Every 26 seconds, the Earth shakes. Not a lot — certainly not enough that you’d feel it — but just enough that seismologists on multiple continents get a measurable little “blip” on their detectors. But even though this pulse has been observed for decades, researchers don’t agree on what’s causing it. The mystery surrounding the phenomenon even has its own XKCD web comic.
The pulse — or “microseism” in geologist lingo — was first documented in the early 1960s by a researcher named Jack Oliver, then at the Lamont-Doherty Geological Observatory. He’s best known for his later work that supplied some important early evidence for shifting tectonic plates. Oliver figured out that the pulse was coming from somewhere “in the southern or equatorial Atlantic Ocean” and that it was stronger in the Northern Hemisphere’s summer months (or, the Southern Hemisphere’s winter).
“Jack didn’t have the resources in 1962 that we had in 2005 — he didn’t have digital seismometers, he was dealing with paper records,” explains Mike Ritzwoller, a seismologist at the University of Colorado, Boulder, whose team would independently come across the strange pulse some decades later.
In 1980, Gary Holcomb, a geologist with the U.S. Geological Survey, looked more closely at the weird microseism, and figured out that it’s strongest during storms. But his and Oliver’s work would mostly be lost to time, while the constant seismic drumbeat would go on, unnoticed, beneath our feet.
Then one day in 2005, then-graduate student Greg Bensen was working with seismic data at his lab at the University of Colorado, Boulder. His advisor walked in and asked him to show him what he was working on. As Ritzwoller tells it, Bensen pulled up some data, and there it was: A strong signal, coming from somewhere far off. “As soon as we saw this, [then-postdoctoral researcher Nikolai Shapiro] and I recognized that there was something weird, but we had no idea what it was,” Ritzwoller says.
Perplexed, the team examined the blips from every possible angle. Was something wrong with their instruments? Or their analyses? Or was this seismic activity really happening? All signs pointed to the latter. They were even able to triangulate the pulse to its origin: A single source in the Gulf of Guinea, off the western coast of Africa. They dug up Oliver’s and Holcomb’s work, too, and published a study in 2006 in Geophysical Research Letters. But even since then, no one has really confirmed the cause of the regular seismic activity. Though many assume it’s caused by waves, some hold out that it’s caused by volcanic activity.
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