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Abstract: Four earthquakes with Mw ≥8 occurred in proximity to 60 Holocene volcanoes, in the Southern Volcanic Zone of the Andes (SVZ) since 1906. We analyzed these events by numerical modeling and field data to understand the key attributes of each volcano that may lead to a seismically-triggered eruption and the general mechanisms by which earthquakes could trigger volcanic new activity. We developed a new approach by resolving the earthquake-induced normal static stress change on the magma pathway of each volcano instead of considering the general crustal volume. We also considered other parameters that may lead to eruption, such as magma chamber depth, magma composition and viscosity, local tectonic settings and volcano dimension. The dataset includes a total of 18 new volcanic activities following large earthquakes (15 eruptions and 3 minor increases in volcanic activity) at 10 volcanoes. Of these, 9 out of 18 events representing unrest occurred at volcanoes that had no activity in the five years before the earthquake. Results indicate that the static stress changes were capable of triggering the observed volcanic phenomena up to a distance of 353km from the epicenter. Eleven out of 18 new volcanic events occurred at volcanoes with shallow magma chambers (2–3km) under conditions of unclamping or very weak clamping. New activity at volcanoes with deeper magma chambers (>7km) occurred only by magma pathway unclamping. Considering the regional tectonics of the SVZ, 5 volcanoes lying along strike–slip faults, 3 on thrust faults and 2 along a transition zone, experienced unrest. Our results show that magma pathway unclamping plays a fundamental role in dictating unrest at volcanoes that are already in a critical state. These studies contribute to possible individuation of those volcanoes that are more prone to seismically-triggered eruptive events. [Copyright &y& Elsevier]

Abstract: We describe the Miocene–Quaternary geological–structural evolution of the region between the Salar de Uyuni and de Atacama, Andes of Chile and Bolivia. We recognized four main tectonic events based on fold geometry, fault kinematics and stratigraphic data. The oldest event, of Miocene age, is characterized by folding and reverse faulting of the sedimentary successions with an E–W direction of shortening in the northern part of the studied area and a WNW–ESE shortening in the southern part. The following two events, of Pliocene age, are characterized by lower shortening amounts; they occurred first by reverse faulting with a NW–SE-trending greatest principal stress (σ 1, computed with striated fault planes) and a vertical least principal stress (σ 3), followed by pervasive strike-slip faulting with the same NW–SE-trending σ 1 and a horizontal NE–SW σ 3. The fourth event, dating to the late Pliocene–Quaternary is characterized by normal faulting: the σ 3 still trends NE–SW, whereas the intermediate principal stress σ 2 exchanged with σ 1. Volcanism accompanied both the contractional, transcurrent and extensional tectonic phases. The Mio–Pliocene compression appears directly linked to a rapid convergence and an apparently important coupling between the continental and oceanic plates. The E–W to WNW–ESE direction of shortening of the Miocene structures and the NW–SE σ 1 of the Pliocene structures seem to be more linked to an intra-Andean re-orientation of structures following the WNW-directed absolute motion of the South-American Plate. The extensional deformations can be interpreted as related to gravity forces affecting the highest parts of the volcanic belt in a sort of asymmetrical (SW-ward) collapse of the belt. [Copyright &y& Elsevier]

Understanding the process of volcanic lateral collapse, which may be governed by numerous possible factors, can be significantly improved by identifying the role of each factor. Here we test the perturbation induced in a volcano by tectonic normal faulting in the basement, magma inflation and multiple flank deformation, based on fieldwork and analogue experiments. Ollagüe Volcano (Chile-Bolivia) provides a good example of a sector collapse with exposed substrate extensional tectonics, and our experiments have been done with conditions consistent with field data. Results show that none of the possible tectonic conditions led to complete lateral failure; in agreement also with the field data, sector collapse of the model cone occurred only when a shallow magmatic intrusion within the volcano was reproduced. We also found out that volcano flank deformation is enhanced if a previous flank destabilization occurred, interleaved by new cone growth. [ABSTRACT FROM AUTHOR]