Urban noise used for pressure and earthquake monitoring

Small seismic waves generated by urban activity in Mexico City provide a glimpse into how low soil can accelerate seismic waves that travel in shallow layers below the city.

With the extraction of groundwater from under the city, the base soil and clay supporting the city became more compacted over time. In some places in the Mexico City Basin, increased subsidence is associated with increases in the velocity of near-surface shear waves, according to a presentation at the annual meeting of the Seismological Society of America.

Seismologists are interested in these changes in part because of their potential effects on building stability during an earthquake. When the frequency of the earth’s motion matches the building’s resonance (the natural period of oscillation back and forth), the oscillations caused by the earthquake are most damaging.

“One of the reasons why the 1985 Michoacan earthquake was so devastating in Mexico City was that soft sediments began to appear in intervals of a few seconds, which is proportional to the resonance periods of tall buildings. If the velocity of the shear wave in the basin increases, it will reduce the resonance periods, on For example, 2.5 to 2 seconds,” explains Laura Ermert of the University of Washington.

“In practical terms, this means that in an earthquake like the 1985 event, shorter buildings can be affected more badly today than they were back then,” she added.

In a collaboration between the National Autonomous University of Mexico and the University of Washington, Ermert and her colleagues used ambient seismic data from human activity in the city — such as the gurgling of traffic on roads and railroads — to monitor changes in seismic velocity around the Mexico City Basin in a time series spanning 15 to 25 months. general.

“Ambient noise is a great source of information because it is free, non-invasive and can give us information every few days to weeks,” Ermert said. “It is a good complement to geodetic data. Geodetic data gives us high-resolution information straight from the surface, while ambient noise gives us integrated sensitivity to what is happening at some depth.”

The researchers found a long-term increase in velocity at stations located in and near the lake’s muddy deposits that accumulate as groundwater was withdrawn from below the city. As the pore pressure in the sediment decreases, the sediment becomes more solid and seismic waves can pass through them more quickly.

Ermert said the team hopes to investigate this descent and velocity relationship in more spatial detail about the pelvis, because not every part of the pelvis shows a similar pattern. “For example, the largest rate of velocity increase that we observed was in an area declining less than the peak rate of decline.”

They also detected seasonal variation in velocity as well as changes in velocity that were affected by the 7.1-magnitude Puebla earthquake in 2017 and the 7.4-magnitude earthquake in 2020 near Oaxaca.

Ihrmert and colleagues were also surprised to find a discrepancy in the velocity changes measured by the different “directions” (east-west, north-south, top-down) of the seismic stations. “For example, the seasonal variation in velocity caused by precipitation and temperature changes will be more pronounced in the north than in the eastern component,” she explained. “This may be related to the complex wave propagation effects of the pelvis.”

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