Browsing by Author "Maringue, Jose"

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    An Empirical Correlation between the Residual Gravity Anomaly and the H/V Predominant Period in Urban Areas and Its Dependence on Geology in Andean Forearc Basins
    (2021) Maringue, Jose; Saez, Esteban; Yanez, Gonzalo
    The study of site amplification effects is crucial to assess earthquake hazards that can produce great damage in urban structures. In this context, the gravity and the ambient noise horizontal-to-vertical spectral ratio (H/V) are two of the most used geophysical methods to study the properties of the subsoil, which are essential to estimate seismic amplification. Even though these methods have been used complementarily, a correlation between them has not been thoroughly studied. Understanding this correlation and how it depends on geology could be important to use one method as an estimator of the other and to make a distinction between the seismic and gravimetric basement. In this research, a comparison between the residual gravity anomaly and the H/V predominant period is performed using a long dataset from different projects on sedimentary basins in a group of the most important cities in Chile. To simplify the geological information, a seismic classification is used for soils, which considers the Vs30 and the predominant period of vibration (T0). The results of this comparison show a direct correlation between both parameters, the higher the negative residual gravity anomaly the higher the H/V predominant period. This correlation improves when only soft soils are considered, increasing the R2 value in more than a 50% in all the individual cities with respect to the overall correlation. When all the cities are considered, the R2 value for soft soils increases up to 0.87. These results suggest that the ideal geological background for this correlation is when a soft soil layer overlies a homogeneous bedrock. Heterogeneities in the bedrock and in the soil column add dispersion to the correlation. Additionally, the comparison between the depth to basement inferred by both methods show differences of less than 15% in soft sites; in denser sites, the difference increases up to 30% and the definition of a clear H/V peak is more difficult. In general, the gravimetric basement is deeper than the seismic one. However, gravimetric depths to basement can be under/over-estimated in zones with a heterogeneous soil column.
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    Geological and geotechnical investigation of the seismic ground response characteristics in some urban and suburban sites in Chile exposed to large seismic threats
    (2022) Maringue, Jose; Mendoza, Laura; Saez, Esteban; Yanez, Gonzalo; Montalva, Gonzalo; Soto, Valeria; Ayala, Felipe; Perez-Estay, Nicolas; Figueroa, Ronny; Sepulveda, Natalia; Galvez, Carlos; Ramirez, Paola; Ovalle, Carlos
    The central area of Chile's Valparaiso Region has been classified as a seismic gap for a major earthquake, which makes it very important to understand the seismic hazard of the zone. Generally, seismic codes consider a qualitative classification of sites to estimate the possible damage in the case of an earthquake scenario. Estimating the values of acceleration could be very important to prevent damages and increase preparedness for these rare events. In this research, a qualitative and quantitative estimation of seismic hazard is performed in the study area (Valparaiso region between Papudo and San Antonio 32 degrees-34 degrees S). This is achieved through an integrated and relatively economical approach which considers the information from Geology, Geophysical experiments (Gravity and seismic methods), and Geotechnical analyses. The results of the geophysical survey and geology information allow dividing the zone into five site types through a new proposal of site classification that depends not only on the V-s30,V- but also on the sites predominant period (T-0), which is an innovation of this work for the Chilean code. The Peak Ground Acceleration (PGA) values in the study zone were estimated using a Ground Motion Predictive Equation developed for the Chilean subduction zone. Additionally, we consider three different seismic scenarios according to the history of events in Central Chile. The results of this quantitative analysis show PGA values up to 0.52 g for the median and 1.2 g for the 84th percentile of the scenarios. Overall, the highest accelerations (PGA) are in zones with low shear wave velocities (< 500 m/s), a long predominant period (> 0.4 s) and where geology establishes the presence of low stiffness soils. The comparison of response spectra from the model against records from 2010 Maule and 1985 Valparaiso earthquakes shows available models tend to overpredict the intensities.
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    Rheological, petrophysical and geometrical constraints of a subduction channel from a numerical model perspective: Insights from La Cabana Paleozoic peridotites, Coastal Cordillera of south-central Chile
    (2022) Sanhueza, Jorge; Yanez, Gonzalo; Barra, Fernando; Maringue, Jose; Figueroa, Ronny; Saez, Esteban
    The emplacement of ultramafic blocks in accretionary complexes poses a geodynamic problem due to their negative buoyancy. In this study, we explore plausible emplacement scenarios for peridotite bodies in the Coastal Cordillera of south-central Chile by combining geophysical observations, numerical modeling and available petrological data for ultramafic blocks exhumed along the subduction channel. The La Cabana massif is the largest serpentinized peridotite complex yet recognized in the Coastal Cordillera, however, its size and petrophysical characteristics remain unknown. The geophysical measurements were performed to determine the size of this body, involving magnetic airborne surveys and electrical resistivity tomography. Inversion data show that the largest ultramafic block in La Cabana is 3 km long, 1.5 km wide and at least 1.2 km deep. This result constrains the characteristics of the block transported by the subduction channel. In the second step, we developed a numerical model for the subduction channel assuming a viscous rheology. In this modeling effort we search for tectonic scenarios that provide adequate conditions for the exhumation of the ultramafic body in La Cabana. These scenarios included a combination of key parameters, subduction angle dip and velocity, subduction channel geometry, rheology and density contrast. Scenarios compatible with the exhumation of La Cabana body type includes channel viscosity range of 10(19-20) Pa s, fast exhumation rates at mantle depths (10-20 mm/yr; >30-40 km), steep subduction angles (30 degrees-60 degrees), subduction channel widths of 3-5 km, density contrast between -200 and -400 kg/m(3) and a body diameter of 1.5 km which is consistent with our geophysical inversions. On the contrary, slow exhumation rates (similar to 1 mm/yr), low subduction angles (15 degrees), high-density contrasts (-400 to -600 kg/m(3)) and bodies larger than 1.5 km wide, are not viable exhumation scenarios. The methodology developed provided insights to infer ancient subduction channel geometries and rheologies that include peridotite.