Publication: Determinación de la fuente sísmica a distancias regionales : aplicación a la serie de El Hierro 2011
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2016-02-18
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Universidad Complutense de Madrid
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Abstract
La determinación del mecanismo focal de los terremotos permite obtener los par ametros del modelo que representa su fuente s smica. En el caso de terremotos ocurridos en islas volc anicas activas, el an alisis de la fuente s smica es de gran utilidad para entender el proceso de ruptura de la corteza terrestre provocado por la intrusi on de los diques de magma e interpretar los cambios en el campo local de esfuerzos de la isla. El objetivo de esta tesis doctoral es la determinación del mecanismo focal de los terremotos de la serie s smica de El Hierro 2011 (17 julio - 10 octubre). Esta serie precedió la primera erupción bien documentada en la isla de El Hierro y la primera erupción completamente monitorizada en el Archipiélago Canario. Debido a la baja magnitud de la sismicidad (MW 4:0), la heterogeneidad de la instrumentación y la escasa cobertura azimutal, se ha resuelto el problema del mecanismo focal aplicando distintas metodologías para obtener la mayor cantidad de información posible. En primer lugar se ha calculado el mecanismo conjunto de las familias de sismos más numerosas de la reactivación utilizando la metodología de la polaridad de las ondas P, que considera una fuente sísmica formada por un doble par de fuerzas. Pese a su sencillez, la aplicación de esta metodología a los datos de El Hierro 2011 ha permitido obtener la evolución temporal de las orientaciones de los planos de falla y los ejes de esfuerzos a lo largo de toda 1 Resumen / Summary la serie, incluyendo los terremotos de los dos primeros meses de reactivación, que no se pueden estudiar con otras técnicas debido a su baja magnitud (mb(Lg) 2:5). En segundo lugar, se ha estudiado el mecanismo focal de los terremotos de mayor magnitud de la serie, (mb(Lg) 3:5) aplicando la metodología de inversión de tensor momento sís mico a los registros de la red de estaciones de periodo corto en la isla ( < 20km). Este método es más completo que el anterior puesto que permite representar fuentes con cambios de volumen en la región focal propios de zonas volcánicas. Debido a las condiciones especificas del problema de inversión en El Hierro, antes de aplicar la metodología a los datos observados, se han realizado diferentes test con datos teóricos para comprobar la estabilidad de la inversión y la fiabilidad de las soluciones...
The determination of the earthquakes focal mechanisms allows to obtain the model parameters that represent the seismic sources. In the case of active volcanic islands, these analysis can help to understand the physical processes of magma migration and the changes of the local stress eld. The aim of this work is to study the focal mechanism of El Hierro 2011 series (17th July - 10th October). This seismic swarm preceded the rst documented volcanic eruption in the 3 Resumen / Summary island of El Hierro which is also the rst fully monitorized eruption in the Canarian Archipelago. Di erent methodologies have been applied in order to obtain as much information as possible from the scarce data available and in order to deal with the low magnitude of the earthquakes (MW 4:0), the strong network asymmetry and the high azimuthal gap. Firstly, a joint source mechanism methodology based on rst motion polarities was applied. Considering a double couple source model, joint mechanism solutions of the most populated families of the unrest were calculated and time evolution of fault planes and principal axes orientation were retrieved. Despite its simplicity, the application of this methodology to El Hierro series, brought insights on the stress orientation during the rst two months of the reactivation, when it was not possible to apply any other technique due to the scarcity of the data and the low magnitude of the earthquakes (mb(Lg) 2:5). Secondly, a point source moment tensor inversion was performed to study the focal mechanisms of the largest earthquakes of the series (mb(Lg) 3:5). Amplitude spectra was t at local distances ( < 20km) using short period data. The use of this methodology allows to determine the possible volume changes that are often related to volcanic earthquakes. Due to the speci c characteristics of the problem studied in this work, di erent tests using synthetic data were carried out in order to corroborate the stability of the inversion solution before applying the algorithm to El Hierro series...
The determination of the earthquakes focal mechanisms allows to obtain the model parameters that represent the seismic sources. In the case of active volcanic islands, these analysis can help to understand the physical processes of magma migration and the changes of the local stress eld. The aim of this work is to study the focal mechanism of El Hierro 2011 series (17th July - 10th October). This seismic swarm preceded the rst documented volcanic eruption in the 3 Resumen / Summary island of El Hierro which is also the rst fully monitorized eruption in the Canarian Archipelago. Di erent methodologies have been applied in order to obtain as much information as possible from the scarce data available and in order to deal with the low magnitude of the earthquakes (MW 4:0), the strong network asymmetry and the high azimuthal gap. Firstly, a joint source mechanism methodology based on rst motion polarities was applied. Considering a double couple source model, joint mechanism solutions of the most populated families of the unrest were calculated and time evolution of fault planes and principal axes orientation were retrieved. Despite its simplicity, the application of this methodology to El Hierro series, brought insights on the stress orientation during the rst two months of the reactivation, when it was not possible to apply any other technique due to the scarcity of the data and the low magnitude of the earthquakes (mb(Lg) 2:5). Secondly, a point source moment tensor inversion was performed to study the focal mechanisms of the largest earthquakes of the series (mb(Lg) 3:5). Amplitude spectra was t at local distances ( < 20km) using short period data. The use of this methodology allows to determine the possible volume changes that are often related to volcanic earthquakes. Due to the speci c characteristics of the problem studied in this work, di erent tests using synthetic data were carried out in order to corroborate the stability of the inversion solution before applying the algorithm to El Hierro series...
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Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra, Astronomía y Astrofísica I (Geofísica y Meteorología), leída el 25-01-2016