[S01-3] Open session III

[S01-3-05] Long Duration of Ground Motion in the Paradigmatic Valley of Mexico

^○Victor M. Cruz-Atienza¹, Josue Tago², Jose David Sanabria-Gomez³, Emmanuel Chaljub⁴, Vincent Etienne⁵, Jean Virieux⁴, Luis Quintanar¹ (1.Instituto de Geofisica, UNAM, Mexico, 2.Facultad de Ingenieria, UNAM, Mexico, 3.Universidad Industrial de Santander, Bucaramanga, Colombia, 4.Institut des Sciences de la Terre, Grenoble, France, 5.ARAMCO, Advanced Research Center, Saudi Arabia)

Built-up on top of ancient lake deposits, Mexico City experiences some of the largest seismic site effects worldwide. Besides the extreme amplification of seismic waves, duration of intense ground motion from large subduction earthquakes exceeds three minutes in the lake-bed zone of the basin, where hundreds of buildings collapsed or were seriously damaged during the magnitude 8.0 Michoacan earthquake in 1985. Different mechanisms contribute to the long lasting motions, such as the regional dispersion and multiple-scattering of the incoming wavefield from the coast, more than 300 km away the city. By means of high performance computational modeling we show that, despite the highly dissipative basin deposits, seismic energy can propagate long distances in the deep structure of the valley, promoting also a large elongation of motion. Our simulations reveal that the seismic response of the basin is dominated by surface-waves overtones, and that this mechanism increases the duration of ground motion by more than 170% and 290% of the incoming wavefield duration at 0.5 and 0.3 Hz, respectively, which are two frequencies with the largest observed amplification. This conclusion contradicts what has been previously stated from observational and modeling investigations, where the basin itself has been discarded as a preponderant factor promoting long and devastating shaking in Mexico City. Supporting our theoretical findings, by means of an array beamforming analysis using data from several subduction earthquakes recorded at hard-rock sites next to the basin we show that most of the coda waves (i.e., between the 30% and 45% of the total seismic energy) in those sites correspond to backscattered energy radiated from the basin. (Work published in www.nature.com/scientificreports ; 2016; DOI: 10.1038/srep38807)