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县北部和滋贺县西北部。该区适合研究孕震层的构造和条件。选取18个点作为该区域的观测点候选点。由于人工电磁噪声的存在，需要采用远程参考的方法。因此，远程站点的选择是本研究的重中之重。我们在第一个候选参考点Mineyama站点进行了MT观测，并在Wachi镇Mizunomi站点进行了初步观测。发现研究区存在大面积的人工噪声污染，同时也发现矿山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大地电磁法转变为宽带大地电磁法与网络大地电磁法的结合方法，使我们能够在有噪声的地区获得长周期的数据。这一结果促使我们将研究区域从京都北部地区转移到滋贺县西北部地区，该地区正好与我们的第一个研究区域相邻，我们也进行了网络MT观测。宽带MT测量是在滋贺县西北地区久月村的三个地点进行的。我们还在奈良县北部的Nosegawa村设置了偏远的地点。距离久月村约150公里。我们使用远程参考方法分析数据。随着地磁变化频率的减小，3个测点的相值均从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。[)300米深，地层底部可能与发震层顶部重合。这一结果是有意义的，因为两次独立的观测结果一致表明，高导电性层盖住了发震层。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