Case studies of electrical and electromagnetic methods applied to mapping active faults beneath the thick Quaternary

It is of considerable importance to explore the geological structure around active faults, especially near-surface unconsolidated layers, to estimate the faults' activity. There are numerous case studies to investigate active faults using geophysical exploration methods; however, only a few cases have been verified in detail by comparison with other geological information. We have applied electric and electromagnetic methods, which can be effective for exploring to several hundred meters depth, to reveal geological structures covered by thick Quaternary formations at four active fault sites in Japan. In this paper, we used the controlled source audio-frequency magnetotelluric (CSAMT) method, the direct current (dc) resistivity method, and the resistivity tomography method. The resistivity profiles were analyzed by two-dimensional inversion techniques, and the resulting models were verified by comparison with geological evidence obtained by drilling or trenching. Our results are as follows. (1) CSAMT is an effective method for defining an outline of geological structures around a fault to several hundred meters deep. It enables us to define a resistivity boundary between different kinds of bedrock with a fault contact. (2) The dc resistivity method distinguishes each sedimentary unit as a different resistivity zone and detects the vertical displacement in the Quaternary formations. (3) The resistivity tomography method is useful to determine in more detail the flexure structure produced by faulting. Using these latter two methods, we can select drilling positions and trenching locations. In addition, it is verified from the data measured along trench walls and electrical logging that the resistivity of soft-sedimentary layers and clayey cataclastic bedrock conforms to the relationship established between the resistivity and the clay content. These resistivity methods have the advantages of detecting clayey layers as very low resistivity zones. The overall conclusion is that the combination of these resistivity methods provides us with more detailed and accurate information for estimating fault activity. (C) 2000 Elsevier Science B.V. All rights reserved.