Every 26 seconds, a subtle seismic pulse reverberates through the Earth. Although this pulse is not strong enough to be felt by humans, it registers as a small but measurable shock on seismological equipment across multiple continents.
This phenomenon, observed by scientists since the early 1960s, remains shrouded in mystery. Despite decades of study, researchers have yet to reach a consensus on its cause. What is known is that this pulse is unrelated to the Schumann resonance, a different natural phenomenon often referred to as the “heartbeat of the Earth.”
The 26-second pulse of the earth
The pulse, often referred to as a ‘microseism’ by geologists, was first documented in the early 1960s by Jack Oliver, a researcher who would later become well-known for providing key evidence of tectonic plate movement. Oliver traced the source of the seismic pulse to a region in the southern or equatorial Atlantic Ocean. He also noted that the pulse was more pronounced during the summer months in the Northern Hemisphere, which corresponds to winter in the Southern Hemisphere. However, in 1962, Oliver was working with the limited technology of the time—digital seismometers were non-existent, and data was still recorded on paper.
Decades later, in 1980, U.S. Geological Survey geologist Gary Holcomb revisited the puzzling microseism. He discovered that the phenomenon was most pronounced during storms. Despite Holcomb’s findings, much of the earlier work by both Oliver and himself was eventually overshadowed as the seismic pulse continued without a clear explanation.
Investigating the Enigma: Benson, Ritzwoller, and Shapiro
In 2005, Greg Bensen, a recent graduate working in a lab at the University of Colorado, Boulder, stumbled upon the mysterious pulse while analyzing seismic data. When his supervisor, Ritzwoller, asked to see what he was working on, Bensen showed him the data, revealing a strong signal originating from a distant location. Intrigued, Ritzwoller brought in postdoctoral researcher Nikolai Shapiro to help analyze the findings. Despite their expertise, the team was baffled. They questioned whether there was an issue with their instruments or if their analysis was flawed. However, after thorough investigation, they confirmed that the seismic activity was real.
Their research led them to revisit the earlier work of Oliver and Holcomb. In 2006, they published a study in Geophysical Research Letters, documenting their findings. Yet, even with this new analysis, the exact cause of the regular seismic activity remained elusive. While many seismologists lean towards the theory that ocean waves are responsible for these pulses, others suggest that volcanic activity could be the cause.
The constant hum of seismic activity
While the 26-second pulse is particularly intriguing, it is not the only form of seismic activity occurring on Earth. Even during periods of relative calm—when there are no earthquakes or volcanic eruptions—there is a continuous background hum of seismic noise.
“Seismic noise is largely driven by the sun,” explains Ritzwoller. The sun’s heat affects the Earth unevenly, warming the equator more than the poles. This temperature difference generates winds, storms, ocean currents, and waves. When ocean waves crash against a coastline, the energy is transferred to the land, creating seismic noise.
Ritzwoller likens this process to tapping a desk. “When you tap your desk, the area near your knuckle feels the impact, but the vibration spreads across the entire surface,” he says. “Someone on the other side of the desk can feel the vibration if they place their hand or cheek on it.”
This constant seismic activity, while subtle, is a reminder of the dynamic processes at play beneath the Earth’s surface. As for the 26-second pulse, its origin remains one of the many unsolved mysteries that continue to captivate scientists, inviting further investigation into the complex workings of our planet.
