Helium isotopes as a tool for detecting concealed active faults

ヘリウム同位体による未知の活断層の検出

梅田 浩司; 二ノ宮 淳

Umeda, Koji; Ninomiya, Atsushi

伏在活断層や低活動性で変動地形の明瞭でない活断層,未成熟な活断層等、いわゆる未知の活断層と呼ばれる断層を概要調査等によって確認することは、地層処分の安全性を確保する点から重要な課題である。これらの調査技術は、近年の変動地形学や地球物理学の進歩によって高い確度で識別されることが予想されるが、さらなる信頼性の向上を図るため、これまで研究事例が少なかった地球化学的アプローチによる活断層の検出手法について検討を行い、有効である可能性が示された。

A magnitude () 7.3 crustal earthquake occurred in western Tottori Prefecture, southwest Japan, on 6 October 2000. However, there was no indication of an earthquake surface fault corresponding to the 2000 earthquake. In order to elucidate the geographic distribution of the He/He ratios around the seismic source region, new helium isotope data for groundwater samples from drinking water wells were acquired. The maximum He/He ratio was observed from the well located nearest to the epicenter of the main shock. In addition, there appears to be a clear trend of decreasing He/He ratios with distance away from the main trace of the estimated fault segments. The observations provide significant evidence that the source fault of the 2000 earthquake is associated with leakage of mantle volatiles through the crust to the Earth's surface. We suggest that helium isotopes can be regarded as a tool for investigating concealed active faults with no surface expression.