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
 Rojstaczer, S., Wolf, S. & Michel R. Permeability enhancement in the shallow crust as a cause of earthquake-induced hydrological changes. Nature, 373, 237–239, 1995. DOI: 10.1038/373237a0.
 Tokunaga, T., Modeling of earthquake-induced hydrological changes and possible permeability enhancement due to the 17 January 1995 Kobe Earthquake, Japan. Journal of Hydrology, 223, 221–229, 1999. DOI: 10.1016/S0022-1694(99)00124-9.
 Sato, T., Sakai, R., Furuya, K. & Kodama, T., Coseismic spring flow changes associated with the 1995 Kobe earthquake. Geophysical Research Letters, 27, 1219–1222, 2000. DOI: 10.1029/1999GL011187.
 Tsuboyama, Y., Tamai, K., Shimizu, T., Kubota, T., Iida, S., Nobuhiro, T. & Sawano, S., Observation of water discharge increase from the forested catchments just after the 2011 Tohoku earthquake. Proceedings of Japan Geoscience Union 2012, U05-07, 2012.
 Ohnuki, Y., Yoshinaga, S., Tsuruta, T., Araki, M., Ito, E., Shichi, K., Matsuura, Y., Ono, K., Okamoto, T. & Stewart, J.B., Distribution of soil thickness and soil physical properties in Katsura Headwater Catchment. Bulletin of Forestry and Forest Products Research Institute, 13, 43–59, 2014 (in Japanese with English summary).
 Ohnuki, Y., Yoshinaga, S. & Noguchi, S. Distribution and physical properties of colluvium and saprolite in unchannelized valleys in Tsukuba Experimental Basin, Japan, Journal of Forest Research, 4, 207–215, 1999. DOI: 10.2136/sssaj1991.03615995005500040006x
 Kabeya, N., Shimizu, A., Nobuhiri, T., Zhang, J., Tsuboyama, Y., Runoff characteristics from forested small basins with thick weathered layer (1) – Observations of outflow from springs. Transactions of Japanese Forestry Society, 115, 806, 2004 (in Japanese).
 Noguchi, S., Kishioka, T. & Kikuya, A. Runoff characteristics of springs in Tsukuba Forest Experimental Watershed. Transactions of Japanese Forestry Society, 101, 605–607, 1990 (in Japanese).
 Tamai, K., Sawano, S., Shimizu, T., Kobayashi, M., Iida, S., Shinomiya, Y. & Kabeya, N., Changes of ‘GWL-Q’ relation caused by the great East Japan earthquake – Comparison of two small mountainous catchments in Ibaraki Pref. Transactions of Japanese Association of Hydrological Sciences, 45–48, 2015 (in Japanese).