A seismic pulse takes place in the Earth every 26 seconds. This earthquake is not strong enough to feel, but it is sufficient to give seismologists a small measurable shock to their measuring equipment on multiple continents.
Although scientists observed this pulse since the early 1960s, researchers disagree as to what causes it. The cause remains a mystery and has nothing to do with the Schumann resonance, a natural phenomenon known as “the heartbeat of the earth.”
The pulse – also called a ‘microseism’ in geologist circles – was first documented in the early 1960s by a researcher named Jack Oliver and then by the Lamont-Doherty Geological Observatory. Oliver was best known for his essential evidence of the shifting of tectonic plates.
Oliver found that the seismic pulse came from somewhere in the southern or equatorial Atlantic Ocean and was stronger during the summer months in the Northern Hemisphere (or during the winter in the Southern Hemisphere). Obviously, in 1962 Oliver did not have the modern measuring equipment we know today. At the time, there were no digital seismometers, and paper files were still used.
In 1980, Gary Holcomb, a geologist with the U.S. Geological Survey, took a closer look at the strange microseism and found that the phenomenon is strongest during storms. But most of his and Oliver’s work would be lost over time as the seismic pulse continued unabated.
Investigation Benson, Ritzwoller, and Shapiro
In 2005, when recently graduated student Greg Bensen was working on his seismic data in his lab at the University of Colorado, Boulder, his supervisor Ritzwoller asked him to show him what he was doing. Bensen showed some data, and there it was: a strong signal, far away from somewhere. Ritzwoller brought in his colleague, the then postdoctoral researcher Nikolai Shapiro. They knew there was something strange, but they had no idea what it was.
Together they analyzed the data and perplexed the team looked at the pulses from every possible angle. Was there something wrong with their instruments? Or their analyzes? And did this seismic activity actually take place at all? Yes, it turned out to be true.
They brought Oliver and Holcomb’s work and published a study in Geophysical Research Letters in 2006. But even since then, no one has confirmed the cause of the regular seismic activity. While many seismologists believe that waves cause the seismic pulses, some maintain that it is caused by volcanic activity.
Always seismic noise, even outside the seismic pulse
While this particular pulse is intriguing, there is also seismic activity during quiet time (that is, not during an earthquake or volcanic eruption). There is always a background rumble of subtle seismic noise around us.
“Seismic noise is caused by the sun,” explains Ritzwoller. The sun heats the Earth more at the equator than at the poles, he says, causing wind and storms, ocean currents, and waves. When a wave hits the coastline somewhere, the energy is transferred to the land.
“It’s like tapping your desk. It warps the area near your knuckle, but then it gets broadcast all over the table,” he says. “So someone sitting on the other side of the table can feel the vibration if he puts his hand or maybe his cheek on the table. ”