Seismic response in the Valley of Mexico: a physical perspective from recent models and observations

Víctor M. Cruz-Atienza | Instituto de Geofísica, UNAM

26-05-2021 4:30 pm.

A common practice in the universe of theory is to develop models either for explaining available observations or solving previously postulated problems. However, several of the most important Scientific Revolutions of our era took place owing to seminal theoretical postulations. One need only cite the heliocentric conception of our solar system by Copernicus and Bruno in the Renaissance or the existence of gravitational waves postulated by Poincaré and Einstein at the beginning of the 20th century, as emblematic examples of disrruptive knowledge detached from pure theory that, to this day, guide the way we observe the Universe in search of data.

The devastation of Mexico City caused by past earthquakes has raised many scientific questions in recent decades. Some have been satisfactorily answered by subsequent models and observations, such as the extraordinary amplification of seismic waves (the largest in the world on record) that can increase ground motion up to 500 times for some critical oscillation periods (for vulnerable structures) in the lake sediments on which much of the city sits. Besides the amplification, duration of intense ground motion in the lake-bed lasts more than three times those recorded in hard-rock a few kilometers away, a phenomenon that has been debated for decades without a plausible answer. In this work we explore the physical reasons explaining the spatial amplification pattern of the basin and the long-lasting motion by means of high-performance computational models and observations. We will show how theoretical predictions of the models led us to search for and find unprecedented seismological observations that support the physical mechanism we propose to solve the so-called "seismological paradox" of the valley, by which there is an apparent impossibility of sustained wave trains in the water-saturated and highly dissipative shallow clays of the lake sediments. The mechanism being the propagation of higher modes of surface waves in the deep and more consolidated layers of the basin. We will also discuss some aspects of the Mw7.1 earthquake of September 19, 2017, which killed 369 people and collapsed 44 one- to ten-story buildings, mostly located near the western edge of the lakebed sediments above the geotechnically known transition zone. Our model indicates that such a distinctive damage pattern (compared to 1985) is most likely due to the propagation of surface-wave fundamental modes generated at the basin edge due to the steep incidence of seismic waves enriched at high frequencies.