The active fault zone on the seafloor off the northern coast of the Noto Peninsula of central Japan is divided into four segments from west to east: Monzen-oki, Saruyama-oki, Wajima-oki, and Suzu-oki. To examine the latest event that occurred in these segments, we investigated the dates and elevations of fossilized intertidal tubeworms along the northern coast of the Noto Peninsula, located on the hanging-wall sides of the faults, using radioactive carbon dating and global positioning measurements. For each fossil, we calculated the difference between the past and present elevation, thereby estimating the elevation of the sea level at the date of the fossil, using a curve for sea level change. This calculation provided us with the elevation change at each site. The radiocarbon dates of the fossils, together with the elevation changes, revealed that the coastal emergence probably occurred between 1600 and 1800 AD. This area of coastal emergence lies adjacent to active faults within the Wajima-oki segment. A model for rectangular faults with three fault planes and a moment magnitude of 6.6 for the Wajima-oki segment reproduced the observed pattern of coastal emergence well. Only one damaging earthquake, that in 1729, is known to have occurred in this part of the northern Noto Peninsula between 1600 and 1800 AD, and there has not been one since 1800 AD. The slip distribution of the earthquake predicted by the model is consistent with the distribution of shaking-related damage documented in 1729. We conclude that rupture of the Wajima-oki segment caused the 1729 earthquake.
Gravity gradient tensor analysis has been a powerful tool for investigating subsurface structures and recently its application to a two-dimensional fault structure has been developed. To elucidate the faulting type and spatial extent, specifically the continuity and the size, of the subsurface fault structure of an active fault through gravity gradient tensor analysis, we analyzed Bouguer anomalies, which were composed of dense gravity measurement data over the land and seafloor, and indices calculated from a gravity gradient tensor around the Togi-gawa Nangan fault (TNF), Noto Peninsula, central Japan. The features of Bouguer anomalies and their first horizontal and vertical derivatives demonstrate clearly that the TNF is a reverse fault dipping to the southeast. Furthermore, the combination of those derivatives and the dimensionality index revealed that the spatial extent of the subsurface fault structure is coincident with that of the surface fault trace and that it shows no evidence of connecting the TNF with surrounding active faults. Furthermore, the dip angle of the subsurface fault structure was estimated as 45°-60° from the minimum eigenvectors of the gravity gradient tensor. We confirmed that this result is coincident with the dip angle estimated using the two-dimensional Talwani's method. This high dip angle as a reverse fault suggests that the TNF has experienced inversion tectonics.
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