Seismic ambient noise tomography to retrieve near-surface properties in soils with significant 3D lateral heterogeneity: the case of Quinta Colorada building in Chapultepec, Mexico

Structural failure due to local soil subsidence is a severe issue in the urban environment, particularly in cities located on thick sediments. We present such a case from Mexico City, where the near-surface soil characteristics beneath a subsided building were examined using 3D ambient noise tomography. We acquired data using dense arrays of 4.5 Hz vertical geophones placed around the building in two geometries. We performed cross-correlation analyses for 12 central frequencies in the range of 4–12 Hz to built tomographic images of surface wave group velocity. The results indicated a low-velocity layer (~ 120 m/s) for the first 10 m depth which we interpret as the typical clay layer of the Lake Zone of the city. Below, a more complex stiff layer (~ 500 m/s) appeared with significant lateral variations. In another independent analysis of HVSR, we observed that the site period varied between 0.4 and 0.55 s, which further signify the complexity of the subsurface. Thus, the conspicuous structural failure was due to the lack of stiffness of the shallow foundation, which subjected to differential motions between consistent materials from the foot of Chapultepec hill, and the soft lacustrine clays of the Lake Zone. As shown in this case, the seismic response variations in short distances, and short periods, for near-source events could be of great importance as demonstrated in Mexico City during the recent Puebla-Morelos Earthquake (M7.1) of September 19th, 2017.