We obtained C-14 ages of about 20,000 yBP from wood fragments collected within lower terrace (L1) deposits, along the Mauchi River at Mochikorobashi, Ohshu City, northern Honshu. The L1 surface at the eastern part of Mochikorobashi may indicate flexural deformation trending north-south with relative scarp height of about 13 meters. Less than 0.5 m high surface ruptures associated with the 2008 Iwate-Miyagi Nairiku earthquake (Mj:7.2, Mw:6.8) had appeared just along middle portion of this flexural slope. The average slip rate of vertical component across the Mochikorobashi flexure is estimated to be 0.65mm / yr in maximum.
The Kakuda-Yahiko fault is a west-dipping blind thrust along the western margin of the Niigata plain, central Japan. The slip rate has not clearly estimated because the fault is buried under thick sediments. We obtained and analyzed three core samples (GS-KNM-1, GS-SGT-1, and AK-1) crossing the fault, and we recognized the Holocene vertical movement on the basis of correlation of detailed description, radiocarbon ages, and diatom assemblages. Based on these analyses, we interpreted nine sedimentary facies: the late Pleistocene marine sediments, meandering river sediments, salt to freshwater marsh sediments, offshore sediments, lower and upper shoreface sediments, foreshore sediments, modern river sediments, dune sediments, and surface soils, in ascending order. The average vertical-slip rate is estimated 3.6-4.6 mm/yr based on the vertical-offsets of the ravinement surface dated 9.4-8.2 cal kyr BP and the pumice layer dated 6-5 cal kyr BP. However, this slip rate might contain at best 1.3 mm/yr compaction effects of unconsolidated muddy sediments. Therefore, the average vertical-slip rate of Kakuda-Yahiko fault would be estimated about 3 mm/yr.
Reconstrucion of paleoseismicity for a long period is very important for the progress of a repetition model of large earthquakes. Previous study (Naruhashi et al., 2004, 2008) successfully reconstructed middle to late Holocene activity of the Kuwana fault based on stepwise changes in the depositional rates of shallow marine sediments controlled by 14C dates and wide-spread tephras. This study examined utility of grain size, electric conductivity (EC) and initial magnetic susceptibility as proxy to detect the faulting events of the Kuwana fault. As a result, the event horizons indicated by Naruhashi et al. (2004, 2008) can be recognized by specific changes in EC, initial magnetic susceptibility and grain size composition of the core samples. These changes reflect the burial process of the fault scarp formed by individual faulting event. Moreover, grain size, EC and initial magnetic susceptibility in the depth of 23-24m and 28-29m on the downthrown side of the fault show similar changes around faulting event horizons on the downthrown side as described above. This result implies that changes of the proxy could indicate two older unknown faulting events.
The Niigataken Chuetsu-oki earthquake in 2007 occurred west of Kashiwazaki in Niigata Prefecture, central Japan on July 16, 2007. We present an overview of crustal deformation and fault models associated with the earthquake estimated from geodetic measurements including GPS, interferometric SAR (InSAR), and leveling. Although it is difficult to determine which plane in two conjugate planes of the focal solution ruptured solely from the observed geodetic data, the data can be explained by slip on a southeast-dipping main-fault suggested by the aftershock distribution. A combination of the southeast-dipping main-fault and a northwest-dipping subfault gives a reasonable explanation of supplemental relation of the coseismic slip and the aftershocks. The Chuetsu-oki earthquake manifests that the present density of the GEONET stations is not enough to select a ruptured plane from two nodal ones for an inland earthquake with a magnitude of 7 and less. InSAR reveals not only a large deformation near the source area of the earthquake but also a local uplift along the anticline axis of an active fold, 15 km east of the earthquake epicenter. This narrow zone of uplift suggests the episodic growth of active folds, which is triggered by a coseismic stress change. The leveling data for about 110 years suggest the uplift have continued with a rate of 2-4 mm/yr for ~40 years. These geodetic data show that the growth rate of active folds temporally changes in short timescale. We propose that understanding of aseismic deformation including active folds is important to assess the earthquake potential in the Niigata-Kobe Tectonic Zone.
This paper summarises the results on active fault studies obtained mainly by the author’s work during the last 10 years. The 1999 Chichi earthquake makes a turning point for active faults study. Many international and interdisciplinary works have been carried out for active faults studies, in which the author has participated in the field of geomorphological study of active fault. Her main contribution is a discussion on the close relationship between the 1999 surface trace and preexisting active fault, estimation of tilt rate and relation between the main fault and subsidiary fault, the possible segmentation based on trenching data (Chelungpu Fault), finding some new active faults and discussion of their implication for the geomorphic evolution, as well as seismotectonic significance (Tunglo Fault System and Touhuanping Fault, northwest Taiwan). Ongoing works on the reverse faults in southwestern Taiwan, are briefly summarized, including significant effect of rapid denudation, that may cause underestimation of length of active faults.
The writer published in 1967 the idea that tectonic movements or crustal deformations are generally characterized by uniform rates or velocities, for long time more than 100 years. For short time less than 100 years, the movements may be accumulations of some stochastic processes including earthquakes. The real tectonic movements for long time can be approximated to the movement of ideal viscous fluid, under a stationary deviatoric stress. This approximation is also probable for the case of a glacier flowing down in a valley, only by its own weight. The regional stress field associated with the tectonic movements appears to be related to the subduction by weight of oceanic lithosphere, at least in the island-arc areas.Consequently, the ultimate cause of stationary tectonic movements might be the gravity of the earth.