Control geológico en el tamaño y recurrencia de grandes terremotos de subducción en Chile.
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Date
2024
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Universidad de Concepción
Abstract
Las zonas de subducción son límites de placas convergentes caracterizadas por la subducción de una placa menos boyante bajo un cuerpo continental (u oceánico) más ligero. Este proceso define una gigantesca falla de corrimiento llamada megathrust, que alberga la mayoría de la sismicidad mundial. La predicción de la sismicidad en esta interfaz ha sido objeto de debate durante décadas, principalmente porque los factores que controlan los procesos geodinámicos que culminan en un terremoto están ocultos por una gruesa capa de roca, lo que hace que el acceso directo a la interfaz entre ambas placas sea actualmente imposible.
En las últimas décadas, se han realizado esfuerzos significativos para relacionar la deformación superficial observada mediante conjuntos de datos GNSS e InSAR con las rupturas del megathrust que han sido registradas por la sismología moderna. Varios estudios han asociado señales específicas de deformación con etapas particulares del ciclo sísmico –que es un proceso centenario o decenal de nucleación de terremotos– asumiendo que algunas regiones de la interfaz de la placa permanecen acopladas, almacenando estrés y produciendo un déficit de deslizamiento que eventualmente se libera durante un terremoto. Sin embargo, a partir de la utilización de sofisticados modelos numéricos y cinemáticos, trabajos recientes han demostrado que el proceso completo de acumulación y liberación de estrés es más complejo de lo que se pensaba anteriormente, involucrando deformación postsísmica explicada por procesos viscoelásticos o poroelásticos.
Por otra parte, observaciones geológicas indican que el megathrust es una amplia zona de falla que involucra deformación métrica a kilométrica, donde diversas litologías interactúan para desarrollar diferentes mecanismos de deslizamiento, desde desplazamiento asísmicos hasta deslizamientos lentos y eventos sísmicos. Estos mecanismos también han sido registrados procesando series temporales de GNSS en diferentes zonas de subducción en todo el mundo. Las señales de estos mecanismos de deslizamiento están impresas en la estructura de la roca de afloramientos de paleo zonas de subducción, revelando que la sismicidad del megathrust involucra interacciones fluido-roca siguiendo episodios de brittle-creep y pressure-solution creep. Estas interacciones generan microestructuras características dentro de la zona dañada, aumentando la complejidad del sistema y, en última instancia, desencadenando un terremoto.
En este marco, este trabajo desentraña el papel de los parámetros clave que afectan el ciclo sísmico de los terremotos en el megathrust. Utilizando modelos numéricos cuasi-dinámicos restringidos por información geológica y geofísica, estudiamos el ciclo sísmico a lo largo de la zona de subducción chilena. Nuestros primeros resultados indican que la estructura geológica, manifestada en la distribución de los parámetros de fricción a lo largo del megathrust, puede explicar la sismicidad, los tiempos de recurrencia y el tamaño de las rupturas registradas en el sur de Chile. Estos hallazgos destacan un aspecto clave del proceso de nucleación de rupturas en las zonas de subducción: los minerales deformados en la interfaz de la placa controlarán el tipo de ruptura que puede desarrollar el ciclo sísmico en el megathrust. A partir de esta idea principal, se infiere que la temperatura y la presión, ambas controlando las reacciones de metamorfismo prógrado en la interfaz entre ambas placas, también tendrán un impacto en la sismicidad al cambiar la estabilidad de ciertos minerales con la profundidad. Además, dado que el esfuerzo normal efectivo en la falla de subducción depende de la razón de presión de poros, la presencia de fluidos influye enormemente en el ciclo sísmico en las zonas de subducción. Nuestros resultados finales muestran que el estado hidráulico medio en la interfaz de la placa, controlado por la porosidad de la falla y la disponibilidad de fluidos, puede permitir la propagación de rupturas o disminuir el tiempo de recurrencia de rupturas específicas en áreas donde la presión de poros está cerca de la litostática, y favorecer secuencias de rupturas pequeñas y en cascada de en regiones donde no se observan fluidos. Estas características destacan las interacciones fluido-roca como un aspecto crucial del ciclo sísmico en las zonas de subducción, mejorando significativamente las evaluaciones de peligrosidad sísmica a lo largo del margen chileno.
Subduction zones are convergent plate boundaries characterized by the subduction of a less buoyant plate beneath a lighter continental or oceanic body. This process delineates a massive thrust fault called the megathrust, which hosts most of the seismicity worldwide. The forecasting of seismicity at this interface has been debated for decades, primarily because the governing factors controlling the geodynamic processes that culminate in an earthquake are obscured by a thick layer of rock, making direct access to the megathrust currently impossible. In recent decades, significant efforts have been made to relate surface deformation observed by GNSS and InSAR datasets with megathrust ruptures recorded by modern seismology. Several studies have associated specific deformation signals with particular stages of the seismic cycle—a centennial or decennial process of earthquake nucleation—assuming that some regions of the plate interface remain coupled, storing stress and producing a slip deficit that is eventually released during an earthquake. However, using sophisticated numerical or kinematic approaches, recent works have shown that the entire process of stress accumulation and release is more complex than previously thought, involving postseismic deformation explained by viscoelastic or poroelastic processes. Geological observations indicate that the megathrust is a broad fault zone involving deformation over meters to kilometers, where various lithologies interact to develop different slip mechanisms, from aseismic slip to slow-slip and seismic events. These mechanisms have also been registered by processing GNSS time-series at different subduction zones worldwide. The signals of these slip mechanisms are imprinted in the rock fabric of paleo-subduction zone outcrops, revealing that megathrust seismicity involves fluid-rock interactions following episodes of brittle-creep and pressure-solution creep. These interactions generate characteristic microstructures within the damaged zone, increasing the system's complexity and ultimately triggering an earthquake. In this framework, this work unravels the role of key parameters affecting the seismic cycle of megathrust earthquakes. By using quasi-dynamic numerical models constrained by geological and geophysical information, we study the seismic cycle along the Chilean subduction zone. Our initial results indicate that the geological structure, as manifested in the distribution of frictional parameters along the megathrust, can explain the seismicity, recurrence times, and size of the ruptures recorded in Southern Chile. These findings highlight a key aspect of the ruptura nucleation process at subduction zones: the minerals deformed at the plate interface will control the type of rupture that the seismic cycle at the megathrust may develop. From this main idea, it is inferred that temperature and pressure, both of which control prograde metamorphic reactions at the plate interface, will also impact the seismicity by changing the stability of certain minerals with depth. Moreover, since the effective normal stress at the subduction fault depends on the pore pressure ratio, the presence of fluids greatly influences the seismic cycle at subduction zones. Our final results show that the mean hydraulic state at the plate interface, controlled by fault porosity and fluid availability, may allow ruptura propagation or decrease the recurrence time of specific ruptures in areas where the pore pressure is close to lithostatic, or favor small and cascading rupture sequences otherwise. These features highlight fluid-rock interactions as a crucial aspect of the seismic cycle at subduction zones, significantly enhancing seismic hazard assessments along the Chilean margin.
Subduction zones are convergent plate boundaries characterized by the subduction of a less buoyant plate beneath a lighter continental or oceanic body. This process delineates a massive thrust fault called the megathrust, which hosts most of the seismicity worldwide. The forecasting of seismicity at this interface has been debated for decades, primarily because the governing factors controlling the geodynamic processes that culminate in an earthquake are obscured by a thick layer of rock, making direct access to the megathrust currently impossible. In recent decades, significant efforts have been made to relate surface deformation observed by GNSS and InSAR datasets with megathrust ruptures recorded by modern seismology. Several studies have associated specific deformation signals with particular stages of the seismic cycle—a centennial or decennial process of earthquake nucleation—assuming that some regions of the plate interface remain coupled, storing stress and producing a slip deficit that is eventually released during an earthquake. However, using sophisticated numerical or kinematic approaches, recent works have shown that the entire process of stress accumulation and release is more complex than previously thought, involving postseismic deformation explained by viscoelastic or poroelastic processes. Geological observations indicate that the megathrust is a broad fault zone involving deformation over meters to kilometers, where various lithologies interact to develop different slip mechanisms, from aseismic slip to slow-slip and seismic events. These mechanisms have also been registered by processing GNSS time-series at different subduction zones worldwide. The signals of these slip mechanisms are imprinted in the rock fabric of paleo-subduction zone outcrops, revealing that megathrust seismicity involves fluid-rock interactions following episodes of brittle-creep and pressure-solution creep. These interactions generate characteristic microstructures within the damaged zone, increasing the system's complexity and ultimately triggering an earthquake. In this framework, this work unravels the role of key parameters affecting the seismic cycle of megathrust earthquakes. By using quasi-dynamic numerical models constrained by geological and geophysical information, we study the seismic cycle along the Chilean subduction zone. Our initial results indicate that the geological structure, as manifested in the distribution of frictional parameters along the megathrust, can explain the seismicity, recurrence times, and size of the ruptures recorded in Southern Chile. These findings highlight a key aspect of the ruptura nucleation process at subduction zones: the minerals deformed at the plate interface will control the type of rupture that the seismic cycle at the megathrust may develop. From this main idea, it is inferred that temperature and pressure, both of which control prograde metamorphic reactions at the plate interface, will also impact the seismicity by changing the stability of certain minerals with depth. Moreover, since the effective normal stress at the subduction fault depends on the pore pressure ratio, the presence of fluids greatly influences the seismic cycle at subduction zones. Our final results show that the mean hydraulic state at the plate interface, controlled by fault porosity and fluid availability, may allow ruptura propagation or decrease the recurrence time of specific ruptures in areas where the pore pressure is close to lithostatic, or favor small and cascading rupture sequences otherwise. These features highlight fluid-rock interactions as a crucial aspect of the seismic cycle at subduction zones, significantly enhancing seismic hazard assessments along the Chilean margin.
Description
Tesis presentada para optar al grado de Doctor en Ciencias Geológicas
Keywords
Terremotos Chile, Zonas de subducción