The lockdown to curtail the spread of the coronavirus has also been reflected in the seismometers records. ICTJA-CSIC researcher, Jordi Díaz, has been analyzing, since last March 13, when lockdown was declared, the variations of the ground vibrations recorded by the seismic stations deployed in Barcelona, both by the 15 sensors deployed within the framework of the SANIMS project in different secondary schools as well as by the permanent seismic sensors from Institut Cartográfic i Geològic de Catalunya (ICGC).

According to Díaz, there is no doubt: the halt of human activity in the city , mainly due to traffic reduction, has significantly lowered the seismic noise levels recorded by different seismometers throughout the last month and a half. According to Jordi Díaz, the main variation has been detected in "the frequency band between 5 and 15 Hz. The drop depends on the precise location of each station, but is generally between 50% to 75% ".

Díaz has also compared the data recorded by seismometers during the quarantine with Google's mobility data and has observed that there is a correlation between them.

The signal registered by the seismometer installed in the ICTJA-CSIC facilities (Gráfico 1), located in North Campus of Univeristy of Barcelona, show a gradual drop during the first lockdown week, then the signal stabilized and then it dropped down again at the begining of April, during the days when the population confinement was total.

Since 14 of April, once the lockdown measures were lighter, the vibrations levels increased again.

Jordi Díaz explains that "the vibrations levels during the lockdown are clearly lower than those registered during August 2019, with lower levels  during summer holyday season." (Gráfico 2)

The signal of another seismometer, installed in the "Josep Pla" secondary school (Gráfico 3), in the Horta neighborhood, shows a sharp drop since 13 May, when the order to close schools was given. The signal keeps a similar level since then during working days and weekends. This effect can also be seen in the  signal by the seismometer located in the "Jaume Balmes" secondary school, close to the city center.(Gráfico 4)

According to Díaz, the seismic ambient noise drop is an oportunity for the seimometers located in urban areas to improve its ability to detect waves in the same frequency range as the noise. It is also an exceptional opportunity to evaluate data acquired by the sensors deployed by SANIMS project. "In the case of the seismic network that we have deployed in Barcelona, the lockdown allows us to detect weak seismic signals that, in a normal day, would be masked by the ground vibrations generated by human activities. One of the SANIMS project goals is to use the seismic noise to study the subsurface properties. We have techniques that allows us to do so in remote areas, but it is still not clear if it is possible in urban areas. The "halt" will allow us to compare the data acquired during a normal period and the data recorded during the lockdown", said Díaz.

The current drop has been a global event that has been noticed in other big cities like Paris, London Brussels, Los Angeles or Roma. Jordi Díaz is in contact with a large group of seismologist from all around the world that are working on the analysis of the acquired data to write a research paper which is leaded by Thomas Lecocq, seismologist at the Royal Observatory of Belgium.

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