The cities have been silenced and with them practically all human activity due to the COVID-19 pandemic. This is how they are revealed by the devices installed throughout the Spanish geography to measure the earthquakes that occur under our feet.
The La Plata Astronomical Observatory in Argentina detected an unusual seismic signal on April 5, 1992. As the station was located about 600 meters from the old Estudiantes football stadium, seismologists determined that the earthquake was caused by the celebration of the goal scored by the Uruguayan player José Perdomo, who from that moment was nicknamed "Terremoto".
The direct free kick from 35 meters, now known as the “earthquake goal”, was the first record of an earthquake during a soccer game. Since then, scientists have repeatedly observed waves caused by the simultaneous joy of thousands of fans of the sport.
From a seismometer installed in the city of Barcelona, theJaume Almera Institute of Earth Sciences (ICTJA-CSIC) already detected these sudden seismic variations in the jumps produced in the stands of the Camp Nou. These vibrations, calledcultural or anthropic noiseThey are not the only ones produced by human activity and perceived by seismologists from cities.
Fireworks or rock concerts also “shake” the Earth every time they occur and we are not aware of it, nor do we perceive natural earthquakes in the ground itself. All these artificial tremors, captured by the devices, generate seismic signals that tower over waves generated by earthquakes.
“Seismometers detect vibrations in the ground, regardless of their origin. Normally we are interested in capturing the waves generated by earthquakes, but the vibrations can also be due to sources such as waves on the high seas, rain and wind or human activities, such as traffic, trains (including subways and trams) or activity industrial ”, explains to SINCJordi Diaz, seismologist at ICTJA-CSIC.
Measure urban noise
For this reason, in general, permanent seismic stations, such as those of the national network of theNational Geographic Institute (IGN), they are installed as far as possible from inhabited areas and the sea, to avoid noise caused by waves. Thus, the center of any large city does not seem to be the most suitable place for the installation of measuring devices, but in the last decade theurban seismology has presented other functions far from the initial intention.
According to a study, published in the journalScientific Reports, the seismic noise, often high, from traffic, machinery, power lines and even the movement of people has become a tool to characterize the urban subsoil, robustly monitor road traffic and activity the metro within the city, among others.
“Seismic instruments are important to us as an opportunity to access society and make our work known. But also, from a more strictly scientific point of view, we hope that they will help us to better understand thesubsurface structure”, Underlines Díaz, who has recently published an article inFrontiers in Earth Science on how seismometers within cities make it possible to connect Earth Sciences with society.
But what happens if the cities go silent? Since the beginning of thequarantine for COVID-19Neither soccer matches make the fans vibrate, nor concerts shake with their music, nor fireworks make the fans throb.
Despite this, the seismologist is in charge of tracking the seismic noise that silenced cities have left, thanks to the stations in cities and other areas with less anthropogenic activity, a large part of them temporarily installed in the research framework called SANIMS.
A similar phenomenon has occurred inMadrid as a consequence of the quarantine. The IGN station located to the southwest of the Retiro Park, near the Atocha Station, has verified drops of up to 3.5 decibels (dB) below the reference level of -92 dB. For seismologists, this gives an idea of the high degree of compliance with the confinement in the city of Madrid.
"The state of alarm decreed on March 14 has paralyzed many human activities that generated ground vibrations and that were recorded by seismometers," he tells SINCBeatriz Gaite Castrillo, seismologist of the IGN, institute that this year celebrates a century and a half of life.
With decreases in cultural seismic noise, researchers can now detect weaker seismic signals that were previously masked. “For example, if a station had high anthropic noise, its reduction will allow better identification of the signal fromsmall nearby earthquakes, which are the ones that produce signals in the range of cultural seismic noise, ”says Gaite Castrillo.
The confinement has caused more obvious changes to be observed in the stations installed in cities or inhabited centers. However, variations of lesser degree have also been perceived in stations in non-urban areas, such as theCanfranc Underground Laboratory (LSC), near Jaca, north of Huesca.
Cultural noises in the open field
Here "the usual seismic noise level is already very low," says Díaz. But, according to a temporary seismometer installed in theTuixent-La Vansa ski resort, near La Seu d’Urgell (Lleida), "seismic noise decreased significantly during daylight hours when the activity of the ski resort ceased," reveals the ICTJA-CSIC researcher.
Outside the cities, seismologists agree that the decrease in seismic noise has also been notable when comparing the weeks of confinement with the equivalent of the previous year.
And this despite the fact that the seismic stations are generally in the open field, “as far as possible from towns, roads, factories, among others, and their installation is carried out in wells, to reduce cultural noise as much as possible and even in caves. , tunnels, abandoned mines and deep boreholes to obtain the cleanest possible record, ”says Gaite Castrillo on the IGN National Seismic Network.
At these stations, scientists have found that during quarantine, the average power density is about 5 dB lower. "This decrease in the amplitude of the recorded signal is observed even in open field stations," emphasizes the IGN seismologist.
At the EARA seismic station located in the municipality of Aranguren, inNavarre, for example, "part of this decrease in signal is due to the decrease in quarry explosions registered in these seismic stations far from towns," he continues.
InCanary Islands, the volcanic surveillance network, which also records seismic noise daily, has also captured a decrease in seismic noise since March 14. In a very marked way since March 27 with the suspension of all non-essential activity by the Government, they emphasize from the IGN.
The result is "very significant" in Jedey, on the island ofThe Palm (CJED station), where the descent coincides with thestop of the improvement work of the LP2 southern ring road. The experts point out, however, that in this station hardly any changes have been observed between March 14 and 27, because the work, and therefore the anthropic noise, remained active.
InLanzarote, the Cueva de los Verdes (CDLV) station, has shown a significant decrease as of March 14, due to its closure and the reduction in nearby human activity. According to seismologists, this decline has also been more pronounced as of March 27. At that time, "levels lower than those of any weekend" were reached, they comment.
Distinguish the signs
This technology not only allows us to better understand the interior of the Earth by registering earthquakes and explosions. Since the end of the 20th century and the beginning of the 21st, its continuous application allows the perception of other seismic noises, previously considered useless.
Now you can capture and record the vibrations of the ground that cause thelandslides of earth, the rise of magma or fluids, therunoff of rivers, thewind, thestorms, thehurricanes, thetyphoons, changes in theice of the planet, the interaction ofsurf with the coast and ocean storms, among others.
“The signal registered in the seismometers contains all the vibrations coming from the different causes. However, each type of signal has specific characteristics such as its shape and frequency at which it is recorded ”, says Beatriz Gaite Castrillo. Thus, certain events, such as earthquakes, are easily recognizable and their signal can be separated from the rest of the record to locate it and calculate its magnitude.
Now, due tostop of human activity produced by the coronavirus crisis, experts will be able to better distinguish thosesigns and identify the mostweak –Lower energy–, hidden by noise of human origin. But since most of the seismic stations are already far from the cities, and therefore have less cultural noise, "we are not expecting a great revolution," Jordi Díaz tells SINC.
This is demonstrated, in part, by the Seismic Network, where the decrease in anthropic noise has not had a significant impact on the detection of the number of localized earthquakes - about 10 a day, say the researchers - nor on the minimum magnitude (<1.0), compared to previous years.
But “this opportunity to analyze a cleaner signal is possible that in the near future will give rise to new advances in the knowledge of our planet”, concludes Gaite Castrillo.
By Adeline Marcos