Aeromagnetic anomalies in the inundation area by the tsunami of the 2011 off the Pacific coast of Tohoku Earthquake, northeast Japan

Abstract

In June 2012, the Geological Survey of Japan (GSJ) conducted an airborne EM and magnetic survey over the inundation area by the tsunami of the 2011 off the Pacific coast of Tohoku Earthquake, northeast Japan. The purpose of the survey was mainly to map the resistivity of the subsurface structure associated with sea water invasion by the tsunami. Airborne EM data were successful for revealing the subsurface resistivity distribution as an aid for groundwater assessment of the study area. Aeromagnetic data were also observed by the survey and processed. However, it turned out that the magnetic data seem to be contaminated by artificial noise probably caused by the survey helicopter. To mitigate directional errors, the generalized mis-tie control method (Nakatsuka and Okuma. 2006b) was applied to the observed magnetic data and magnetic anomalies were reduced onto a smoothed observation surface. According to the preliminary aeromagnetic anomaly map, magnetic highs lie over the Cretaceous granitic rocks outcropping on the north-trending Wariyama Mountains, which may constrain the groundwater flow system. The magnetic highs also extend NE and reach the Pacific coast, implying the existence of Cretaceous granitic rocks.