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

观测最密集的地区之一是京都县北部和滋贺西北部。该区适合研究发震构造和发震条件。在该地区选择了18个点作为观测点的候选点。由于人工电磁噪声的存在，远参考法是必要的。因此，远程站点的选择是本研究的重中之重。我们在我们的第一个候选参考点峰山进行了MT观测，并在和知町的水见进行了初步观测。结果表明，研究区普遍存在人工噪声污染，峰山站也受到异常大地噪声电流的影响。因此，我们必须改变观测方法，从单一的宽带MT方法向宽带MT方法和网络MT方法相结合的方法发展 关于我们 这种方法使我们能够在噪声区获得长周期的数据。因此，我们将研究区从京都县北部移至与第一个研究区相邻的滋贺西北部，并在滋贺西北部的Kutsuki村的三个地点进行了MT观测。我们还将远程站点设置在奈良县北部的野濑川村。距离Kutsuki村约150公里。我们使用远程引用方法分析数据。随着地磁场变化频率的降低，三个测点的相位值从45度增加到60度，视电阻率逐渐降低。还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。[）300m深，层底与发震层顶重合。这一结果是有意义的，因为两个独立的观测结果一致表明，高导层覆盖孕震层。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