The complex 3D multi-segmented rupture of the 2014 Mw 6.2 Northern Nagano Earthquake revealed by high-precision aftershock locations

Recent estimates of the rupture scenarios of moderate-to-large earthquakes highlights the need of incorporating segmented fault surfaces into seismic hazard models rather than individual, planar ones. The 2014 Northern Nagano earthquake sequence provides an opportunity to rethink the fault system geometrical complexities of the Itoigawa-Shizuoka tectonic line, one of the most inland active faults in Japan, and their impact on the fault rupture nucleation and development. For this study we used continuous records of 37 seismic stations to detect and locate 2500 events that occurred from 1st October 2014 to 31 December 2014. By refining the automatic arrival time picks, based on the cross-correlation and the hierarchical clustering, we relocated the earthquake hypocenters with the double-difference locationand a three-dimensional (3D) velocity model. We obtained a high-quality earthquake location catalog and the composite focal mechanisms which served for constraining the 3D geometry of the segmented fault system. We found that the early aftershocks are distributed along two sub-parallel, NNE-SSW striking, en-echelon, left-stepping fault segments and analyses on the spatio-temporal evolution of fault activation suggest that the rupture propagates in a clockwise direction, involving first the shallow northern portion of the segmented fault system, then the deeper northern portion of it, and finally the deeper and the shallower parts of the southern portion. Results suggest that a relationship exists between the rupture mechanism and the 3D segmented fault geometry. This relationship between initial structure of the segmented fault controls the earthquake rupture becomes crucial to assess reliable scenarios in seismic hazard.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)