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An increase of outflow of 1.70–4.66 mm was observed in five mountain small basins within areas that sustained a tremor of lower 6 on the Japanese intensity scale in Ibaraki during the 7–8 days immediately after the 2011 earthquake off the Pacific coast of Tohoku. The groundwater level dropped as much as about 1m in 8 days after the earthquake at a well in one of these watersheds. Comparison of the relation between outflow and groundwater level in this watershed for one year before and after the earthquake suggests that the earthquake has encouraged outflow of water of underground origin. however, the flow increment immediately after the earthquake was not so great. Computational analysis results indicate that the outflow might have been compensated by decline in a groundwater level of about 1.5% of the area of the basin area, at most. accordingly, the decline in the groundwater level is regarded as a phenomenon occurring in an extremely limited area within the watershed.
Aftermath of the 2011 To-hoku earthquake and tsunami, Head water area
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