Seismic Wave Propagation and Site Response in Mexico City Due to 19 September 2017 Earthquake

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Seismic amplification in Mexico City is a widely studied phenomenon. The peculiar soft clays layers deposited in the ancient lake area cause considerable dynamic soil amplification of incident seismic motion. According to the seismic microzonation of the city, the lake sites with local period between 1-3 s period are likely to have more amplification than those where the dominant period is smaller than 1 second. The recent September 19, 2017, Earthquake (Mw 7.1), which occurred less than 150 km away from Mexico City, caused damages in zones where the local dominant period is smaller than 1.5 s. In this research, we explore the relationship between the incident wavefield and the seismic response of the Mexico basin by analyzing the accelerometric data recorded at Mexico City. We assign absolute time to records using arrival times of the dominant pulse in the range between 8 and 12 s period. Those waveforms show Rayleigh waves in the vertical and radial components and Love waves in the transverse component. Our analysis is based on f-k spectra computed for each ground motion component using the stations located in transition and hill zones. We observe that between 2 and 5 s period, Rayleigh waves travel with slightly higher phase velocity than that predicted by the subsoil structure model. The horizontal components suggest that the waves in that period range did not originate at the source (SE direction) but come from the SW edge of the basin. The phase velocities associated with these waves are comprised between 700 and 1200 m/s, which correspond to Vulcano-sedimentary deposits with depths between 0.5 and 1.0 km. For periods smaller than 2 s, it is not possible to define phase velocities or propagation directions; there are no coherent wavetrains in the records, and therefore f-k is not useful to understand the wavefield around the period of the collapsed buildings (0.6 s). The results suggest that destructive ground motion in Mexico City is a combination of local surface waves with the 1D amplification of soft soil deposits.

Presenting Author: Martin Cardenas-Soto

Authors