A Time‐Dependent Model of Elastic Stress in the Central Apennines, Italy

Abstract

Seismicity in the Central Apennines is characterized by normal faulting with dip NE‐SW near 45°. If the stress at the hypocenter of the 2016 Norcia (Mw = 6.5) and 2009 L'Aquila (Mw = 6.3 on the Paganica fault) earthquakes originated only from stress transfer from previous historical events, the orientation of the principal stress axes would have been inconsistent with the observed tensional regime. The additional contribution of a regional stress is thus required, but Global Navigation Satellite System geodesy provides only stress rates. We empirically estimate a time multiplier for the regional stress rate, computed with a dense Global Navigation Satellite System network, such that the principal stress axes resulting from the sum of the stress transferred by previous events and the regional stress rate multiplied by the empirical temporal scale are consistent with normal faulting, both at the L'Aquila and Norcia hypocenters. Based on a Catalogue of 36 events of magnitude larger than 5.6, we estimate the total Coulomb stress at depths and along planes parallel to those of L'Aquila and Norcia. We provide evidence of an asymmetry of the Coulomb stress leading to a stress concentration near the hypocenter of the two events just prior of the 2009 and 2016 earthquakes. This stress anomaly disappeared after the two events. Similar stress patterns are observed for earlier events, which took place in 1461 at L'Aquila, 1703 on the Montereale plain, and in 1703 at Norcia/Valnerina. The 1997 sequence of Colfiorito exhibits a similar, anisotropic Coulomb stress pattern. Other areas with a similar stress anisotropy could be seismic gaps.

Spatiotemporal Clustering of Paleoseismicity in the Walker Lane, USA: An Extensional Supercycle?

“…Similar earthquake clusters in normal faulting environments have been observed before in both the Apennines (Figure 5a, Caporali et al., 2019; Ferrarini et al., 2015) and the Walker Lane/Eastern California Shear Zone/Central Nevada Seismic Belt (Figure 5b, Bell et al., 2004). In both examples, major earthquakes “jump around” rather than forming a cascading pattern.…”