The complex 4D multi-segmented rupture of the 2014 Mw 6.2 Northern Nagano Earthquake revealed by high-precision aftershock locations

Abstract

Enhanced earthquake sequence seismic catalogs have the potential to reveal fault surface complexities at depth (e.g., segmentation, bends). Past analyses were generally restricted to 2D analysis considering faults discontinuous along their strike and do not include segment temporal evolution. Our work provides a comprehensive methodology, for reconstructing the 3D fine-scale (few kilometers length) geometry of a segmented fault surface and to characterize the triggering of the segments during an earthquake mainshock-aftershock sequence We analyzed the 2014 Northern Nagano (Japan) (M_w 6.2) earthquake sequence using high-resolution seismic catalogs. We automatically detected and located about 2500 events between October and December 2014 with a magnitude range between –0.6 and 6.2. We refined the automatic picks, based on cross-correlation and hierarchical clustering, and we relocated the hypocenters with the double-difference technique in 3D velocity models optimized for the area. Moreover, we calculated the composite focal mechanisms of the main clusters, crucial to constrain the kinematics and the 3D geometry of the fault segments, and rupture directivity that we interpreted jointly with the seismicity and the fault slip. We find that a segmented fault surface comprising, at least, 9 distinct segments, ruptured during 3 successive activation phases. The different segments exhibit a different rupture mechanism based on their spatial and temporal occurrence, influencing seismicity evolution and rupture length. Our method, based on deep analysis of relocated aftershocks sequence, can be used in other study cases for the fault surface reconstruction to better understand the complexity of the earthquake rupture.