Coulomb stress transfer as an explanation for a XVI-century earthquake cascade in the Eastern Betics Cordillera, Spain; Insights from viscoelastic relaxation of the lithosphere and postseismic stress triggering.

The Eastern Betics Cordillera embraces a zone of low-to-moderate seismic activity located at the SE of the Iberian Peninsula. However, a major active fault system which crosses the area, ca. 500 km long, known as the Eastern Betics Shear Zone (EBSZ), has been responsible for the occurrence of several large historical earthquakes (Mw> 6.0) since the beginning of the historical record. Finding physics-based evidence for relations between significant historical events in such a moderate-slipping fault system would help us narrate them as long-term cascades. Such a perspective provides valuable insights into faults interactions over time and, thus, until the contemporary periods.

Some authors have examined static Coulomb failure stress changes (ΔCFS) to explain the triggering influence of moderate instrumental earthquakes in this region. However, the applied approach in this study, which implies the estimation of postseismic ΔCFS, is the first attempt of this kind to identify triggering connections between historical earthquakes in EBSZ.

This study addresses a sixteenth-century cascade of three large earthquakes that occurred in less than 13 years within a radius of 100 km in the southern section of the EBSZ. It includes the 1518 Vera (Mw~6.2), the 1522 Alhama de Almería (Mw~6.5 -7.1) and the 1531 Baza (Mw~6.5) earthquakes, each one associated with a different causative fault, namely the N-S strike-slip Palomares fault, the NE-SW strike-slip Carboneras fault and the N-S to NW-SE normal Baza fault, respectively. We aim to explore the Coulomb stress transfer along the occurrence of this cascade and the plausible rupture scenarios that could favour or not a triggering connection between the causative faults.

First, a simple smoothed slip model is performed to simulate the earthquake ruptures. The applied slip models respect existing information on the attributes and hypotheses based on seismological and paleoseismic studies. Then, the multilayered viscoelastic relaxation modelling by Wang et al. (2006) is used to calculate the time-dependent deformation fields (since the 1518 Vera earthquake) across the crust and the lithospheric mantle. Finally, the cumulative co+postseismic ΔCFS are solved for the kinematics of the Carboneras and Baza fault planes in 1522 and 1531, respectively.

Our results strongly suggest a sequential stress-triggering connection between these three large events. According to our models, the 1531 Baza earthquake occurred along with an increase in the ΔCFS due to the viscoelastic relation over time. We further explore the implication of the characteristic curved-shape of the Baza fault when considering different rupture scenarios of the 1522 event at the Carboneras fault. We found that the northern NS-oriented section of the Baza fault remains more exposed to positive cumulative co+postseismic ΔCFS and, indeed, was more prone to rupture in 1531 rather than the southern NW-SE section. We believe our results would pave the way for understanding the relationship between many other major historical earthquakes in the Betics Cordillera.