此，边远地区的选址问题就成为本研究的重点。我们在Mineyama遗址进行了MT观测，这是我们第一个作为参照点的候选遗址，并在和池镇的Mizunomi遗址进行了初步观测。研究发现，我们的研究区普遍受到人为噪声的污染，同时还发现Mineyama站受到异常大地噪声电流的影响。因此，我们必须将观测方法从单纯的宽带MT方法改为宽带MT方法和网络MT Me…相结合的方法使我们能够在噪声较大的地区获得长周期数据。这一结果使得我们的研究区域从京都北部地区转移到与第一个研究区域相邻的志贺县西北部地区，并进行了网络大地电磁观测。我们还把地点设在奈良县北部的小谷川村。它距离Kutsuki村约150公里。我们使用远程引用的方法来分析数据。随着地磁变化频率的降低，三个地点的相位值都从45°增大到60°，视电阻率逐渐减小。并进行了一维模型分析。最合适的模型表明，在1000(]sy以下存在高导电层。-.[)埋深300m，其底部可能与孕震层的顶层重合。这一结果是有意义的，因为两个独立的观测显示了一致的结果，即高导电层覆盖了孕震层。较少

One of the most dense observation of micro earthquakes covers the north area of Kyoto prefecture and northwestern area of Shiga prefecture. This area is suitable for the study of the structure and condition of seismogenic layr. Eighteen points were selected as candidates of observation points in thisarea. Remote reference method is necessary for this study because artificial electro-magnetic noise should exist. Therefor the selection of remote site is on the most important item of this study. We carried out MT observation at Mineyama site which is the our first candidate as the reference site and also carried out preliminary observation at Mizunomi site in Wachi town. It was founded that our study area is widely contaminated by artificial noise and also found that Mineyama station is affected by unusual telluric noise current. Therefore we have to change the observation method from the broad-band MT method only to the combination method of the broad-band MT method and the Network MT me … More thod, which make us to enable to get long period data in noisy area. This result lead us to move our study area from the north area of Kyoto prefecture to the northwestern area of Shiga prefecture which is just adjacent to our first study area and we also carried out the Network MT observation.The broad-band MT measurements was made at threes sites in Kutsuki village, northwestern area of the Shiga prefecture. We also set the remote site at the Nosegawa village, north area of the Nara prefecture. It is about 150Km from the Kutsuki village. We analyze data using the remote referencemethod. Phase value of all of three sites increase from 45 degree to 60 degerr and apparent resistivity decrease gradually as frequency of geomagnetic variation decrease. One dimensional model analysis was also carried out. The most suitable models show that highly conductive layr exist below 1000(]SY.+-。[)300m in depth and the bottom of the layr may coincide with the top layr of the seismogenic layr. This result is meaningful because two independent observation show the consistent results that highly conductive layr cap the seismogenic layr. Less