Urban noise in Mexico City reveals the changing seismic hazard of a sunken city

Scientists use noise from human activities to determine how the rate of subsidence of densely populated Mexico City affects earthquake risk.

Written by Meghomita Das, McGill University (@meghomita)

Citation: Das, AD, 2022, Mexico City urban hype reveals changing seismic risk of submerged city, Temblor, http://doi.org/10.32858/temblor.255

The land is sinking under the residents of Mexico City. This phenomenon, known as subsidence, can lead to buildings collapsing and streets deformed. A new study, presented last week at the annual meeting of the American Seismological Society, used the noise of the city itself to monitor how this subsidence occurred. Seismologists can use the creative tool to reassess seismic risk from densely populated, earthquake-prone Mexico City.

The city lies on a former lake basin, called Lake Texcoco, which was drained by the Spanish conquistadors during the construction of the city. After the water flowed in, the sediment that filled the basin began to dry up. In addition, city authorities pump groundwater to meet the needs of its residents. The area of ​​the underground pores – previously occupied by water – began to close up, as the weight of the ground above them compressed the sediments. Combined, these processes have caused Mexico City to sink at a rate of up to 20 inches (50 cm) per year.

Using satellite and ground observations, the researchers predicted that the pressure would persist for the next 150 years and could add 30 meters to the existing subsidence the city experienced.

Using ambient noise monitoring allows researchers to obtain a continuous record of the interaction between Earth’s pressure rates and the seismic velocity of vibrations transmitted through the Earth’s interior. It is another tool that can be used to reassess the seismic hazard of Mexico City as the impact of subsidence can alter the behavior of the Earth near the surface which is important for seismic hazard analysis.

Actively bustling and quiet, Mexico City lies on the dried-up ancient Lake Texcoco. Credit: Maranda P via Pixabay, CC-Public Domain Mark 1.0

To better understand how the problem of lake subsidence affects a city’s risk assessment, a team of researchers used urban vibrations to monitor the subsurface surface beneath the city.

“Human activity is causing persistent low-level shaking for many urban areas, and Mexico City is no exception,” said Laura Irmert, a seismologist at the University of Washington. She and a group of researchers have taken advantage of small seismic waves generated by traffic, vehicles, and everyday events in major cities.

“By comparing the appearance of these small waves, also known as ambient noise, over time and tracking their changes in velocity, we can determine the effect of subsidence at the surface,” Ermert says.

Map of the Basin of Mexico, circa 1519, after the arrival of the Spaniards. Credit: Semhur, via Wikimedia Commons, CC-Public Domain Mark 1.0

The velocity of seismic waves varies with the characteristics of the rocks they pass through. It increases when rocks are more compact and denser, and decreases if there are more holes or fluid-filled pore space inside the rock.

By comparing the speed of waves moving through different parts of the basin and during different parts of the year, the group found that small waves travel through subterranean soil faster during the summer, or drier, months and slower during the rainy season, or winter months. This happens because precipitation adds water to the Earth’s interior, slowing down S waves (shear waves), according to Ermert. The team also discovered that waves travel faster within a certain mud-rich layer of the basin. This may be due to the compaction of the clay layers and be responsible for more changes in velocity within the basin over time, according to Ermert.

The researchers found that seismic waves moved more slowly after the 2017 Puebla earthquake – a similar observation that has been documented in other earthquake-prone regions. Ermert says the group is not sure what causes this drop in velocity, but they suspect it is related to a change in the friction properties under the surface after a strong vibration.

Effects of the 2017 Puebla earthquake. Credit: Anto Fran, via Wikimedia Commons, CC-Public Domain Mark 1.0

“We want to increase our analysis over a longer period of time to make sure that the increase in velocity due to pressure is indeed as strong as we think,” says Ermert. The team plans to use data collected by Mexico’s National Seismological Service and continue the monitoring process in the hope that this long-term analysis will help develop a more detailed view of the Earth’s hazard assessment in Mexico City.

In-depth reading:

Simon, M.; (2021, May 17). Mexico City can sink up to 65 feet. wired. https://www.wired.com/story/mexico-city-could-sink-up-to-65-feet/

O’Hanlon, L.; (2021, May 17). The looming land sinking crisis in Mexico City. Eos. https://eos.org/research-spotlights/the-looming-crisis-of-sinking-ground-in-mexico-city

What Are The Main Benefits Of Comparing Car Insurance Quotes Online

LOS ANGELES, CA / ACCESSWIRE / June 24, 2020, / Compare-autoinsurance.Org has launched a new blog post that presents the main benefits of comparing multiple car insurance quotes. For more info and free online quotes, please visit https://compare-autoinsurance.Org/the-advantages-of-comparing-prices-with-car-insurance-quotes-online/ The modern society has numerous technological advantages. One important advantage is the speed at which information is sent and received. With the help of the internet, the shopping habits of many persons have drastically changed. The car insurance industry hasn't remained untouched by these changes. On the internet, drivers can compare insurance prices and find out which sellers have the best offers. View photos The advantages of comparing online car insurance quotes are the following: Online quotes can be obtained from anywhere and at any time. Unlike physical insurance agencies, websites don't have a specific schedule and they are available at any time. Drivers that have busy working schedules, can compare quotes from anywhere and at any time, even at midnight. Multiple choices. Almost all insurance providers, no matter if they are well-known brands or just local insurers, have an online presence. Online quotes will allow policyholders the chance to discover multiple insurance companies and check their prices. Drivers are no longer required to get quotes from just a few known insurance companies. Also, local and regional insurers can provide lower insurance rates for the same services. Accurate insurance estimates. Online quotes can only be accurate if the customers provide accurate and real info about their car models and driving history. Lying about past driving incidents can make the price estimates to be lower, but when dealing with an insurance company lying to them is useless. Usually, insurance companies will do research about a potential customer before granting him coverage. Online quotes can be sorted easily. Although drivers are recommended to not choose a policy just based on its price, drivers can easily sort quotes by insurance price. Using brokerage websites will allow drivers to get quotes from multiple insurers, thus making the comparison faster and easier. For additional info, money-saving tips, and free car insurance quotes, visit https://compare-autoinsurance.Org/ Compare-autoinsurance.Org is an online provider of life, home, health, and auto insurance quotes. This website is unique because it does not simply stick to one kind of insurance provider, but brings the clients the best deals from many different online insurance carriers. In this way, clients have access to offers from multiple carriers all in one place: this website. On this site, customers have access to quotes for insurance plans from various agencies, such as local or nationwide agencies, brand names insurance companies, etc. "Online quotes can easily help drivers obtain better car insurance deals. All they have to do is to complete an online form with accurate and real info, then compare prices", said Russell Rabichev, Marketing Director of Internet Marketing Company. CONTACT: Company Name: Internet Marketing CompanyPerson for contact Name: Gurgu CPhone Number: (818) 359-3898Email: [email protected]: https://compare-autoinsurance.Org/ SOURCE: Compare-autoinsurance.Org View source version on accesswire.Com:https://www.Accesswire.Com/595055/What-Are-The-Main-Benefits-Of-Comparing-Car-Insurance-Quotes-Online View photos