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Groundwater and Borehole Strain Monitoring for the Prediction

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Groundwater and Borehole Strain Monitoring for the Prediction P06 Groundwater and borehole strain monitoring for the prediction research of the Tonankai and Nankai earthquakes Norio Matsumoto ([email protected]) , Naoji Koizumi, Makoto Takahashi, Yuichi Kitagawa, Satoshi Itaba, Ryu Ohtani and Tsutomu Sato Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ, AIST) 1. Nankai and Tonankai earthquakes Great earthquakes about magnitude 8 or more along the Nankai trough, off central to southwest Japan have been recognized nine times since 684 by ancient writings. Recent events were the 1944 Tonankai (M 7.9) and the 1946 Nankai (M 8.0) earthquakes along the Nankai trough after 90 - 92 years from the 1854 Ansei Tokai (M 8.4) and the 1854 Ansei Nankai (M 8.4) earthquakes. 2. Groundwater anomalies before and after the Nankai and Tonankai earthquakes Hydrological anomalies related to the past Nankai-Tonankai Preseismic hydrological anomalies at fifteen wells several days before the 1946 earthquakes were repeatedly reported in and around Shikoku Nankai earthquake were reported by Hydrographic Bureau (1948). Reported and Kii Peninsula by ancient writings. In particular, discharges anomalies were turbid groundwater and/or decreases of groundwater level or hot of hot spring stopped or decreased at the Dogo and Yunomine spring discharge. The manuscript also reported that there were legends in which hot springs after four and five of the nine Nankai-Tonankai decreases of groundwater level might happen before the occurrence of the Nankai- earthquakes, respectively. The 1946 Nankai earthquake caused Tonankai earthquakes around the wells where the preseismic anomalies were 11.2 m drop of well water level at the Dogo hot spring. observed. Furthermore, Shigetomi et al. (2005) found several ancient writings that report preseismic decreases of groundwater level before the 1854 Nankai earthquake. For these reasons, we think that these preseismic hydrological anomalies before the Four and Five decreases in discharge / water level in ancient writings historical Nankai-Tonankai earthquakes are iteratively occurred. Decrease in groundwater level in shallow well Decrease in hot water discharge Getting turbid Color rectangles: Slip distribution of the 1946 Nankai earthquake (Tanioka and Satake, 2001) Hydrographic Bureau (1948); Umeda (2003); Tanioka and Satake (2001) Figure 2 Preseismic groundwater anomalies before the 1946 Nankai earthquake (hydrographic Bureau, 1948) and a fault model of the 1946 Nankai earthquake Figure 1 Occurrence of the Nankai, Tonankai and Tokai earthquakes and hot spring anomalies. (Tanioka and Satake, 2001). 3. Possible mechanism of coseismic and preseismic anomalies of groundwater level We think that static steps of the coseismic strain cause the coseismic groundwater level changes at the Dogo hot spring. A 0.1-1.9 m increase was also observed after the 2001 Geiyo earthquake (Mw 6.8) at the same well. The strain sensitivity of the groundwater level at the well is 1.73 mm/nstrain determined by the tidal analysis, and it is consistent with the coseismic groundwater-level changes that caused by the coseismic strain steps. The mechanism of the preseismic anomalies is less understood. Hashimoto (2003) proposed a fault model of the preseismic slip before the 1946 Nankai earthquake to explain the preseismic groundwater-level changes along the coast. He assumed 10 -25 % dislocation of several sub-faults of the coseismic fault model proposed by Sagiya and Thatcher (1999) as the preseismic slip, and then he showed that the induced changes of areal strain were ~ 0.5 ppm (Figure 4). If the strain sensitivities of the groundwater levels are assumed to be 1 mm/nstrain at the wells tapping a confined aquifer, induced changes in groundwater level are ~ 50 cm, which might be all or part of the observed preseismic groundwater-level decreases. However, the fifteen wells, where the preseismic hydrological anomalies were reported, are shallow and tapping unconfined aquifers, and strain sensitivities of the shallow well water levels are much smaller than 1 mm/nstrain. We need to consider other mechanism to explain such large decreases in shallow well water levels. 200 km Hashimoto (2003) Figure 3 Relationship among coseismic groundwater-level anomalies, coseismic Figure 4 A fault model of a possible preseismic slip and distribution of the inferred strain change induced strain steps and strain sensitivity estimated by tidal analysis. by the preseismic slip before the 1946 Nankai earthquake (Hashimoto, 2003). A scale of groundwater level is calculated if the strain sensitivity of the groundwater level is assumed to be 1 mm/nstrain 4. New wells for the monitoring groundwater and borehole strain We are constructing new wells to monitor groundwater and borehole strain at Hongu and Kumano, near the epicenters of 1944 Tonankai and 1946 Nankai earthquakes (Figure 5). The 30, 200 and 600 m-depth wells are constructed in one observation site. Groundwater level and groundwater temperature will be observed at each well, and multi-component borehole strainmeter and borehole tiltmeter will be installed at the bottom of the 600 m-depth well. We expect to observe groundwater levels to understand the preseismic hydrological anomalies, and groundwater level and/or strain changes associated with transient slow slips, preslips, coseismic and afterseismic crustal deformations. Modified from NEID (2006) Figure 7 Inferred principal strain of the short- term slow slip model occurred on January, Figure 5 Location and structure of the new wells 2006 (NIED, 2006) at Hongu and Kumano. Figure 6 Detectability test of a hypothetical M5.5 slip at the plate boundary using the four component borehole strainmeters at Hongu and Kumano The Sixth Joint Meeting of the UJNR Panel on Earthquake Research, Tokushima, Japan, 9 – 10 November 2006. http://www.aist.go.jp/RIODB/gxwell/GSJ_E.
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