Browsing by Author "Tassara, Andres"

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    Mosaicking Andean morphostructure and seismic cycle crustal deformation patterns using GNSS velocities and machine learning
    (2023) Yanez-Cuadra, Vicente; Moreno, Marcos; Ortega-Culaciati, Francisco; Donoso, Felipe; Baez, Juan Carlos; Tassara, Andres
    We use unsupervised machine learning techniques to analyze continental-scale crustal motions in areas affected by the seismic cycle of large subduction earthquakes along the Chilean Trench. Specifically, we use the agglomerative clustering algorithm as an exploratory tool to investigate spatial patterns in GNSS regional velocities without the complexity of modeling a physical source. We present a continental-scale velocity field including all available GNSS data for two-time windows (pre-2014, 2018-2021) that represents two periods with different deformation patterns of the seismic cycle. We test two different pre-processing methodologies for the design of machine learning features from the GNSS-derived velocities. The first method uses the direction and magnitude of the secular rates as input features to the clustering algorithm. These results show a clustering spatially related to seismic cycle deformation, separating latitudinal segments with different velocities in the fore-arc and back-arc, as well as regions affected by postseismic relaxation. Thus, highlighting the effectiveness of this method for mapping first-order patterns of active deformation in a subduction zone, that are particularly related to variations on interplate coupling and postseismic transient deformation. In a more sophisticated approach, we use surface strain and rotational rates from GNSS velocities as features in the second methodology. Here, we develop a novel methodology to estimate strain and rotation rates accounting for the spatial heterogeneity of the GNSS-network. We determine the spatial scale at which these features are estimated by least squares inversions, by using a Bayesian model class selection method. The distribution of stations allows to identify heterogeneities in strain and rotation rates at spatial scales larger than 50 km, being particularly notorious the main features of regional deformation at scales > 100 km. Interestingly, the results show a spatial correlation between seismic segmentation in the fore-arc and geologic and structural domains in the arc and back-arc. Our results demonstrate the ability of the combination of inverse and machine learning methods to efficiently identify active deformation patterns and their relationship to the subduction seismic cycle and regional-scale geological structures. Furthermore, our analysis suggests that Andean geological structures influence the observed deformation field.
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    Recurrence time and size of Chilean earthquakes influenced by geological structure
    (2024) Julve, Joaquin; Barbot, Sylvain; Moreno, Marcos; Tassara, Andres; Araya, Rodolfo; Catalan, Nicole; Crempien, Jorge G. F.; Becerra-Carreno, Valeria
    In 1960, the giant Valdivia earthquake (moment magnitude, Mw, 9.5), the largest earthquake ever recorded, struck the Chilean subduction zone, rupturing the entire depth of the seismogenic zone and extending for 1,000 km along strike. The first sign of new seismic energy release since 1960 occurred in 2017 with the Melinka earthquake (Mw 7.6), which affected only a portion of the deepest part of the seismogenic zone. Despite the recognition that rupture characteristics and rheology vary with depth, the mechanical controls behind such variations of earthquake size remain elusive. Here we build quasi-dynamic simulations of the seismic cycle in southern Chile including frictional and viscoelastic properties, drawing upon a compilation of geological and geophysical insights to explain the recurrence times of recent, historic, and palaeoseismic earthquakes and the distribution of fault slip and crustal deformation associated with the Melinka and Valdivia earthquakes. We find that the frictional and rheological properties of the forearc, which are primarily controlled by the geological structure and fluid distribution at the megathrust, govern the magnitude and recurrence patterns of earthquakes in Chile.
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    Site Effects and Damage Patterns
    (EARTHQUAKE ENGINEERING RESEARCH INST, 2012) Assimaki, Dominic; Ledezma, Christian; Montalva, Gonzalo A.; Tassara, Andres; Mylonakis, George; Boroschek, Ruben
    A set of observations on site effects and damage patterns from the M-w 8.8 Maule, Chile, earthquake is presented, focusing on identification of structural damage variability associated with nonuniform soil conditions and subsurface geology. Observations are reported from: (1) the City of Santiago de Chile (Americo Vespucio Norte Ring Highway, Ciudad Empresarial business park), (2) the Municipality of Vina del Mar, and (3) the City of Concepcion, extending over 600 km along the Chilean coast. Reconnaissance information and ground motion recordings from the megathrust event are combined with site investigation data in the regions of interest. Comparisons against macroseismic observations related to uneven damage distribution from the M-w 8.0 1985 Valparaiso earthquake are discussed. Complexities associated with identifying the mechanics and underlying physical processes responsible for the manifestation of these effects are elucidated. [DOI: 10.1193/1.4000029]