In order to study active tectonics and related matters, it is very important to know the actual distribution and nature of active faults. The western boundary faults of the northern part of the Kitakami lowland are composed of MBF (Main Boundary Fault), FAF (Frontal Active Fault), and SAF (Secondary Antithetic Fault). The former two are west-dipping reverse faults. SAF is east-dipping antithetic reverse fault located between MBF and FAF. There was a disagreement about the existence of another west-dipping active reverse fault to the east of FAF. We surveyed fluvial terrace surfaces and observed a new outcrop. In addition, we examined geomorphic features based on 5 m-mesh DEM. The results are summarized as follows: (1) The average vertical slip-rate of FAF, located in the east of MBF, is ca. 0.3 mm/yr. (2) Geologic structures to the east of FAF do not coincide with west-dipping reverse fault. Fluvial terrace surfaces do not display any tectono-geomorphic feature indicative of active faulting there. Therefore, we deny a possibility of active fault to the east of FAF.
Based on field surveys and satellite-geodetic observations, small surface ruptures associated with the 2016 Kumamoto earthquake sequence were widely identified on unknown distributed faults up to ～15 km further from the main rupture zone. In the Aso caldera, we observed small surface ruptures, which were firstly identified as the short discontinuous traces of fringes of interferometric synthetic aperture radar (InSAR) images, around Miyaji, Aso City, Kumamoto Prefecture. These surface ruptures show vertical and dextral displacements ＜10 cm. To reveal the characteristics of the subsurface continuity of surface ruptures and cumulative slip, we excavated a pit across the surface breaks. As a result, the pit walls exposed flower structures that are composed of near-vertical faults and indicates a dextral slip. At a depth of 1.5 m, a measured vertical separation of the strata is 15-20 cm, which is three times larger than that on the ground surface. We then depicted the relation between the depth and the vertical displacement, suggesting that fault displacement decreases toward ground surface. Consequently, we interpret that the dextral fault movement resulted in surface ruptures around Miyaji. In addition, the tephra and radiocarbon dating revealed that the 2016 Kumamoto earthquake sequence is the only surface-rupturing event at the pit site since 1700-1900 cal. BP.
Precise location and distribution of active faults provide essential information for predicting damage caused by fault displacement. However, in regions where human activity continues for a long time, faulted landforms indicative of the fault trace might have been destroyed or modified by past land improvement, which makes precise mapping of the active faults difficult. Examinations of local documents concerning the intense cultivated land consolidation works carried out in the late 1920s and old 1: 20,000 topographic maps issued by Japanese Imperial Land Survey in the 1900s allow us to revise the location of the Kita-amagi fault, a structural element of the Uto section of the Futagawa fault zone in central Kyushu, which has been mapped primarily based on interpretation of aerial photographs taken in the 1940s or later. Validity of the revision is verified by appearance of the surface ruptures on the Kita-amagi fault formed in association with the 2016 Kumamoto earthquake sequence, although the surficial slips are small and scattering. To assess whether the tectonic landforms along the Kita-amagi fault have been created by repetition of small and faint surficial slips as observed during the 2016 event or repetition (or combination) of other events that has been accompanied by more large and continuous surface deformation, further comprehensive investigations including paleoseismic trenching should be needed.