The water level in wells along the Paci.c coast from the Kii peninsula to Shikoku showed remarkable decreases a few days before the 1946 Nankai earthquake. If pre-slip occurred on a deep portion of the earthquake fault, an uplift .eld would cover the area of the decreased well water. However, the expected uplift is only a few centimeters. Can the drastic changes of well water be explained by the slight uplift? By our field surveys, we confirmed that the wells were located in a small delta. The seawater permeates under the delta which faced the sea, and the fresh water .oats on the seawater due to the different density. The Ghyben-Herzberg’ law shows that the depth between the horizon and the sea-and fresh water boundary is balanced to 40 times of the height between the horizon and fresh water head which is called groundwater table. This law implies that a slight uplift of the fresh water in a delta induces a 40 times drop of the sea-and fresh water boundary. In this case, much fresh water will be required to suspend the slight uplift of fresh water. If the new fresh water does not be supplied from the outside of delta, the much fresh water flows from the upper level of delta to take the new balance. Then, the well water on the upper level of the delta must decrease or dry up. The decrease of well water before the great earthquake was also con.rmed at the time before the 1854 Nankai earthquake (M 8.4). The reproducibility of the decrease of well water level caused by the pre-slip of Nankai earthquakes is fairly high. These understandings are effective for the prediction of the next great Nankai earthquake.

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The Tegano fault is an active fault, which is important from the perspective of the evolutionary process of reverse faults and for earthquake disaster prevention. It is believed that the longer and older Byobusan fault, which runs side by side with the Tegano fault, affects the formation of the Tegano fault. We performed precise gravity measurements to study the origin of the fault. Then, we estimated the basement structure in conjunction with a preexisting shallow re.ection survey as the constraining condition. By adopting differential filtering, we were able to see that the dip angle of the Tegano fault is low, whereas the angle of the Byobusan fault is high. The inferred schematic pro.le of the Tegano fault is consistent with a preexisting theory of the evolution of a reverse fault; this supports the hypothesis that the Tegano fault was derived from the deep part of the Byobusan fault. Moreover, we found that most of the fault plane of the Tegano fault is shallower than 1 km below the surface. Although the tectonic landform in the Tegano fault is young (or, the Byobusan fault is old), the deep part of the Tegano fault or the Byobusan fault may be capable of producing an earthquake. When assessing an active fault, it is important to comprehensively consider not only geomorphologic and ser.cial-geologic surveys but also gravimetrical ones.

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A trial to integrate various datasets obtained by observation into those with a spatially and temporarily high-resolution gridded data format is presented. Compiling, conservation, and share of the database with the uni.ed gridded data format for different seismotectonic indices, such as seismicity, strain rate, gravity anomaly, heat flow, etc., lead to promote further understanding of crustal activity through statistical examination on their spatiotemporal relationships. The use of one seismotectonic index with the gridded data format makes it possible to compare with any other seismotectonic index whether or not this has the gridded data format. Our .nal goal is comprehensive understanding of crustal activity through the statistical approach using gridded seismotectonic indices. We here introduce a first attempt to accomplish it. First, we converted the hypocenter catalog compiled by the Japan meteorological Agency and the data of GPS operated by the Geographical Survey Institute, Japan, into various geodetic (e.g., dilatation rate and maximum shear strain rate) and seismic (e.g., seismic energy and the number of earthquakes) indices with the gridded data format. Next, we drew scatter diagrams to overview the relationships between seismic and geodetic indices. Third, we introduced new statistical indices to quantitatively represent the temporal changes in the relationships.

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The Philippine Sea plate has subducted beneath southwest Japan at the Nankai Trough, causing megathrust earthquakes every 100-140 years. The subduction process shows lateral variations along the trough, such as a bending of the plate interface beneath the Kii Peninsula and rapid changes of strike and dip off the east coast of Kyushu. We invert three-dimensional GPS velocities using ABIC geodetic inversion technique to estimate interseismic slip de.cit distribution at the Nankai Trough. The studied area is roughly from Kii peninsula to eastern Kyushu (131.5-137E, 31-35N). In the ABIC geodetic inversion some initial constraints are needed to stabilize solutions, such that the slip at the outer margin of the fault is set to zero. For the estimation of interseismic slip de.cit on the plate interface, however, these constraints are unrealistic because slip de.cit distribution continues later.ally to the outside of the target area. In this study we propose a repeated inversion method that shifts target area laterally along the plate boundary. This method averages slip de.cit estimates at the same point obtained from more than three consecutive inversions. The estimates obtained near the lateral margin, which may be strongly affected by the non-slip constraints at the outer margin, are excluded from the averaging. As a result final solutions are stabilized regardless the setting of the region. The final slip de.cit distribution at the Nankai Trough shows high contrast of strong and weak plate locking zones. Down-dip variation of the slip de.cit is emphasized more clearly but the lateral variation becomes moderate compared with the single inversion result.

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