Evaluation of the Coulomb stress changes between the 1960 Concepción and Valdivia earthquake in southern Chile.

Chile is one of the most seismically active countries in the world. In May 1960, two large earthquakes occurred along the subduction interface in southern Chile. The first took place on May 21st in Concepción, with a magnitude of Mw=8.1, and the second happened on May 22nd and corresponded to the world's most significant event recorded in instrumental history with a magnitude of Mw=9.5, popularly known as the Valdivia earthquake. Both events provoked considerable structural damage in some of the most important cities in the center and south of Chile, and the second one produced a transpacific tsunami, with casualties in Japan and Hawaii. Here, we investigate the interaction between the Concepción and Valdivia earthquakes, which occurred within a mere 33 hours of each other. According to previous studies, both earthquakes were initiated at similar locations and depths below the Arauco peninsula. We compute the Coulomb stress changes between these two seismic events to explore the increase of the regional stress produced by the Concepción earthquake into the Valdivia segment. We hypothesized that the Concepción earthquake promoted the rupture initiation of the second largest event, modifying their stress field and giving the conditions that lead to the mainshock nucleation. Our preliminary analysis indicates an interesting relationship between both earthquakes, which allows us to characterize and quantify these events' interactions at near distances and along the same subduction zone. Finally, a better understanding of the stress transference between these historical earthquakes gives us key information on the physical conditions for the nucleation of the largest earthquake ever recorded.