TEPCO’s Investigation on the Yunotake Fault

Objective

We have confirmed that some normal earthquake fault has emerged along the Yunotake Fault by the earthquake (M7.0) which occurred on April 11th on Hama-dori in Fukushima Prefecture, which we did not think need to be considered as active faults in terms of the aseismic design.

Based on this, in order to check whether there have been activities of Yunotake Fault after the Late Pleistocene (after approximately 120,000 ~ 130,000 years ago), we have been Conducting boring and trench investigations around the fault

Status of investigation (quick report)

In the past we did not consider Yunotake Fault to be active fault that needs to be taken into account in terms of aseismic design based on the condition of the surface of the grand and the evaluation on fault activities, as it is far from the nuclear power plant sites (approximately 50 km from Fukushima Daiichi and approximately 40 km from Fukushima Daini) and its impact will be small in terms of fault length.

However, since the earthquake fault has emerged along Yunotake Fault after the earthquake, we have been conducting the boring investigation and the investigation in the area shown in the left lower as the investigations by which we can understand the status of the basement, in order to confirm in more detail if there have been activities of Yunotake Fault after the Late Pleistocene.

As a result, we have found some evidence (slips) in the sediment that we evaluated became deposited after the Late Pleistocene, which implicates Yunotake Fault acted in the past. Now we have been analyzing the time of the sediment and once we have finished it, we will announce the result.

See full report here

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