Evidence for HV peaks superposition leading to extreme horizontal ground motion amplification revealed by the Xochimilco ambient noise tomography

SUMMARY Mexico City is one of the largest cities in North America, facing high seismic hazards and water supply problems. This paper presents an ambient seismic noise tomography of the city's south area in Xochimilco, where large amplifications have already been registered during subduction earthquakes. Eighty-four seismic stations have been installed, and their records processed. The tomography method combines the inversion of the horizontal-to-vertical spectral ratios (HVSR) and multimodal dispersion curves. The importance of considering a multimodal approach is justified in light of the complex geological setting. The dispersion curves analysis shows that the surface wave energy is divided over the fundamental and the higher modes, particularly between 50 and 300 m s−1, and in the whole frequency range analysed. We observe a spatially continuous decrease of the dominant peak frequency of the HVSRs toward the lake interior but a heterogeneous amplification. By analysing the velocity profiles associated with the highest amplifications, we discovered that these latter result from the superposition of several resonance peaks. Their coincidence in frequency is due to the overall constant linear gradient velocity in the sedimentary basin crossed by several low-velocity anomalies due to high water content or high-velocity anomalies due to lavas. Although most of the shallow water is trapped in clay sediment, the velocity model also allows for identifying deeper water reserves. All these analyses are of fundamental importance for the correct seismic mitigation in Mexico City but might also be extended to other cities built on top of sedimentary basins.