Seismic interferometry at the urban scale: imaging the Quito basin

Description

Temporary seismic networks installed in urban areas provide a powerful tool for investigating the shal-low geological structure and assess the seismic hazard using seismic interferometry. Quito, the capital of Ecuador, is located in a high seismic zone, placed at 200 km from the Pacific sub-duction zone and surrounded by crustal-faults prone to generate significant earthquakes. The city has been built on a basin, on the hanging wall of a system of active reverse faults. The high population density (around 3 million inhabitants), together with the lack of planning in most of its buildings, make Quito a metropolis with a high seismic risk. In Quito, the filling of the basin has been described as volcano-sedimentary sequences consisting of lavas, lahars, lacustrine and pyroclastic deposits. However, the thickness of the in-fill material, its spa-tial arrangement, and the deep structure of the basin remain poorly known. Between May 2016 and July 2018, 20 broad- and mid-band seismological stations were deployed pro-gressively throughout the city in an irregular array to record ambient seismic noise to investigate the upper-most geological structure of the basin. Here, we present the results of ambient noise cross-correlations of simultaneous operating seismic stations to retrieve inter-stations surface waves Green's functions in the frequency band 0.1 - 2 Hz. Careful analyses of day-night variations in noise spectral power have been done to select optimal time windows for the cross-correlations. Rayleigh and Love phase- velocity dispersion curves were inverted to obtain shear wave velocity profiles throughout the city. Love wave trains traveling in the longitudinal direction of the basin (NNE-SSW) are much clearer than Rayleigh wave trains. The surface waves Green's functions and their inversions mark a clear difference in the structure of the basin between the northern and southern parts. At the north, we detect the seismic basement at a depth of about 500 meters whereas at the south it appears much deeper around 1300 meters. This strong difference could be the main explanation for the low-frequency amplification (at 0.3 Hz) highlighted in the southern part of the basin on the recording of earthquakes (the April 2016, Mw 7.8 Pedernales earthquake being an example) and by the analysis of spectral ratios (Laurendeau et al., 2017).

Abstract

International audience

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