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

通过层析成像等地震速度研究，得知日本东北部火山前缘附近的中、下地壳低速区附近是低频地震的震源区。这表明地壳中的流体与低频地震的发生有关。电阻率的认识对于研究流体在地震发生中的作用是非常有用的。在本研究中，沿沿着三条测线进行了宽频带MT测量，这三条测线穿过屋石岳、栗驹山和鸣子火山附近的深低频地震震中区。结果表明：1.火山前缘以西的中、下地壳存在明显的低阻区; 2.深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 Ohm-m。4.上地壳的微震几乎都发生在电阻率区。5. 1962年北方宫城和1970年东北部秋田大地震的震源区与中地壳的低阻区相连。6.中、下地壳的低电阻率区与GPS测量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.