Detailed description of the thesis topic

During the ERC ABYSS project, 24 acquisitions of 150 kilometers will be acquired on the different submarine fiber optic cables along the Chilean margin. From these new data and the ambient seismic noise recorded at any point along the fiber, high-resolution 2-D shear wave velocity profiles will be computed by inverting the multi-mode dispersion curves. These images will provide unprecedented details on the margin and allow better observation of the structural heterogeneities that drive differences in the subduction behavior. This will improve our understanding of fault segmentation and the seismic cycle. In addition, the resulting images will provide better crustal velocity models which will increase the accuracy of the seismic event locations that will be recorded during the project.

The use of ambient seismic noise to image the structure of the crust has demonstrated its potential in different geological contexts including using DAS data. For example, using this approach, we performed a 2-D shear wave velocity image of a sedimentary basin off Methoni, Greece (Lior et al. 2022). This study allowed us to identify its internal structure, geometry and also its spectral response.

Several challenges are considered in this thesis. (1) The distribution of noise sources will have to be taken into account to achieve unbiased images as the acquisition times will only be a few weeks on each fiber. (2) The mechanical coupling of the optical fiber to the ground will have a direct impact on the quality of the recorded signals. This can change the response of the fiber to a given signal and thus be a source of error on the estimates of the source parameters of the earthquakes that will be recorded. A preliminary step to the imaging will be to characterize the response of the fiber optic telecom cables of the Chilean network from well-characterized sources (earthquakes, known acoustic noise, ocean gravity waves). Finally (3) several field missions to deploy the instrument on the different fibers of the GTD network in Chile will take place during the thesis.

Work context

In the last decades, the observation and measurement of the Earth's dynamics has made important progress with the development of dense seismological networks. A new metrological approach that allows to exploit telecom optical fibers as very high density (typically one sensor every meter of fiber) and range (several tens of kilometers) sensor networks opens new perspectives in the understanding of the functioning of giant faults. These measurements are made from an optical system connected at the end of the fiber and commonly called DAS.

This recruitment is done in the framework of the ERC ABYSS project funded by Europe. This project aims at transforming the observation capabilities of the Chilean giant subduction fault, using three DAS instruments that will provide a continuous and dense measurement of ground motion over 400km, using the submarine fiber optic network of the Chilean GTD telecom network.

The thesis is part of the activities of the SEISMES team of the Geoazur laboratory, which is supervised by the CNRS, the Observatoire Côte d'Azur, the Université Côte d'Azur and the IRD.

The employer of the recruited PhD will be the Observatoire de la Côte d'Azur and the place of work will be located in Valbonne Sophia Antipolis (06).

The Observatoire de la Côte d'Azur is a national public institution of higher education and research with an administrative character (EPA) and carries by its statutes the missions of scientific research, observation services and dissemination of knowledge of an Observatory of Sciences of the Universe (OSU). The Observatoire de la Côte d'Azur brings together and pilots research activities in Earth and Universe sciences in the Côte d'Azur region thanks to its mixed services unit, Galilée, and its three multi-supervisory research units (OCA, CNRS, UNS, IRD): Artémis, Géoazur and Lagrange.

The Geoazur laboratory is a mixed multidisciplinary research unit composed of geophysicists, geologists and astronomers working together on major scientific issues: telluric hazards (seismic, gravity and tsunamigenic) and associated risks, lithosphere dynamics and Earth imaging, geodesy and metrology of the Earth and the near Universe. It includes about 170 people (researchers, teacher-researchers, engineers, doctoral students...). It is spread over 2 sites: the Azur CNRS campus in Valbonne Sophia Antipolis and the Calern - Caussols site of the Côte d'Azur Observatory.

The person recruited will join a dynamic group of scientists working on DAS (members of the SEISME team), who are experts in the use of DAS data for multiple applications, in particular for seismology of the ocean floor. In addition, the ABYSS project will provide very rich interactions with a group of nine people on a wide range of seismological applications of DAS data.

Constraints and risks:

Several missions in Chile.

Duration in months:

36 months

Hiring date:

October 2022