Characteristics of a Sensitive Well Showing Pre-Earthquake Water-Level Changes
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Water-level data recorded at a sensitive well next to a fault in central Japan between 1989 and 1998 showed many coseismic water-level drops and a large (60 cm) and long (6-month) pre-earthquake drop before a rare local earthquake of magnitude 5.8 on 17 March 1997, as well as 5 smaller pre-earthquake drops during a 7-year period prior to this earthquake. The pre-earthquake changes were previously attributed to leakage through the fault-gouge zone caused by small but broad-scaled crustal-stress increments. These increments now seem to be induced by some large slow-slip events. The coseismic changes are attributed to seismic shaking-induced fissures in the adjacent aquitards, in addition to leakage through the fault. The well’s high-sensitivity is attributed to its tapping a highly permeable aquifer, which is connected to the fractured side of the fault, and its near-critical condition for leakage, especially during the 7 years before the magnitude 5.8 earthquake.
KeywordsSensitive site water well water level aquifer earthquake slow-slip event seismic geodetic coseismic change pre-seismic change subduction zone seismogenic fault Japan plate
I am thankful to my former colleagues at the University of Tokyo for providing the data used in this study, especially Y. Hitagawa for the Okada-model-based calculation of volumetric strain at Tono for the 1996–97 slow-slip event. I am also thankful to M. Manga and three anonymous reviewers for comments, which have helped to improve the presentation significantly.
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