By Allison Kubo Hutchison
Earthquakes are well known to cause seismic activity in the Earth but you probably aren’t familiar with “football quakes”. Anthropogenic seismic sources such as football games also produce detectable seismic noise that seismologists can pick up on their instruments. In urban settings, road traffic, public transport have all been studied as seismic sources. This is an important consideration as scientists listen for warnings of earthquakes and volcanic eruptions within the noise of human activity. We have to be able to tell what signal is fluid moving in the crust potentially indicating volcanic activity and what signal is a touchdown.
Since the declaration of the global pandemic in March lockdown measures have resulted in as much as a 50% decrease in urban seismic noise. The sounds of ambient life have quieted as people stay in their homes and disband large gatherings. A recent study released Sept. 11 quantifies the global quietness as we all observe this pandemic. They studied 337 broadband seismometers as well as citizen-operated seismometers which provide a look into the populated areas.
Decreases in seismic noise occur regularly during weekends and holidays but it has been even quieter during the pandemic. There were particularly large drops at school and universities due to closures and the levels of high-frequency noise seismic ambient noise (hiFSAN) were up to 20% lower than school holidays. A drop in seismic noise in tourist hotspots quantifies the lack of global tourism in locations such as Barbados and ski resorts in Europe which experienced a 50% drop in hiFSAN. These reductions in the surface activity of cars were even measured deep underground at borehole seismometers installed in New Zealand’s Auckland Volcanic Field.
Although it is clear how human activities affect the surface and atmosphere, this quiet period also emphasizes how humans affect the subsurface. This period of quiescence will be useful to seismologists for many years to come. The reduction in hiFSAN has allowed scientists to pick up small earthquakes and by using this period they can further tune seismometers to ignore human-produced seismic noise and focus on the small earthquakes. Although the record should be dominated by small earthquakes which are several magnitudes more common than any quakes we can actually feel, small quakes are under-recorded. In populated areas, it can even be hard to detect larger quakes due to hiFSAN. By using the Great Quarantine Quiet, scientists can detect the full range of earthquakes while working from home.
Anthropogenic seismic noise can travel kilometers so even sheltered stations can be affected by it. It is believed that humans can produce shallow subsurface Rayleigh waves due to cars and public transport. This study confirms that hiFSAN is strongly correlated to human mobility and travel. In fact, hiFSAN is a good additional measure of human mobility that can be used in the future. Urbanization in the future will complicate hazards monitoring efforts as increased seismic noise clouds the signals of geologic origin.
Finally, the authors of the study emphasized the importance of citizen-operated seismometers in measuring hiFSAN in urban areas. If you are looking for a quarantine hobby I recommend citizen seismometers but it might be pretty quiet.