Abstract
AbstractThe Geological Survey of Japan, AIST has been monitoring groundwater using the observation network of about 40 stations in and around the Kinki and Tokai districts for earthquake prediction research. The Noto Hanto Earthquake in 2007 (MJMA6.9) occurred in the northwestern part of the Noto Hanto Peninsula, Japan on March 25, 2007. The epicentral distance to the nearest and furthest observation well is about 100 km and 400 km, respectively. Changes in groundwater level related to the Noto Hanto Earthquake were detected at many stations. Most of these were changes in the long-term trend and/or short-term oscillation; there were a few steplike changes. The coseismic static volumetric strain changes calculated from the fault model ranged between 10−9and 10−10at most of the observation stations. These changes were therefore considered to be groundwater changes caused by the ground shaking because groundwater level sensitivity to crustal volumetric strain is a few centimeters to 10−8strain at the most. We compared the coseismic groundwater level changes for the Noto Hanto Earthquake with those for five recent large earthquakes and obtained rough characteristics of the effect of the ground shaking on the groundwater level at each of the observation stations.
Highlights
Earthquake-related groundwater level changes have been reported repeatedly (Wakita, 1975; Roeloffs, 1988, 1996; Koizumi et al, 1996)
There are several possible factors that could explain why the patterns of the oscillations in the groundwater levels are different: (1) the ground shaking was different at the different stations; (2) aliasing probably occurred because the sampling interval (2 min) is generally longer than the period of the oscillation; (3) there possibly occurred a kind of resonance so that the oscillation at a certain period, which depends on the geometrical structure of the well, is amplified (Cooper et al, 1965; Kunugi et al, 2000)
Coseismic groundwater level changes expected from those volumetric strain changes are smaller than 1 mm at most of the stations where the coseismic trend changes were observed (Table 1)
Summary
Earthquake-related groundwater level changes have been reported repeatedly (Wakita, 1975; Roeloffs, 1988, 1996; Koizumi et al, 1996). As the hypocentral or epicentral distance is larger, the effect of ground shaking on groundwater level changes becomes larger than that of coseismic static strain changes. We show groundwater changes related to the Noto Hanto Earthquake in 2007 and evaluate the effects of ground shaking on the groundwater level. The upper and western end is situated at 136.61E, 37.23E; the depth of the upper end is 2.0 km, the depth of lower end is 12.4 km, the strike is 58◦, the dip is 60◦, the rake is 117◦, and the slip amount is 1.2 m, which is equal to the earthquake of Mw 6.5 Using this model, we calculated the coseismic volumetric strain change at each station using the program of Okada (1992). The results of these analyses were compared with those of the Noto Hanto Earthquake
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