LABONNE Claire

An analysis of seismic coda phases recorded by five small aperture arrays in Kazakhstan and Russia is performed. The characteristics of the coda phases are obtained using array processing providing estimates of direction of propagation, frequency content, and apparent propagation velocity of coherent arrivals propagating through the arrays. A statistical analysis, using the coda characteristics of the whole dataset (far‐regional earthquakes) recorded by the arrays, is performed leading to a time‐azimuth distribution of the detected waves. Long Lg‐coda wavetrains clearly dominate the far‐regional seismic records. The later part of an Lg‐coda wavefield is generally expected to consist of omnidirectional and multiple scattered waves. still, in the present study, preferential propagation directions of scattered waves are identified in seismic signals, even for a lapse time larger than 1200 s. The comparison of the Lg‐coda characteristics for two different groups of earthquakes and for the different arrays shows that the geometrical configuration of the event array strongly influences the back‐azimuthal distribution of detected waves. More precisely, temporal variations of the Lg‐coda characteristics are identified: (1) as a transitory regime in the early part of the coda where arrival directions deviate from the epicenter back azimuth (eastward or westward). (2) as a final regime where the later part of the Lg coda observed by the two northern arrays exhibits incoherent phases with random propagation directions. and (3) as a final regime for the three southern arrays, which reveals coherent arrivals propagating from array‐specific back azimuths not related to the epicenter back azimuth.

A full polarization parameter system in 3D space is presented to characterize the state of polarization of a seismic wavefield and to parametrize any type of elliptical polarized seismic wave including extreme linear and circular polarizations. This parameter system does not require the a-priori knowledge of the orientation of the polarization plane and provides access to all parameters required in most polarization studies. Two groups of angular and vectorial parameters are defined, which can be easily related to one another. The knowledge of particle motion polarization opens the field of applications for 3-component seismic records contributing to improvements in phase identification. An example of a time-frequency visualization of the polarization parameters on a teleseimic earthquake recorded at a seismic station of LSBB is presented.

The seismic wavefield can be approximated by a sum of elliptical polarized motions in 3D space, including the extreme linear and circular motions. Each elliptical motion need to be described: the characterization of the ellipse flattening, the orientation of the ellipse, circle or line in the 3D space, and the direction of rotation in case of non-purely linear motion. Numerous fields of study share the need of describing an elliptical motion. A review of advantages and drawbacks of each convention from electromagnetism, astrophysics and focal mechanism is done in order to thereafter define a set of parameters to fully characterize the seismic wavefield polarization.

The analysis of the wave field is an essential prerequisite to the study of seismic wave propagation, which in turn improves our understanding of the physical processes related to the nature of the source and our knowledge of the propagation media. The objective of this thesis is to develop signal processing techniques to improve the exploitation of the information provided by the stations and 3-component antennas in order to characterize the seismic wave field. It focuses on polarization analysis, its extension to 3-component antennas and its use in conjunction with conventional antenna processing. The thesis reviews existing approaches that attempt to extend antenna processing to 3 components. These existing methods are complex and their use remains limited, the thesis suggests two alternative methods combining successively antenna processing and polarization. In order to make the best use of the polarization analyses, a standardized system of parameters describing the polarization is developed and associated with a visualization solution allowing to group all the parameters essential to the interpretation on a single figure. Finally, a polarization study on the LSBB (Laboratoire Souterrain Bas Bruit) 3-component antenna demonstrates the possibility of using the spatial coherence of the polarization as an aid for the interpretation of seismograms.