Research on resistivity structure of the source region of deep low frequency earthquakes in the crust

通过对地震速度的研究，如层析成像，可以知道低频地震的震源区在日本东北部靠近火山前缘的中下地壳低速区附近。这表明地壳中的流体与低频地震的发生有关。电阻率的了解对于研究流体在地震发生中的作用是非常有用的。在这项研究中，沿着三条测线进行了宽带大地电磁测深，这三条测线穿过位于谷西大克山、栗马山和那鲁科火山附近的深部低频地震的震中地区。结果表明：1.各测线火山前缘以西的中下地壳均存在明显的低电阻区；2.深部低频地震震源区位于中下地壳低阻区边缘附近；3.中下地壳低阻区的电阻率小于10欧姆/米。4.上地壳几乎所有的微震都发生在电阻区。5.1962年宫城北部和1970年秋田东南部大地震的震源区与中地壳的低电阻率区相连。断裂带中注入的流体与大地震的发生密切相关。6.中下地壳低阻区与GPS观测揭示的高应变率区重合。

By the research in seismic velocity, for example tomography, it is known that the source region of low frequency earthquakes is near the low velocity area of the middle or the lower crust near the volcanic front in Northeastern Japan. It suggests that fluids in the crust are concerned to occurrence of low frequency earthquakes. The knowledge of the resistivity is very useful to investigate the role of the fluids to occurrence of the earthquake. In this research, wide band MT surveys were performed along three survey lines which pass through the epicentral areas of deep low frequency earthquakes near Mt.Yakeishi-dake, Mt.Kurikoma and Naruko volcano. The results are summarized as follows :1.There exist remarkable low resitivity area in the middle and the lower crust to the west of the volcanic front in each three survey line.2.Source area of deep low frequency earthquakes are near the rim of low resistivity area of the middle or the lower crust.3.Resistivity of the low resistivity area in the middle or the lower crust is less than 10 Ohm-m. That means the majority of fluids consists of brine.4.Almost all of microearthquakes in the upper crust occur in resistive areas.5.Source regions of big earthquakes of 1962 Northern Miyagi and 1970 Southeastern Akita are connected to low resistivity areas of the middle crust. It suggests that fluid injected into the fault zone relates strongly the occurrence of big earthquakes.6.Areas of low resitivity area in the middle and the lower crust, coincide areas of high strain rate revealed by GPS surveys.