**5.3. Superposition of second inversion**

An eastward motion of the Amur plate happened to commence in association with the structural development of Himalaya - Tibetan plateau since the end of Miocene at around 7 Ma. Then the Tohoku, Izu and Seinan arcs started collisions mutually in central Japan, forming a core site of orogeny from where compressional tectonics has gradually spread to *far from the central Fossa Magna to the Niigata*. Therefore, sedimentary basins in the Hokuriku-Shin'etsu area developed deformation structures with reducing the deposition area of clastic sediments and being restricted into the present coastal plains and inland fault-basins. In the Hokuriku-Shin'etsu area after the late Miocene, overlapping of faults and folds in the three structural trends of north-south, east-west, and northeast-southwest is well recognizable and such a superposed structure has been illustrated also by current geomorphology. This intersecting feature has a broad expanse throughout the eastern margin of Japan Sea and is displayed in the seafloor topography conspicuously in particular along the continental slope of Okushiri and the Sado ridges. In addition, Present crustal earthquakes of moderate magnitudes occur by reverse faulting with a sense to promote the geomorphology development of the northeast - southwest direction of hills and mountains surrounding coastal plains evolved from the Miocene sedimentary basin.

#### **5.4. Lateral variation in modes of active tectonics**

**Figure 5.** Regional seismogenic stress provinces in Japanese Islands. Inset is a simplified model for variation of fault

**Figure 6.** P-wave perturbation structures beneath the central Japan. This section image obtained by high-density seis‐ mic stations by seismic tomography using a viewer software developed by NIED to estimate the 3D seismic velocity structure typical of under Japan. Data are quoted from [73]. Tohoku and Seinan arcs are distinctive in rheology struc‐

ture due to difference in plate configuration.

types due to the along-arc stress gradient of horizontal compression. After [72] with a slight addition.

120 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

This paper also noticed the present-day deformed structures and the spatial variety about the existence of basin inversion is also recognized. Namely, in the reverse fault province of Hokuriku, the inversion structure by earlier normal fault which formed the Miocene sedi‐ mentary basin is not seen, but typical basin inversion structures are seen in the reverse fault province such as the faults along the western margin of Nagano basin and the middle and northern segment of ISTL in the northern Fossa Magna area.

This cause can be considered as area characteristics of the principal stress axes arrangement by the stress gradient in the seismogenic upper crust. In the former province, stress field is in a state of strong horizontal compression (σ2>>σ3=σV) and the latter state is somehow neutral (σ2 ≈ σ 3) where a strike-slip faulting is easily exchanged into a reverse faulting ( [72]; see Figure 5). Moreover, [35, 48, 74, 75] presented a possible model for the deeper geologic structure, where high-angled block faults among tectonic provinces originated as transform faults and rooted in vertical weak zones in the lower crust beneath the basement of the sedimentary basin.

Based on the sedimentary basin evolution discussed in this paper and in accordance to the results of GPS geodesy and related studies [3-5, 76], the hypothesis of tectonic belt along the eastern margin of Amur Plate [70] is promising for the origin of strain concentration belt running oblique through the zone. This hypothesis includes an eastward motion of the Amur plate with convergence along the east Japan margin and transpression along the west Japan margin as well as its collision in central Japan [1, 2, 71].
