The complex 4D multi-segmented rupture of the 2014 Mw 6.2 Northern
Nagano Earthquake revealed by high-precision aftershock locations
Abstract
Neglecting fault segmentation in hazard assessments leads to
underestimated potential hazard. Moreover, integrating the temporal
evolution of fault segments activations in hazard assessment improve
scenario’s reliability. In this view, enhanced seismic catalogs have
potential in revealing previously neglected fault complexities. Past
efforts were restricted to the 2D view analysis without involving the
segment temporal activation. Our work provides a comprehensive approach,
reconstructing 3D fault fine-scale geometry and segments activation
evolution. We analyzed the 2014 Northern Nagano (Japan) (Mw 6.2)
earthquake sequence using high-resolution seismic catalogs. We
automatically detected and located about 2500 events between October and
December 2014. We refined the automatic picks, based on
cross-correlation and hierarchical clustering, and we relocated the
hypocenters with the double-difference in 3D velocity models optimized
for the area. Moreover, we calculated the composite focal mechanisms of
the main clusters, crucial to constrain the 3D geometry of the fault
segments, and rupture directivity that we interpreted jointly with the
seismicity and the fault slip. We found that the multi-segmented fault
system, is comprised of, at least, 9 distinct segments, that ruptured
during 3 successive phases. Different segments exhibit a different
rupture mechanism based on their spatial and temporal occurrence,
influencing seismicity evolution and rupture length. The presented
analysis can be used to improve the reliability of probabilistic hazard
assessment in the high seismic potential area of the Itoigawa-Shizuoka
fault system. The possibility of fault segment interaction and mutual
triggering processes should be considered when drawing reliable seismic
hazard scenarios.