We conducted drilling survey to re-examine near surface geometry and a rate of the vertical deformation on the Kamishiro fault, northern part of the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL), central Japan. 40- and 45-m-long core samples, extracted from the hanging wall of the Kamishiro fault, consist of fluvial sediments (alternation of sand-mud and sand-gravel layers). In the two cores, bedding plane is almost horizontal at uppermost part, gently sloped at upper to middle part, upstanding at lower part, and mildly sloped or horizontal at lowest part, respectively. The core samples enable us to estimate horizon of penetration of the Kamishiro fault on the basis of change with sharp boundary from upstanding strata older than 30,000 cal BP to underlying mildly sloped or horizontally laminated strata younger than 30,000 cal BP. Geologic cross-section on the basis of correlation of both the two cores with previously known stratigraphy, indicates that estimated fault is a reverse fault with dip of about 30 degrees. Altitude of facies boundaries and over fifty radiocarbon ages show cumulative vertical displacements of 14-20 m during the past 10,000 years, indicating average vertical displacement rate of 1.4-2.0 mm/yr. And, the net slip rate is estimated to be 2.8-4.0 mm/yr by using 30 degree dip of fault. The average vertical displacement rate in this study is smaller than that in the previous study where data of lithofacies and radiocarbon age is insufficient on the upthrown side. Because the Kamishiro fault is associated with drag folding near the surface, previously reported net slip rate during the past 10,000 years is considered to be underestimated. The largeness of net slip rate in this study relative to previously estimated rate is consistent with the forecast that the previous estimation is underestimated. The lowness of Holocene average vertical displacement rate, compared with previously estimated rate during the past 28,000 years, suggests decrease in activity of the Kamishiro fault during the past 10,000 years. And, the Holocene average vertical displacement rate shows that average recurrence interval is 150-360 years assuming that faulting with vertical displacement of 0.3 to 0.5 m as same as that at 2014 Nagano-ken-hokubu earthquake has repeated. This interval is shorter than that obtained from previous paleoseismic study, implying unknown paleoseismic event with small vertical displacement can be detected through reconsideration of paleoseismic survey.
The Itoigawa-Shizuoka Tectonic Line (ISTL) fault zone extends for ca. 158 km from Otari Village to Hayakawa Town, central Japan. The northern part of the fault zone (from Otari to Akashina in Azumino) has the potential for an earthquake larger than M 7. A M 6.7 earthquake called the Naganoken-hokubu earthquake occurred in 2014 in the northern section of this part. Most of the coseismic surface ruptures appeared along the pre-identified active fault traces. However, the others emerged at sites where no active fault traces had been revealed by previous studies. We have to continue detailed mapping of fault-related landforms to understand active fault distribution of the northern part of the ISTL fault zone. In this study, we report the existence of a ca. 1 km long, NNW-trending trace of possible active fault in central Omachi City, Nagano Prefecture, revealed by geomorphic analysis using high-resolution digital elevation models from airborne LiDAR data issued by Geospatial Information Authority of Japan. Our evidence for this trace is (1) slope-direction anomaly in the fan of the Kashima River identified by detailed elevation contours, (2) east-side-up vertical displacement indicated by topographic profiles, and (3) bend of an abandoned channel near the fault trace. The drainage system anomaly and irregular aqueduct network probably reflect the existence of this trace. This trace would have emerged prior to the construction age of the aqueducts drawn from the Nogu River to the area of the present central Omachi City, based on previous studies on geography and history of the city.
We described newly found fault exposures of the Shiraiwa and Ota faults and obtained dating materials from exposures including that of Senya fault, on which the surface ruptures appeared at the 1896 Rikuu earthquake. Based on radio-carbon dating and paleoseismic interpretation, we examined the event horizon of 1896 earthquake. We also estimated the exact trace of 1896 surface ruptures of Ota fault, using the comparison between before-and-after large scale topographic maps on artificial modification. Each of two new fault exposures and a previously reported fault exposure demonstrates the fault movement of a single event related to the 1896 earthquake. We confirmed that all faults in the exposures described here were activated at the 1896 earthquake and produced continuous fault scarps. The fault scarp of Shiraiwa fault on the foot of mountains is precisely identified along the boundary between higher farming fields and lower paddy fields depicted in the large scale topographic maps drawn before 2000s’ artificial modification.
During the 16 April Kumamoto earthquake (Mj7.3), ～30-km-long surface ruptures with right-lateral slip appeared along the previously mapped Futagawa and Hinagu faults. In Mashiki Town, surface ruptures also appeared north of the Futagawa fault across the alluvial plain of the Kiyama River where no tectonic geomorphic features were identified. In order to reveal shallow subsurface structure and past movements of the 2016 surface ruptures north of the Futagawa fault, we conducted a trenching survey at Jichu, Mashiki Town. Fluvial deposits derived from the Kiyama River and Aso-4 pyroclastic flow deposits were deformed by moderately to steeply south-dipping reverse faults. While the vertical offset during the 2016 earthquake was ～30 cm up on the south, older strata exposed on the trench walls were offset more than 2 m, suggesting that reverse faulting events occurred repeatedly in the late Quaternary. Based on the deformational features of the exposed strata, we identified three surface-rupturing events, including the 2016 earthquake. Radiocarbon dating of the strata suggests that at least two faulting events occurred after ～9,000 yBP. Since there were no strata deposited after ～8,500 yBP, it is possible that there were more seismic events than we identified during the middle to late Holocene. We suggest that the small vertical displacement associated with individual events, sedimentation/erosion by the Kiyama River, and artificial modification contributed to no tectonic geomorphic features along the surface rupture that appeared across the alluvial plain of the Kiyama River.
The Republic of Korea (hereinafter referred to as South Korea) is located in the Eurasia Plate, unlike Japan where is on the plate boundary. Consequently, most people of South Korea had not concerned about earthquake disaster until the 2004 Indian Ocean earthquake. Earthquake observation in South Korea has started since 1978, and the number of earthquakes has been increasing since then. For example, 2016 Kyeongju earthquake (M＝5.8) is the largest earthquake since the beginning of earthquake observation. Several tens of people were injured and many houses were damaged. Under such background in South Korea, “EARTHQUAKE RECOVERY PLANS ACT” was legislated at last in 2008 to “prescribe matters necessary for the observation of, prevention of, provision against and action against earthquakes and tsunami, earthquake-proof measures, and research and technical development or such, to reduce earthquake disasters in order to protect the life and property of the people and major infrastructure from disasters due to earthquakes and tsunami.” This paper aims to introduce “the Act” by translating Korean into Japanese directly and sum up the contents to understand them easily.