Quantitatively estimating denudation is generally difficult because it essentially involves the removal and loss of materials in situ. The denudation rate of mountainous areas in Japan has commonly been studied from the volume of sediment in a basin or catchment. Nonetheless, the availability of these methods is constrained spatially by upstream area and temporally by depositional age. In the last few decades, thermochronometric methods that evaluate thermal history using radiometric-dating methods have been used to evaluate the denudation and tectonic history of orogenic belts around the world. The advantages of thermochronometric methods are that we can calculate the denudation rate at each sampling point and that combining multiple methods and/or target minerals enables us to calculate denudation rates in multiple periods. However, thermochronometric methods have been applied to areas with extraordinarily high denudation rates in island arc areas such as Japan. Thus, the effectiveness of thermochronometric methods for estimating denudation rates in island arc areas has not been demonstrated. We applied apatite and zircon fission-track thermochronometry to granitic rock samples collected from outcrops and a borehole to estimate the tectonic history of the Rokko area, southwest Japan. Previous studies suggested that the Rokko Mountains have been uplifted by active faulting along their northern and southern margins during the Rokko movements, a Quaternary tectonic movement in the Kinki district. However, the tectonic history of the Rokko area prior to 1 Ma has not been revealed due to a lack of prevalent geologic markers. We estimated average denudation rates in various periods based on apatite and zircon FT ages and previously reported radiometric ages. We obtained denudation rates at about 0.04-0.10 mm/yr after about 30 Ma,

The 2008 Iwate-Miyagi Nairiku earthquake (Mw 6.9) struck an area where no active fault was known prior to the earthquake. The earthquake was shallow and large enough to produce scattered and discontinuous surface ruptures. To better understand the source processes and seismotectonic setting of the earthquake, we mapped the coseismic surface ruptures observed at Oshu and Ichinoseki Cities, Iwate Prefecture. The NNE-to NNW-trending surface ruptures appeared at three localities: (1) between Mochikorobashi and Ochiai, (2) Itagawa, and (3) southwest of Shinyu hot spring. Most of the surface ruptures are fold scarps up on the west, which are consistent with source fault models based on geodetic and seismological data. These surface ruptures appear to be associated with a reverse slip on west-dipping faults imaged by seismic reflection profiling, which originated as Miocene normal faults and have been reactivated as reverse faults since the Pliocene. Some of the ruptures appeared along preexisting tectonic scarps, although these scarps are short and discontinuous. Geomorphic expression of the causative faults of the 2008 earthquake is weak. Integration of geomorphic, geologic and geophysical data is necessary to identify active faults in areas where crustal strain is distributed to multiple fault strands and thus the tectonic landform of each fault is not well developed.

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