Profiling the Quito basin (Ecuador) using seismic ambient noise

Abstract

Summary Quito, the capital of Ecuador, with more than 2.5 M inhabitants, is exposed to a high seismic hazard due to its proximity to the Pacific subduction zone and active crustal faults, both capable of generating significant earthquakes. Furthermore, the city is located in an intermontane piggy-back basin prone to seismic wave amplification. To understand the basin’s seismic response and characterize its geological structure, 20 broad and medium frequency band seismic stations were deployed in Quito’s urban area between May 2016 and July 2018 that continuously recorded ambient seismic noise. We first compute horizontal-to-vertical spectral ratios to determine the resonant frequency distribution in the entire basin. Secondly, we cross-correlate seismic stations operating simultaneously to retrieve inter-stations surface-wave Green’s functions in the frequency range of 0.1 - 2 Hz. We find that Love waves traveling in the basin’s longitudinal direction (NNE-SSW) show much clearer correlograms than those from Rayleigh waves. We then compute Love wave phase-velocity dispersion curves and invert them in conjunction with the HVSR curves to obtain shear-wave velocity profiles throughout the city. The inversions highlight a clear difference in the basin’s structure between its northern and southern parts. In the center and northern areas, the estimated basin depth and mean shear-wave velocity are about 200 m and 1800 ms−1, respectively, showing resonance frequency values between 0.6 and 0.7 Hz. On the contrary, the basement’s depth and shear-wave velocity in the southern part are about 900 m and 2500 ms−1, having a low resonance frequency value of around 0.3 Hz. This difference in structure between the center-north and the south of the basin explains the spatial distribution of low-frequency seismic amplifications observed during the Mw 7.8 Pedernales earthquake in April 2016 in Quito.

Improved 3D Shallow‐Deep Vs Structure in Tongzhou, Beijing (China), Revealed by Dense Array Ambient Noise Tomography

“…(2022) and Pacheco et al. (2022) showed that, for different stacking periods, the SNR values of NCFs from the ts‐PWS are higher than that from the linear stack. To show the surface wave clearly, in Figure 3a, we only plot 500 NCFs that vary with the interstation distance in the 0.3–8s period band.…”

“…The ts-PWS can greatly improve the signal-to-noise ratio (SNR) and Empirical Green's functions (EGFs) convergence compared with the linear stack (Ventosa et al, 2017). The results of Shekar et al (2022) and Pacheco et al (2022) showed that, for different stacking periods, the SNR values of NCFs from the ts-PWS are higher than that from the linear stack. To show the surface wave clearly, in Figure 3a, we only plot 500 NCFs that vary with the interstation distance in the 0.3-8s period band.…”

Characterizing the seismic response and basin structure of Cusco (Peru). Implications for the seismic hazard assessment of a World Heritage Site

“…Major earthquake disasters such as Mexico City (1985), Kobe (1995) and more recently Turkey-Syria (2023) have demonstrated the increase in hazard caused by the ampli cation of seismic waves in soft soils (e.g., Kramer, 1996). However, this topic remains in its infancy in the Andes, limited almost exclusively to the capital cities (Santiago: Pilz et al, 2009; Lima: Calderon et al, 2014; Quito: Pacheco et al, 2022). As a fast-growing touristic town with uncontrolled planning, the city of Cusco turns out to be a crucial case study.…”