ALBERT

Beschreibung: After 137 years without a great earthquake, the Mw 8.1 Pisagua event of 1 April 2014 occurred in the central portion of the southern Peru–northern Chile subduction zone. This megathrust earthquake was preceded by more than 2 weeks of foreshock activity migrating ∼3.5 km/day toward the mainshock hypocenter. This foreshock sequence was triggered by an Mw 6.7 earthquake on a reverse fault in the upper plate that strikes at a high angle to the trench, similar to well-documented reverse faults onshore. These margin-oblique reverse faults accommodate north-south shortening resulting from subduction across a plate boundary that is curved in map view. Reverse slip on the crustal fault unclamped the subduction interface, precipitating the subsequent megathrust foreshock activity that culminated in the great Pisagua earthquake. The combination of crustal reverse faults and a curved subduction margin also occurs in Cascadia and northeastern Japan, indicating that there are two additional localities where great megathrust earthquakes may be triggered by upper plate fault activity.

Beschreibung: We performed an integrated analysis of the coseismic slip, afterslip and aftershock activity of the 2014 Mw 8.1 Pisagua earthquake. This earthquake seems to be spatially located between two major historical earthquakes, the 1868 Mw 8.8 earthquake in southern Peru and the 1877 Mw 8.5 earthquake in northern Chile. Continuous GPS data were used to model the coseismic slip of the mainshock and the largest aftershock (Mw 7.6). The afterslip was modeled for 273 days (end of year 2014) after the largest aftershock, revealing two patches of afterslip: a southern patch between the mainshock and the largest aftershock and a patch to the north of the mainshock. Observations from the seismic network indicate that aftershocks were concentrated near the southern patch. Conversely, the northern patch contained hardly any aftershocks, indicating a dominant aseismic slip. The Pisagua earthquake occurred within a prominent, curved section of the Andean subduction zone. This section may have acted as a barrier for the largest historical earthquakes and as an isolated segment during the Pisagua earthquake.

Beschreibung: Boiling mud ponds, hot springs, and geysers are the scenic surface expression of rising thermal fluids, often emerging in clusters. The details on the spatial appearance and structural control of such geothermal objects as well as on the variability of their locations are rarely investigated, however. Here we use Unmanned Aerial Systems (UAS) to acquire close-range optical and thermal infrared data over the El Tatio geothermal field (Chile), one of the largest geyser fields in the world. From high-resolution aerial images, processed using the Structure from Motion (SfM) method, we compute spatial image data at 1.5 to 14.5 cm resolution for a ∼ 2 km2 area. We identify 1863 objects related to geothermal activity, providing an unprecedented catalog of the geothermal area. Out of these, 148 were classified as topography objects (e.g. cone geysers), 415 showed signs of fluid discharge, and 1091 were characterized by a thermal signature exceeding background temperatures. The geothermal objects were further analyzed regarding their spatial distribution and clustering, suggesting a high degree of organization in 5 main groups on a broader scale, and clustering in specific vent arrangements resembling two main orientations on a smaller scale. The 5 zones show significant differences considering their orientation, types of geothermal objects located within, but also their eruptive characteristics, and thermal energy release. We discuss these, considering the structural setup and hydrological setting of the El Tatio geothermal field. Over 90% of the mapped geothermal objects are located within a 100 m distance to an estimated trend line oriented NE-SW. We thus hypothesize a possible structural arrangement controlling the location and activity of geothermal objects at El Tatio with important implications for other geothermal areas worldwide.

Beschreibung: Many volcano summits host craters that are partially overlapping. The formation of such nested craters has been commonly interpreted as vent migration. Here, we present an additional mechanism that may explain the geometry of nested craters at volcanoes. Láscar Volcano, the most active volcano of the Central Volcanic Zone in the Chilean Andes, hosts ENE-WSW trending summit craters that are partially overlapping (nested). Details on the evolution and interaction between the different craters remain unclear. To create a robust dataset, Terrestrial Laser Scanner (TLS) data were collected at the summit of Láscar in 2013. The resulting topographic data set, consisting of more than 15 million data points with centimetre sampling, allows visualising almost the complete eastern edifice of the volcano's summit. From the TLS data, a Digital Elevation Model (DEM) and a slope map were generated allowing us to create a lineament map and quantify the observed morphological and structural features. To further improve our understanding of the processes responsible for the formation of the craters and geomorphology, we designed sandbox analogue models. Results suggest that one of the craters is a ‘parasite’ crater, formed as a consequence of ongoing activity in the adjacent crater. Our data suggest that the nested craters have all been modified since the last major eruption in 1993, by near surface effects associated with cooling, compaction and gravitational sliding of the crater floor infill. As the active crater deepens, the adjacent inactive crater extends and partially slumps towards the active one. Understanding the structural development of these nested craters is relevant for assessing potential future eruption sites, thus making Láscar a dynamic target for a detailed morphology study. These findings may similarly be applied to other volcanoes, where nested craters have developed.

Beschreibung: We analyzed the coseismic and early postseismic deformation of the 2015, Mw 8.3 Illapel earthquake by inverting 13 continuous GPS time series. The seismic rupture concentrated in a shallow (〈20 km depth) and 100 km long asperity, which slipped up to 8 m, releasing a seismic moment of 3.6 × 1021 Nm (Mw = 8.3). After 43 days, postseismic afterslip encompassed the coseismic rupture. Afterslip concentrated in two main patches of 0.50 m between 20 and 40 km depth along the northern and southern ends of the rupture, partially overlapping the coseismic slip. Afterslip and aftershocks confined to region of positive Coulomb stress change, promoted by the coseismic slip. The early postseismic afterslip was accommodated ~53% aseismically and ~47% seismically by aftershocks. The Illapel earthquake rupture is confined by two low interseismic coupling zones, which coincide with two major features of the subducting Nazca Plate, the Challenger Fault Zone and Juan Fernandez Ridge.

Beschreibung: Quaternary deformation in the northern Chile forearc is controlled by trench parallel shortening along reactivated Mesozoic faults. Dextral strikes-slip is expressed in NW–SE striking faults of the Atacama Fault System, and reverse displacement dominates in E–W faults. This deformation results of the convergence in a concave-seaward continental margin. On September 11th, 2020, a Mw 6.3 earthquake and its subsequent aftershocks took place in the coastal region of northern Chile, revealing the reactivation of the deepest segment of a WNW–ESE striking upper plate fault. The reactivation of this fault occurred after the Mw 8.1 Iquique earthquake, and it seems to be connected to a N–S interplate locking segmentation of the plate margin, which is clearly shown by the locking pattern before the Iquique earthquake. This poses the question of how heterogeneous locking influences upper plate seismicity and how it relates to trench-parallel shortening.