Publication: "APORTES PARA UNA NUEVA ZONIFICACIÓN GEOTÉCNICA DE VIÑA DEL MAR: ESTIMACIÓN DE LA PROFUNDIDAD DEL BASAMIENTO ROCOSO EN LA CUENCA DEL MARGA MARGA A PARTIR DE MEDICIONES ESPECTRALES H/V"
Loading...
Date
2017
Authors
SERAFINI SAN CELEDONIO, FRANCO JAVIER
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
RESUMENLos daños en edificios en la ciudad de Viña del Mar luego de los terremotos de 1906, 1985y 2010 han puesto en evidencia la ocurrencia de fenómenos de amplificación sísmica local(efecto de sitio) como resultado de la interacción de ondas de cuerpo con estratos de suelosblandos constituidos por sedimentos marino-fluviales no consolidados provenientes delestero Marga-Marga y la costa.En años recientes, diversas investigaciones han estudiado el fenómeno de propagación deondas utilizando modelos numéricos basados en el método lineal equivalente y/o el métodode elementos finitos. Una de las condiciones de borde más importantes en estos estudios esla profundidad y morfología del basamento rocoso, la que suele estimarse mediantedescripciones geológicas, gravimetría o sondajes de prospección. Esta última técnica es la másconfiable, sin embargo, su alto costo y elevados tiempos de ejecución limitan su aplicacióngeneralizada.El método H/V o técnica de Nakamura (1989) es un método no intrusivo que plantea queel peak de la relación espectral horizontal/vertical del ruido sísmico ambiente es unaaproximación confiable de la frecuencia fundamental de vibración de depósitos de suelossedimentarios siempre que exista un marcado contraste de impedancia (>3) entre el estratorocoso y el estrato sedimentario.Por otro lado, el análisis de propagación unidimensional de ondas de corte proporciona lafrecuencia fundamental teórica, que corresponde al primer peak de la función detransferencia, obtenida a partir de las propiedades del suelo (e.g. módulo de corte) y delespesor del estrato sedimentario.De acuerdo a lo anterior, se estudió la aplicabilidad del método H/V combinado con lateoría de propagación unidimensional de ondas para obtener una estimación de la morfologíay profundidad del basamento rocoso en el sector Plan de Viña del Mar.Para el desarrollo del estudio se cuenta con información del perfil de velocidad de ondasde corte ()s V y caracterización estratigráfica del sector obtenida a partir de ensayos depenetración estándar, información proporcionada por LEMCO y LEPUCV. De maneraadicional, se realizaron 63 mediciones de ruido sísmico ambiente utilizando un geófonotriaxial y un sismógrafo, las que fueron procesadas de acuerdo a los criterios establecidos porSESAME (Bard y SESAME Team, 2004), permitiendo determinar frecuencias fundamentales abajos niveles de deformación.Por otra parte, el fenómeno de propagación de ondas de corte se modeló con el softwareEERA, basado en el método lineal equivalente, de modo de determinar una relaciónfrecuencia-profundidad característica del sector. Esta relación se combinó con las frecuenciasfundamentales obtenidas mediante el método H/V de modo de estimar la profundidad delbasamento rocoso en los distintos puntos de medición.Los resultados indican que la hipótesis de propagación unidimensional es válida en elsector y por lo tanto los valores de frecuencias fundamentales son representativos yconfiables. Las profundidades obtenidas fueron contrastadas con profundidades conocidaspor sondajes obteniendo un error menor a 13%. Mediante una interpolación se determinó lamorfología del basamento rocoso de la cuenca del Marga-Marga, obteniéndose unaprofundidad máxima de 120 metros.Además, se determinó que el sector en estudio es altamente susceptible a amplificaciónsísmica debido a tres factores principales: suelos blandos, efectos 3D y desplazamiento de lafalla geológica Marga-Marga, ubicada en el eje de la cuenca.
ABSTRACTSeismic damage of buildings in Viña del Mar after the 1985 and 2010 earthquakes haveshown the occurrence of local seismic amplification (site effects) like a result of theinteraction between body waves and soft soils constituted by coast and fluvial sediments.In recent years, several researches have studied the wave propagation phenomenon usingnumerical models based on the equivalent linear response method and/or the finite elementsmethod. One of the most important border conditions in these studies is the depth andmorphology of the bedrock. It is usually estimated by geological descriptions, gravimetry orboreholes. This last technique is the most reliable one, but high costs and time required limitits general application.The Horizontal to Vertical Spectral Ratio (HVSR) or Nakamura’s technique (1989) is a notintrusive method which states that the main peak of de HVSR of seismic ambient noise is areliable approach of the fundamental frequency for sedimentary soils. This is valid only if astrong impedance contrast (>3) exists between the sedimentary layer and bedrock.On the other hand, wave propagation study provides the theoretical fundamentalfrequency, which corresponds to the first main peak of the transfer function, obtained fromthe soil properties (e.g. shear modulus) and bedrock’s depth.Therefore, we studied the applicability of the HVSR method combined with the theoretical1D wave propagation to obtain the bedrock’s morphology and depth approach in the Marga-Marga Basin.Information of the shear wave velocity profile ()s V and stratigraphy is available fromgeophysical and standard penetration tests. This information was provided by the civilengineering laboratories of the Federico Santa Maria Technical University and CatholicalUniversity of Valparaíso.Furthermore, we performed 63 ambient seismic noise measurements using a triaxialgeophone/velocity sensor and a seismograph. This measurements were processed accordingto the SESAME (Bard y SESAME Team, 2004) criteria, allowing to determine fundamentalfrequencies in low strain range.On the other hand, wave propagation was modeled with EERA software, based in theequivalent linear approach. This allowed to obtain a characteristic theoretical relationshipbetween frequency and depth of sedimentary layer of this zone. This relationship wascombined with the fundamental frequencies obtained from HVSR, so as to obtain thebedrock’s depth in all measurements points.Results indicate that 1D wave propagation hypothesis is valid, therefore the fundamentalfrequency values are reliable and representative. Obtained depths were compared withdepths known by boreholes obtaining an error less than 13%. An interpolation was performedto determine the bedrock’s morphology, obtaining a maximum depth of 120 meters.Additionally we conclude that Marga-Marga basin is highly susceptible to seismicamplification due to three main factors: soft soils, 3D effects and displacement of the Marga-Marga geological fault, located on the basin’s axis.
ABSTRACTSeismic damage of buildings in Viña del Mar after the 1985 and 2010 earthquakes haveshown the occurrence of local seismic amplification (site effects) like a result of theinteraction between body waves and soft soils constituted by coast and fluvial sediments.In recent years, several researches have studied the wave propagation phenomenon usingnumerical models based on the equivalent linear response method and/or the finite elementsmethod. One of the most important border conditions in these studies is the depth andmorphology of the bedrock. It is usually estimated by geological descriptions, gravimetry orboreholes. This last technique is the most reliable one, but high costs and time required limitits general application.The Horizontal to Vertical Spectral Ratio (HVSR) or Nakamura’s technique (1989) is a notintrusive method which states that the main peak of de HVSR of seismic ambient noise is areliable approach of the fundamental frequency for sedimentary soils. This is valid only if astrong impedance contrast (>3) exists between the sedimentary layer and bedrock.On the other hand, wave propagation study provides the theoretical fundamentalfrequency, which corresponds to the first main peak of the transfer function, obtained fromthe soil properties (e.g. shear modulus) and bedrock’s depth.Therefore, we studied the applicability of the HVSR method combined with the theoretical1D wave propagation to obtain the bedrock’s morphology and depth approach in the Marga-Marga Basin.Information of the shear wave velocity profile ()s V and stratigraphy is available fromgeophysical and standard penetration tests. This information was provided by the civilengineering laboratories of the Federico Santa Maria Technical University and CatholicalUniversity of Valparaíso.Furthermore, we performed 63 ambient seismic noise measurements using a triaxialgeophone/velocity sensor and a seismograph. This measurements were processed accordingto the SESAME (Bard y SESAME Team, 2004) criteria, allowing to determine fundamentalfrequencies in low strain range.On the other hand, wave propagation was modeled with EERA software, based in theequivalent linear approach. This allowed to obtain a characteristic theoretical relationshipbetween frequency and depth of sedimentary layer of this zone. This relationship wascombined with the fundamental frequencies obtained from HVSR, so as to obtain thebedrock’s depth in all measurements points.Results indicate that 1D wave propagation hypothesis is valid, therefore the fundamentalfrequency values are reliable and representative. Obtained depths were compared withdepths known by boreholes obtaining an error less than 13%. An interpolation was performedto determine the bedrock’s morphology, obtaining a maximum depth of 120 meters.Additionally we conclude that Marga-Marga basin is highly susceptible to seismicamplification due to three main factors: soft soils, 3D effects and displacement of the Marga-Marga geological fault, located on the basin’s axis.
Description
Catalogado desde la version PDF de la tesis.
Keywords
CUENCA DEL MARGA MARGA, V REGION , GEOTECNIA , MEDICIONES ESPECTRALES