Browsing by Author "Yanez, G."

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    Nature and tectonic significance of co-seismic structures associated with the Mw 8.8 Maule earthquake, central-southern Chile forearc
    (PERGAMON-ELSEVIER SCIENCE LTD, 2011) Arriagada, C.; Arancibia, G.; Cembrano, J.; Martinez, F.; Carrizo, D.; Van Sint Jan, M.; Saez, E.; Gonzalez, G.; Rebolledo, S.; Sepulveda, S. A.; Contreras Reyes, E.; Jensen, E.; Yanez, G.
    The Mw 8.8 Maule earthquake on February 27, 2010 affected the central-southern Chilean forearc of the Central Andes. Here we show the results of field investigations of surface deformation associated with this major earthquake. Observations were carried out within three weeks after the seismic event, mostly in the central and northern part of the forearc overlying the rupture zone. We provide a detailed field record of co-seismic surface deformation and examine its implications on active Andean tectonics. Surface rupture consisted primarily of extensional cracks, push-up structures, fissures with minor lateral displacements and a few but impressive extensional geometries similar to those observed in analogical modeling of rift systems. A major group of NW-WNW striking fractures representing co-seismic extensional deformation is found at all localities. These appear to be spatially correlated to long-lived basement fault zones. The NW-striking normal focal mechanism of the Mw 6.9 aftershock occurred on March 11 demonstrates that the basement faults were reactivated by the Mw 8.8 Maule earthquake. The co-seismic surface ruptures show patterns of distributed deformation similar to those observed in mapped basement-involved structures. We propose that co-seismic reactivation of basement structures play a fundamental role in stress release in the upper plate during large subduction earthquakes. The fundamental mechanism that promotes stress relaxation is largely driven by elastic rebound of the upper plate located right above the main rupture zone. (C) 2011 Elsevier Ltd. All rights reserved.
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    Reactivation of Fault Systems by Compartmentalized Hydrothermal Fluids in the Southern Andes Revealed by Magnetotelluric and Seismic Data
    (2020) Pearce, R. K.; Sanchez de la Muela, A.; Moorkamp, M.; Hammond, J. O. S.; Mitchell, T. M.; Cembrano, J.; Araya Vargas, J.; Meredith, P. G.; Iturrieta, P.; Perez-Estay, N.; Marshall, N. R.; Smith, J.; Yanez, G.; Ashley Griffith, W.; Marquardt, C.; Stanton-Yonge, A.; Nunez, R.
    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 Planchon-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.
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    Soil electrical resistivity monitoring as a practical tool for evaluating irrigation systems efficiency at the orchard scale: a case study in a vineyard in Central Chile
    (2021) Vargas, J. Araya; Gil, P. M.; Meza, F. J.; Yanez, G.; Menanno, G.; Garcia-Gutierrez, V; Luque, A. J.; Poblete, F.; Figueroa, R.; Maringue, J.; Perez-Estay, N.; Sanhueza, J.
    In many orchards, irrigation scheduling is designed based on data from meteorological networks and considering homogeneous soil properties. Such assumptions may result in inefficient irrigation, which is difficult to constrain without expensive or invasive techniques. Here we have evaluated the ability of the electrical resistivity tomography (ERT) for detecting meter-scale irrigation uniformity and deep percolation during irrigation. The spatiotemporal variability of soil volumetric water content (VWC) in a vineyard located near Santiago (Chile) was inferred using ERT monitoring of two irrigation cycles. The electrical resistivity structure up to 4 m depth was estimated using two-dimensional inversion of ERT data. ERT results were verified by comparing resistivity models with VWC measured with soil moisture sensors, soil properties mapped in a 2 m-depth soil pit, and the spatiotemporal evolution of VWC obtained by solving numerically Richards equation. Largest temporal variations of resistivity were observed within the root depth (1 m) and are consistent with expected relative changes in VWC during irrigation. ERT images exhibit lateral changes in resistivity at these depths, likely indicating non-uniform infiltration of water controlled by observed soil texture variations. Resistivity changes were also observed below the root zone, suggesting that a fraction of the irrigation water percolates downward. These findings can be explained by an excess of irrigation water applied during the monitoring, which was planned considering regional evapotranspiration (ET) data that overestimated the actual ET measured at the vineyard. Altogether, our results suggest that ERT monitoring during irrigation is a cost-effective tool to constrain the performance of irrigation systems.
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    The Role of Temperature in the Along-Margin Distribution of Volcanism and Seismicity in Subduction Zones: Insights From 3-D Thermomechanical Modeling of the Central Andean Margin
    (2021) Araya Vargas, J.; Sanhueza, J.; Yanez, G.
    The distribution of volcanic and seismogenic zones is segmented along the trench-parallel direction in the Central Andes, and factors controlling their clustering are not fully understood. Here we present a 3-D thermomechanical model of the subduction zone at 18 degrees-26 degrees S to examine the role that temperature and mantle flow play in the distribution of active volcanoes and seismicity. We applied a steady state approach in which solid-state flow is driven by a kinematically prescribed slab with realistic geometry (including changes along the Bolivian Orocline) and using a 3-D model of the continental crust thickness. The obtained temperature distribution is consistent with proxies for isotherms derived from independent geophysical data, except below the Eastern Cordillera at 21 degrees-23 degrees S. The computed mantle flow pattern reveals the presence of along-margin dynamic pressure gradients. This 3-D preferential flow results in mantle temperatures of 1200-1400 degrees C at 80-100 km depth below the arc, with comparatively higher temperatures at similar to 22 degrees-25 degrees S. The obtained along-margin variations in temperature and in estimated melt velocity suggest that the subarc mantle south of 22 degrees S exhibits more favorable conditions for generation and upward migration of partial melts. This segment coincides with the higher concentration of active arc volcanoes and the presence of the Altiplano-Puna Volcanic Complex in the backarc. Intermediate-depth seismicity concentrates roughly below where the slab top is at 400-800 degrees C, suggesting that temperature exerts some control on the first-order distribution of intraslab seismicity. However, most intraslab seismicity occur at pressure-temperature conditions which are outside of the stability field expected for key dehydration reactions in slabs.