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  • English  (11)
  • 1
    Publication Date: 2020-02-12
    Description: A temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike-slip Liquiñe-Ofqui fault zone (LOFZ) in southern Chile. Seventy five shallow crustal events with magnitudes up to M(tief)w 3.8 and depths shallower than 25 km were observed in an 11-month period mainly occurring in different clusters. Those clusters are spatially related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the surface expression of the LOFZ. Focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike-slip mechanisms indicating a NE–SW direction of the P-axis for the LOFZ at this latitude. The seismic activity reveals the present-day activity of the fault zone. The recent M(tief)w 6.2 event near Puerto Aysén, Southern Chile at 45.4°S on April 21, 2007 shows that the LOFZ is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region.
    Keywords: 550 - Earth sciences
    Language: English
    Type: info:eu-repo/semantics/article
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  • 2
    Publication Date: 2021-10-14
    Description: During earthquakes, structural damage is often related to soil conditions. Following the 2014 April 1 Mw 8.1 Iquique earthquake in Northern Chile, damage to infrastructure was reported in the cities of Iquique and Alto Hospicio. In this study, we investigate the causes of site amplification in the region by numerically analysing the effects of topography and basins on observed waveforms in the frequency range 0.1–3.5 Hz using the spectral element method.We show that topography produces changes in the amplitude of the seismic waves (amplification factors up to 2.2 in the frequency range 0.1–3.5 Hz) recorded by stations located in steep areas such as the ca. 1-km-high coastal scarp, a remarkable geomorphological feature that runs north–south, that is parallel to the coast and the trench. The modelling also shows that secondary waves—probably related to reflections from the coastal scarp—propagate inland and offshore, augmenting the duration of the ground motion and the energy of the waveforms by up to a factor of three. Additionally, we find that, as expected, basins have a considerable effect on ground motion amplification at stations located within basins and in the surrounding areas. This can be attributed to the generation ofmultiple reflected waves in the basins, which increase both the amplitude and the duration ofthe ground motion, with an amplification factor of up to 3.9 for frequencies between 1.0 and 2.0 Hz. Comparisons between real and synthetic seismic waveforms accounting for the effects of topography and of basins show a good agreement in the frequency range between 0.1 and 0.5 Hz. However, for higher frequencies, the fit progressively deteriorates, especially for stations located in or near to areas of steep topography, basin areas, or sites with superficial soft sediments. The poor data misfit at high frequencies is most likely due to the effects ofshallow, small-scale 3-D velocity heterogeneity, which is not yet resolved in seismic images of our study region.
    Language: English
    Type: info:eu-repo/semantics/article
    Format: application/pdf
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  • 3
    Publication Date: 2020-10-12
    Description: We examine the intra‐arc crustal seismicity of the Andean Southern Volcanic Zone. Our aim is to resolve interseismic deformation in an active magmatic arc dominated by both margin‐parallel (Liquiñe‐Ofqui fault system, LOFS) and Andean transverse faults. Crustal seismicity provides information about the schizosphere tectonic state, delineating the geometry and kinematics of high strain domains driven by oblique‐subduction. Here, we present local seismicity based on 16‐month data collected from 34 seismometers monitoring a ~200‐km‐long section of the Southern Volcanic Zone, including the Lonquimay and Villarrica volcanoes. We located 356 crustal events with magnitudes between Mw 0.6 and Mw 3.6. Local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain, suggesting a convex shape of the crustal seismogenic layer bottom. Focal mechanisms indicate strike‐slip faulting consistent with ENE‐WSW shortening in line with the long‐term deformation history revealed by structural geology studies. However, we find regional to local‐scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity, within three distinctive latitudinal domains. In the northernmost domain, seismicity is consistent with splay faulting at the northern termination of the LOFS; in the central domain, seismicity distributes along ENE‐ and WNW‐striking discrete faults, spatially associated with, hitherto seismic Andean transverse faults. The southernmost domain, in turn, is characterized by activity focused along a N15°E striking master branch of the LOFS. These observations indicate a complex strain compartmentalization pattern within the intra‐arc crust, where variable strike‐slip faulting dominates over dip‐slip movements.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 4
    Publication Date: 2020-12-14
    Description: Magma is transported through the lithosphere as dykes which, during periods of unrest, may feed eruptions at the surface. The propagation path of dykes is influenced by the crustal stress field and can be disturbed by crustal heterogeneities such as contrasting rock units or faults. Moreover, as dykes propagate, they themselves influence the surrounding stress field through processes of stress transfer, crustal deformation and seismic failure. The result is the formation of arrested dykes, as well as contrasting strike and dip angles and dyke segmentation. Here, we study the mechanisms of dyke injection and the role played in modifying the stress field and potential propagation paths of later dyke injections. To do this we combine field data from an eroded and well-exposed shallow feeder dyke swarm with a suite of two-dimensional FEM numerical models. We mapped 35 dyke segments over a ~1 km long dyke swarm exposed ~5 km to the East of Pellado Volcano, in the Tatara-San Pedro-Pellado (TSPP) volcanic complex, Southern Volcanic Zone of the Andes. Detailed mapping of the swarm elucidates two preferential strike orientations, one ~N80°E and the other ~N60°E. Our numerical models simulate both the TSPP volcanic complex and the studied dyke swarm as zones of either magmatic excess pressure or as a rigid inclusion. The crustal segment hosting the volcanic complex and dykes is modelled using an elastic domain subjected to regional compression in select model cases. Model outputs provide the stress and strain fields resulting from the different geometries and applied boundary loads. The model results indicate that individual dyke injections can locally rotate the principal stresses such as to influence the range of orientations over which later dykes will form. The orientation of σ1 at the dyke tip ranges over 60° (±30° either side of the dyke tip) indicating that the strike orientation of later dykes will fall within this range. The effect of adding a bulk regional compression is to locally increase the magnitude of favorably oriented tensile stresses in the bedrock but to reduce the range of σ1 orientations to 40° (±20°). This implies that under a far-field transpressive stress regime, as is common in Andean settings, regional dyke swarms will tend to maintain their strike orientation parallel to the regional bulk stress. These results should be accounted for when studying periods of volcanic unrest in order to discern the location and orientation of potential fissure eruptions in active volcanic areas such as the Southern Volcanic Zone of the Andes.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 5
    Publication Date: 2020-07-10
    Language: English
    Type: info:eu-repo/semantics/conferenceObject
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  • 6
    Publication Date: 2022-03-28
    Description: The Liquiñe-Ofqui fault system (LOFS) in south-central Chile provides a natural laboratory to assess the interplay between magma/hydrothermal fluid flow and crustal deformation. Understanding these processes is of paramount importance for geothermal energy exploration and seismic hazard assessment. We deployed a dense seismic network (Sielfeld et al., 2019) at the northern termination of the LOFS in south-central Chile (~38°S) between 2014 March and 2015 June. The main aim was to better understand the significance and implications of seismic activity in relation to geological information such as the complex fault-fracture network, volcanoes, and the stress field estimated from geological data. As a result, the network was designed to monitor the northern segment of the LOFS on a more regional scale rather than concentration on the activity of one volcano. The network covered a ~200‐km‐long section of the Southern Volcanic Zone, including several Holocene stratovolcanoes (Callaqui, Copahue, Caviahue Caldera, Tolhuaca, Lonquimay, Llaima, Sierra Nevada, Sollipulli, Villarrica, Quetrupillán, Lanín (La), and Mocho‐Choshuenco). Waveform data are open and available under network code 3H from the GEOFON data centre under license CC BY 4.0.
    Language: English
    Type: info:eu-repo/semantics/workingPaper
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  • 7
    Publication Date: 2022-07-06
    Language: English
    Type: info:eu-repo/semantics/conferenceObject
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  • 8
    Publication Date: 2021-04-29
    Description: In active volcanic arcs such as the Andean volcanic mountain belt, magmatically sourced fluids are channeled through the brittle crust by faults and fracture networks. In the Andes, volcanoes, geothermal springs, and major mineral deposits have a spatial and genetic relationship with NNE trending, margin‐parallel faults and margin‐oblique, NW trending Andean Transverse Faults (ATF). The Tinguiririca and Planchón‐Peteroa volcanoes in the Andean Southern Volcanic Zone (SVZ) demonstrate this relationship, as their spatially associated thermal springs show strike alignment to the NNE oriented El Fierro Thrust Fault System. We constrain the fault system architecture and its interaction with volcanically sourced hydrothermal fluids using a combined magnetotelluric (MT) and seismic survey that was deployed for 20 months. High‐conductivity zones are located along the axis of the active volcanic chain, delineating fluids and/or melt. A distinct WNW trending cluster of seismicity correlates with resistivity contrasts, considered to be a reactivated ATF. Seismicity occurs below 4 km, suggesting activity is limited to basement rocks, and the cessation of seismicity at 9 km delineates the local brittle‐ductile transition. As seismicity is not seen west of the El Fierro fault, we hypothesize that this structure plays a key role in compartmentalizing magmatically derived hydrothermal fluids to the east, where the fault zone acts as a barrier to cross‐fault fluid migration and channels fault‐parallel fluid flow to the surface from depth. Increases in fluid pressure above hydrostatic may facilitate reactivation. This site‐specific case study provides the first three‐dimensional seismic and MT observations of the mechanics behind the reactivation of an ATF.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 9
    Publication Date: 2023-05-02
    Language: English
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  • 10
    Publication Date: 2023-02-27
    Description: Geothermal systems are commonly genetically and spatially associated with volcanic complexes, which in turn, are located nearby crustal fault systems. Faults can alter fluid flow in their surroundings, potentially acting as barriers or conduits for fluids, depending on their architecture and slip-rate. However, this fundamental control on fluid migration is still poorly constrained. Most previous modeling efforts on volcanic and hydrothermal processes consider either only fluid flow in their formulations, or only a mechanical approach, and seldom a full, monolithic coupling between both. In this work, we present a poro-elasto-plastic Finite Element Method (FEM) to address the first-order, time-dependent control that a strike-slip crustal fault exerts on a nearby geothermal reservoir. For the model setting, we selected the Planchón-Peteroa geothermal system in the Southern Andes Volcanic Zone (SAVZ), for which the geometry and kinematics of a potentially seismogenic fault and fluid reservoir is constrained from previous geological and geophysical studies. We assess the emergence and diffusion of fluid pressure domains due to fault slip, as well as the development of tensile/dilational and compressive/contractional domains in the fault' surroundings. Mean stress and volumetric strain magnitudes in these domains range between ±1 [MPa] and [-], respectively. Our results show the appearance of negative and positive fluid pressure domains in these dilational and contractional regions, respectively. We also investigate the spatial and temporal evolution of such domains resulting from changes in fault permeability and shear modulus, fluid viscosity, and rock rheology. These variations in fluid pressure alter the trajectory of the reservoir fluids, increasing migration to the eastern half of the fault, reaching a maximum fluid flux of 8 to 70 times the stationary flux. Pressure-driven fluid diffusion over time causes fluid flow to return to the stationary state between weeks to months after fault slip. These results suggest that the mechanism that exerts a first-order control is similar to a suction pump, whose duration heavily depends on fault permeability and fluid viscosity. We also show how a von Mises plasticity criterion locally enhances fluid flow. The transient process analyzed in this work highlights the importance of addressing the solid-fluid coupling in numerical models for volcano-tectonic studies.
    Language: English
    Type: info:eu-repo/semantics/article
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