北部フォッサマグナの地殻構造と2014年11月22日長野県北部の地震

抄録

The Nagano-ken-hokubu earthquake (Mw 6.2) occurred on 22 November 2014. A 9.2-km-long surface rupture appeared in association with the earthquake along the pre-existing trace of an active fault (the Kamishiro fault), which constitutes the northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault zone. The ISTL bounds the western margin of the Northern Fossa Magna Basin, which is the southern extension of the Uestu depositional basin developing on the back-arc side of the Northeast Japan (NEJ) arc. In the back-arc region of the NEJ arc, normal faults, that originally formed in conjunction with the Japan Sea opening in the early and middle Miocene, were reactivated as reverse faults under compressive stress since the Pliocene. In order to understand the present-day tectonics of the Uetsu-Northern Fossa Magna basin, we should take into account the Miocene extension and the subsequent tectonic inversion. In this paper, we review the crustal evolution of the Uetsu-Northern Fossa Magna rift basin by retrogressively restoring the contractive structures since Pliocene time and extensional structures of Miocene time along a deep-seated detachment fault, and then discuss the geometry, behavior, and loading process of the source fault that generated the 2014 earthquake on the basis of the aftershock distribution, results of SAR interferometry, and GPS horizontal velocity data. We found that: (1) coseismic slip occurred only on the listric reverse fault, which merges down-dip onto the deep-seated detachment fault, and (2) before the 2014 earthquake, aseismic slip was likely to have occurred on the detachment fault