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.
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