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Etude de la structure superficielle et de l'activité sismique du volcan Arenal, Costa Rica

Abstract : Arenal volcano presents a continuous strombolian explosive activity characterized by permanent lava flows, sporadic pyroclastic flows and intense seismic activity. This activity is monitored since 1974 by the "Observatorio Sismologico y Volcanologico de Arenal y Miravellas (OSIVAM)" of the Instituto Costarricence de Electricidad. In the first part of this work we study the volcanic activity between 1984 to 2000 using data obtained by OSIVAM that we grouped in a unique database. Data were analysed focusing on the volcano's behavior at the time scale of several years. So far it is difficult to establish whether the observed variations of different parameters represent real changes in volcanic activity or not. We highlight limitations of volcanic monitoring system and proposed improvements. The next part of this work consists in a study of the volcanic structure and seismic activity using different dense array data (triangles, semi-circles, profiles, L-array).We first analyzed local structure effects on seismic signals by using data of two profiles deployed at E and W of the volcano. Strong amplitude variations of spectral peaks were observed locally and were studied using the H/V spectral ratio method. We that found the spectral ratios form is independent of the seismic signal used. At certain sites, resonance effects are observed at frequencies that correspond to those of the observed amplifications. S-waves theoretical transfer functions were calculated at each site using seismic refraction velocity models. Both H/V spectral ratios and theoretical transfer functions show a peak at the same frequency if the impedance contrast between the shallow structure and the deep layers is high enough. This shows that the observed amplifications are closely related to site effects. The shallow structure was determined using a spatial correlation method that is based on correlation coefficient estimation between stations of semi-circular arrays. The correlation coefficient inversion method was improved in order to have a better estimate of the surface wave phase velocities. At a site of the W flank of the volcano Rayleigh wave phase velocities vary from 765 m s-1 at 1 Hz to 300 m s-1 at 12 Hz and for Love waves between 780 and 295 m s-1 in the same frequency band. At site at the NE flank, Rayleigh wave velocities range from 1386 to 300 m s-1 and those of Love from 1983 to 315 m s-1. Dispersion curves were inversed in order to obtain velocity models that take available geological data into account. Velocity models are coherent with seismic refraction data and spectral ratios observations. The analysis of continuous broadband records shows a wide range of tremor and explosive event behaviors. Results show the existence of two non-steady oscillating processes having different spectral characteristics that can be activated simultaneously. This suggests that two tremor sources can be active at certain moments. Autoregressive methods bring robust and high-resolution frequency and temporal measures of the spectral peaks. They show that frequencies and quality factors change rapidly with time, reflecting variations of physical properties of the seismic sources.
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https://tel.archives-ouvertes.fr/tel-00766832
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Submitted on : Wednesday, December 19, 2012 - 10:15:27 AM
Last modification on : Friday, September 30, 2022 - 4:10:23 AM
Long-term archiving on: : Sunday, December 18, 2016 - 5:25:15 AM

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  • HAL Id : tel-00766832, version 1

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Mauricio Mora Fernandez. Etude de la structure superficielle et de l'activité sismique du volcan Arenal, Costa Rica. Volcanologie. Université de Savoie, 2003. Français. ⟨NNT : ⟩. ⟨tel-00766832⟩

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