2020-10-31 22:30:53
At 5:47 a.m. on May 23, 2020, the accelerometric station of the National Geographic Instituteinstalled inside the enclosure of the Royal Observatory of Madrid, detected a low level disturbance. Although the station is designed to detect earthquakes, that disturbance had a very different origin. A few seconds before, the dome that crowned the Críspulo Moro building, located barely 80 meters from the observatory, had collapsed. The collapse caused no casualties, but the debris hit the ground with such force that the rumble was picked up by the seismometer.
Under normal conditions, the vibration would have gone unnoticed by seismologists because the disturbance was so small that it would have been hidden under the noise that usually occurs in a large city like Madrid, but on that occasion another event made a difference: the pandemic. Since mid-March 2020, the Spanish government had decreed a state of alarm that confined the population and reduced economic and social activity in the country to a minimum. The movement restrictions had an unforeseen consequence: noise sources originating from road traffic, trains, planes or industrial activities were reduced, and the same happened with the vibrations produced by mass events or holidays. Thus, the noise that usually overlapped the weak signals in the instruments that measure seismic movements dropped to unprecedented levels. Compared to pre-lockdown levels, there was a “seismic silence” during the spring, making it possible to detect sporadic disturbances that, under normal conditions, would have been hidden.
Seismic noise has been an inevitable companion in seismometer and accelerometer measurements, says Beatriz Gaite Castrillo, a seismologist at the National Geographic Institute and a guest at Hablando con Científicos. Seismometers detect ground vibrations regardless of their cause. Those instruments are very sensitive and quickly pick up the vibrations produced by earthquakes that have a great intensity, but seismic movements are sporadic and of short duration, but once they are over, the instruments are not silent, they continue to detect a signal of low intensity that was long dismissed as nuisance noise because it interfered with weaker seismic signals.
Now things have changed, comments Beatriz Gaite during the interview. That seismic noise is a source full of valuable information that could be used to study other natural phenomena and human activities. Seismometers located in urban areas can be used to measure or value aspects of road traffic, suburban or railway communications, sporadic events, such as landslides or attacks. Another source of information originates from industrial, cultural or festive activities, whether they are mass demonstrations, rock concerts, fireworks or football matches. Yes, as you read, football matches.
At a seismic station located at the Barcelona Geosciences Institute (CITY–CSIC) installed to introduce visitors to Earth sciences during science fairs, traffic variations on Av. Diagonal are usually detected during the day, night and weekend. But the records obtained in May 2015, during the Champions League Semifinal match between Barcelona and Bayern Munich, were particularly striking. The data obtained clearly shows the ripples created by the fans when they jumped from their seats to celebrate the three goals that the home team scored during the last 15 minutes of the game.
The study of seismic noise has a very wide field of present and future applications. It can be used not only to discriminate between the different sources of vibrations in urban or natural environments, but also to study and better understand the mechanisms involved in the generation of seismic signals. The detection of noise signals between two distant stations makes it possible to compare them and extract information about the interior of the Earth below those points. The use of interferometry techniques makes it possible to obtain information on the ground, detect areas where there are fluid sources, whether groundwater or hydrocarbon deposits, and monitor their evolution over time.
Beatriz Gaite Castrillo, a seismologist from the National Geographic Institutean institution that this year is now celebrating its 150th anniversary.
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seismic noise
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