Last week, residents of San Diego reported another mysterious "boom" noise, accompanied by the unsettling rattling of windows. The boom is the second to take place in the city in the past three weeks, which have been unaccompanied by earthquakes, according to the United States Geological Survey.
The booms are far from the first. For hundreds of years there have been reports of unidentified boom noises across the US. Sometimes accompanied by earthquakes, sometimes not, they have been heard during the New Madrid earthquakes of 1811-1812 right up until January of 2020. They're often described as a "rushing" or "rolling" sound, and ocassionally are associated with cold temperatures rather than earthquakes.
There have been an unusual number of such reports this year around the world, including a Darth Vader breathing-like noise in Bratislava and a boom noise heard by residents of Texas, though most of these new reports could be accounted for by the eery silence during lockdown allowing people to hear background meteorological sounds such as wind and thunder, no longer being drowned out by the noise of traffic and travel.
The booming sounds aren't just limited to the US. Around the world they are known as “Bansal guns” in the Ganges delta and the Bay of Bengal, “yan” in Shikoku, Japan, and “mistpouffers” (fog belches) in Belgium.
Loud banging noises have been known to occur particularly often near Lake Seneca in the Finger Lakes region of New York. Known as the Seneca Guns, the sounds are so loud they ocassionally can rattle windows and doors, and go back to the Charleston earthquake in August 1886 when the noises were heard for several weeks after the event, coinciding with the many aftershocks.
Scientists are now using seismic data from the EarthScope Transportable Array (ESTA) to attempt to explain the noises around the US, comparing it to accounts of the noises from 2013 onwards.
The team from University of North Carolina at Chapel Hill trawled back through news reports from North Carolina, where reports of of the noises have occurred fairly frequently. The team hoped to be able to verify the noises with seismo-acoustic data taken from ESTA. They didn't find any events that coincided with earthquakes.
"Generally speaking, we believe this is an atmospheric phenomenon - we don't think it's coming from seismic activity," researcher Eli Bird told Live Science. "We're assuming it's propagating through the atmosphere rather than the ground."
The researchers, who presented their findings at the annual meeting of the American Geophysical Union on December 7, focussed instead on listening to infrasound data - low-frequency sound that isn't audible to humans. They did pick up signals - varying between 1 and 10 seconds long, Live Science reports - associated with reported booms.m
However, we're not a lot closer to an explanation for the noises, nor whether the noises are caused by the same type of event around the Earth. Many could be sonic booms from aircraft breaking the sound barrier, rather than unknown natural causes. Possible explanations for other events range from storm waves and tsunamis being amplified in a particular direction and ignition of methane gas released from methane hydrate beds.
One promising possibility is bolides in the upper atmosphere - meteoroids producing a sonic boom, where the meteor goes unseen and isn't noticed until we hear the noise it creates. For now, until more data is gathered, the noises remain unsolved.