Defended Theses

Seismic Attenuation Analysis using Lg waves and ambient noise recordings. Application to the Iberian Peninsula and Morocco.

Details
  • Thesis title: Seismic Attenuation Analysis using Lg waves and ambient noise recordings. Application to the Iberian Peninsula and Morocco.
  • Author: Raquel Noriega
  • Thesis supervisors: Dr. Antonio Villaseñor (ICTJA-CSIC) and Dra. María José Jurado (ICTJA-CSIC)
  • Date: Sep, 16 2016 11:30 am
  • Place: Sala de Graus de la Facultat de Física  (UB)

Abstract

Three different earthquake methods have been implemented to estimate the quality factor Q (inversely proportional to attenuation): the two-station (TS) method, the coda normalization (CN) method and the spectral amplitude decay (SAD) method. These approaches measure the spectral amplitude of the Lg wave of regional events. Among all the methods evaluated, the TS method allows imaging the spatial variations of the Lg wave attenuation in the Iberian Peninsula whereas the CN and the SAD methods only estimate average attenuation values as well as its frequency dependence. For the Iberian Peninsula, the first regional map that images the lateral variation of Lg Q has been estimated. This map reflects high Lg Q values in the stable Iberian Massif in western Iberia, while lower values are mainly found in the Pyrenean Range and in eastern and southern Iberia. For Morocco, the CN and the SAD methods produce similar results, indicating that the Lg Q models are robust to differences in the methodologies. The frequency-dependent Q estimates represent an average attenuation across a broad region of different structural domains and correlate well with areas of moderate seismicity. Additionally, I have studied the Lg propagation efficiency across the IMR. Results reflect an inefficient or even blocked propagation across the Gulf of Cádiz and for most paths crossing the western Alboran basin. The continental crust of the Iberian Peninsula and Morocco shows efficient Lg propagation. I have also investigated the potential of using ambient noise measurements to retrieve information about the anelastic structure of the Earth s crust. To verify the reliability of the attenuation results I have examined the influence of the distribution of noise sources and receivers on Q estimates. Azimuthally and spatially averaged Q values derived from noise recordings were estimated and compared with earthquake attenuation measurements. Results reveal that the average Q estimates are in concordance with previous long-period surface-wave measurements in the central part of the Iberian Peninsula obtained from earthquake data. Accurate Q estimates are also found in Morocco.

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