Seismic noise variability as an indicator of urban mobility during the COVID-19 pandemic in the Santiago metropolitan region, Chile

On 3 March 2020, the first case of COVID-19 was confirmed in Chile. Since then, the Ministry of Health has imposed mobility restrictions, a global policy implemented to mitigate the propagation of the virus. The national seismic network operating throughout Chile provides an opportunity to monitor the ambient seismic noise (ASN) and determine the effectiveness of public policies imposed to reduce urban mobility in the major cities. Herein, we analyse temporal variations in high-frequency ASN recorded by broadband and strong-motion instruments deployed throughout the main cities of Chile. We focus on the capital, Santiago, a city with more than 7 million inhabitants because it is seismically well instrumented and has high levels of urban mobility due to worker commutes inside the region. We observed strong similarities between anthropogenic seismic noise and human mobility indicators, as shown in the difference between urban and rural amplitudes, long-term variations, and variability due to the COVID-19 outbreak. The same results are observed in other cities such as Iquique, La Serena, and Concepción. Our findings suggest that the initially implemented public health policies and the early end to confinement in mid-April 2020 in the metropolitan region caused an increase in mobility and virus transmission, where the peak in anthropogenic seismic noise coincides with the peak of the effective reproductive number from confirmed positive cases of COVID-19. These results confirm that seismic networks are capable of recording the urban mobility of population within cities, and we show that continuous monitoring of ASN can quantify urban mobility. Finally, we suggest that real-time changes in ASN amplitudes should be considered part of public health policy in further protocols in Santiago and other high-density cities of the world, as has been useful during the recent pandemic.