Seismic Imaging Using Auto- and Cross-correlation of seismic noise in the Quito (Ecuador) basin

Temporary seismic networks installed in urban areas provide a powerful tool for investigating shallow geological structures and assessing the seismic hazard using passive seismic methods, including auto- and cross-correlation of seismic noise. To examine the feasibility of the methods to image the uppermost geological structures, 20 broad- and mid-band seismological stations were deployed progressively throughout Quito in an irregular array to record ambient seismic noise between May 2016 and July 2018.

Quito, the capital of Ecuador, is located in a high seismic zone, 180 km from the Pacific subduction zone and surrounded by crustal-faults prone to generate significant earthquakes. 

The city is built on a sedimentary basin, located on the hanging wall of a system of active reverse faults. The high population density (around 2.5 million inhabitants) and the lack of planning of most of its buildings, make Quito a metropolis exposed to high seismic risk. In Quito, the basin's filling has been described as volcano-sedimentary sequences consisting of lavas, lahars, lacustrine, and pyroclastic deposits (Alvarado et al., 2014). However, the thickness of the in-fill material, its spatial arrangement, and the basin's deep structure remain poorly known. 

This study presents the results of ambient noise auto- and cross-correlation of simultaneous operating seismic stations to retrieve: 1)  zero-offset high frequency body-wave crustal reflections, and 2) inter-station, surface-wave Green's functions in the frequency band 0.1 - 2 Hz. 

Auto-correlation of seismic noise indicated at least one reflection within the first 2.5 s from the surface. 

Careful analyses of day-night variations in noise spectral power were carried out to select optimal time windows for the cross-correlation. Additionally, 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-wave Green’s functions and their inversions suggest a clear difference in the basin's structure between the northern and southern parts. In the north, we detect the seismic basement at a depth of about 300 meters, whereas in the south, it appears much deeper at around 1000 meters. This significant difference could be the main explanation for the low-frequency amplification (at 0.3 Hz) highlighted in the southern part of the basin from earthquake recordings (e.g., the Mw 7.8 Pedernales earthquake on April 2016) and by the analysis of spectral ratios (Laurendeau et al., 2017).

References:

Alvarado, A., et al. (2014). Active tectonics in Quito, Ecuador, assessed by geomorphological studies, GPS data, and crustal seismicity, DOI: 10.1002/2012TC003224.

Laurendeau, A., et al. (2017). Low-frequency seismic amplification in the Quito basin (Ecuador) revealed by accelerometric recordings of the RENAC network, DOI: 10.1785/0120170134.