This study reviews the relationship between geological structure and major earthquakes along the eastern margin of the Japan Sea, and considers the possibility of inferring the location and geometry of earthquake source faults from geological structures. The source areas of major earthquakes in this region correspond to the locations of asymmetric anticlines that are 10-20 km wide and that have grown during the past 2-3 million years. These observations suggest that the anticlines are fault-related folds located above the source faults. This hypothesis was partly confirmed by a comparison between a fault model based on fold geometry and the distribution of aftershocks in the source area of the 2004 Niigata Chuetsu Earthquake. It is important to assess whether this model of fault-related folding is widely applicable to folds at the eastern margin of the Japan Sea. If we could infer the location and geometry of future earthquake source faults based on analyses of geological structure, this would greatly improve our ability to evaluate ground motions due to earthquakes.
The backarc side of northern Honshu is marked by an arc-parallel fold-and-thrust belt. Historical and instrumental records suggest that this belt is the site of frequent, shallow, damaging crustal earthquakes. To improve our estimates of strong ground motions and to evaluate the risk of damaging earthquakes, it is important to identify the locations of seismogenic source faults beneath the thick cover of Neogene sediments in this region. Based on deep seismic reflection profiling and the distribution of aftershocks associated with damaging earthquakes, we assessed the relationship between an earthquake source fault and geological structure at several areas in northern Honshu. Reverse movement along Miocene normal faults is dominant along the fore-arc side of the Miocene backarc rift system. Source faults dip at moderate to high angles, reflecting their origin. On the backarc side, a shallow detachment is developed within upper Miocene mudstone, and thin-skinned tectonics are dominant within the sediment cover. The relationship between the deep-seated source fault in the Niigata sedimentary basin and the sedimentary cover remains a topic of controversy, mainly because of the occurrence of >6 km of sediments within the basin and the possible existence of a ductile mudstone layer within the lower part of the succession. Given the great thickness of the sedimentary cover, deep seismic profiling is required to understand the nature of the source fault.
An interferometric analysis of Synthetic Aperture Radar (InSAR) using PARSAR data of the Advanced Land Observations Satellite (ALOS PALSAR) reveals the spatial pattern of deformation associated with the 2007 Niigataken Chuetsu-oki Earthquake (Mj 6.8). The analysis reveals not only regional deformation in the focal area, but also a narrow uplifted zone (1.5 km in width and 15 km in length) along the axis of an active anticline in the Nishiyama hill, located 15 km east of the epicenter of the mainshock. The deformation provides direct evidence for episodic growth of the fold, triggered by the earthquake. In addition, InSAR data reveal non-tectonic ground deformation, such as soil liquefaction and lateral soil flow in the central part of Kashiwazaki City.
We investigated ruptures in the asphalt and concrete pavement of roads in the uplifted belt of the western wing of the Oginojo Anticline, where crustal deformation associated with the 2007 Niigataken Chuetsu-oki Earthquake (Mj 6.8) has been detected using synthetic aperture radar (SAR) interferometry. In the region of uplift (displacement >8 cm) on the western limb, the pavement was ruptured at many places, but not by faults. Open cracks are dominant in the uplifted belt, whereas cracks generated by gravity sliding or sagging of roads were observed over large parts of the study area. Striations on the bedding planes on the western limb indicated flexural slip folding mechanism; however, we observed no bedding plane slips. These observations suggest that open cracks of the uplift region were generated in response to local coseismic uplift.
The Hokuriku and Shin’etsu region, which represents the northern boundary zone of the Northeast and Southwest Honshu arcs, is characterized by frequent earthquakes as large as magnitude 6.8-6.9 (Richter scale) along a latest Cenozoic thrust-and-fold belt. Based on a recently updated tephrochronology, this paper seeks to reconstruct the geomorphologic and geologic evolution of the region. It is shown that the present tectonic geomorphology in the region has been influenced by the superposition and intersection of three tectonic trends (N-S, E-W, and NE-SW; these trends are defined by the orientations of tectonic features such as faults, fold axes, and elongate basins) since the Miocene. Sedimentary basins that have been active since the Early Miocene have developed into Quaternary coastal plains. Reverse faults along the foot of areas of mountains and hills surrounded by coastal plains have produced relatively strong crustal earthquakes, thereby promoting the development of the most recent (NE-SW) tectonic trend. Typical tectonic inversion has occurred in Northeast Japan, especially in the northern Fossa Magna district, where the boundary faults of Miocene sedimentary basins have been reactivated as reverse faults, although basin inversion has yet to occur in the reverse-fault province of the Hokuriku district. By comparing the evolution of geomorphologic and geologic structures on both sides of north-central Japan, this paper emphasizes the million-year scale of reversals in the stress field and its migration, which is important in understanding the origin of strain concentration in north-central Japan and the cause of crustal earthquakes in the Hokuriku and Shin’etsu region.