Preprints
https://doi.org/10.5194/egusphere-2022-1361
https://doi.org/10.5194/egusphere-2022-1361
 
10 Jan 2023
10 Jan 2023
Status: this preprint is open for discussion.

Probing environmental and tectonic changes underneath Ciudad de México with the urban seismic field

Laura Ermert1,a, Enrique Cabral-Cano2,, Estelle Chaussard3,, Dario Solano-Rojas4,, Luis Quintanar2,, Diana Morales Padilla4, Enrique A. Fernandez-Torres2, and Marine A. Denolle1 Laura Ermert et al.
  • 1Department of Earth and Space Sciences, University of Washington, Seattle WA, USA
  • 2Instituto de Geofísica, Universidad Nacional Autónoma de México, CDMX 04510, Mexico
  • 3Independent Researcher
  • 4Facultad de Ingeniería, Universidad Nacional Autónoma de México, CDMX 04510, México
  • anow at: Swiss Seismological Service, ETH Zürich, Zürich, Switzerland
  • These authors contributed equally to this work.

Abstract. The subsurface materials of Ciudad de México have unique mechanical properties that give rise to strong site effects. We investigated temporal changes in the seismic velocity at strong-motion and broad-band seismic stations throughout Mexico City, including sites with different geologic characteristics ranging from city center locations situated on lacustrine clay to hillsite locations on volcanic bedrock. We used autocorrelations of urban seismic noise, enhanced by waveform clustering, to extract subtle seismic velocity changes by coda wave interferometry. We observed and modeled seasonal, co-, and postseismic changes, as well as a long-term linear trend in seismic velocity. Seasonal variations can be explained by self-consistent models of thermo-elastic and poro-elastic changes in the subsurface shear wave velocity. Overall, sites on lacustrine clay-rich sediments appear to be more sensitive to seasonal surface temperature changes, whereas sites on alluvial and volcaniclastic sediments and on bedrock are sensitive to precipitation. The 2017 Mw 7.1 Puebla and 2020 Mw 7.4 Oaxaca earthquakes both caused a clear drop in seismic velocity followed by a time-logarithmic recovery that may still be ongoing for the 2017 event at several sites, or that may remain incomplete. The slope of the linear trend in seismic velocity is correlated with the downward vertical displacement of the ground measured by Interferometric Synthetic Aperture Radar, suggesting a causative relationship and supporting earlier studies on changes in the resonance frequency of sites in the Mexico City basin due to groundwater extraction. Our findings show how sensitively shallow seismic velocity, and in consequence, site effects, react to environmental, tectonic and anthropogenic processes. They also demonstrate that urban strong-motion stations provide useful data for coda-wave monitoring given sufficiently high-amplitude urban seismic noise.

Laura Ermert et al.

Status: open (until 24 Feb 2023)

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Laura Ermert et al.

Model code and software

Processing tools for seismic noise monitoring L. Ermert, M. Denolle https://github.com/lermert/ruido

Monte Carlo model for seismic noise monitoring L. Ermert https://github.com/lermert/cdmx_dvv

Laura Ermert et al.

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Short summary
Ciudad de México is built on a unique soil containing the clay-rich deposits of ancient lake Texcoco. Continuous, imperceptible shaking of these and deeper deposits by the traffic and other sources allows us to monitor changes in the subsurface seismic wave speed. Wave speed varies seasonally, likely due to temperature and rain effects; it temporarily drops after earthquakes, then starts to recover. Throughout the studied period, it increased on average, which may be related to soil compaction.