Zahlen, bitte! The earth pulsates every 26 seconds
For over 60 years, researchers have been investigating a strange phenomenon: global stations measure a pulsation in the earth at exact intervals of 26 seconds.
In 2020, our Numbers, please! - article on the mysterious Earth pulse moved readers: it was widely read and actively discussed in the heise forum. We have therefore brought it up to date and are republishing it as Numbers, please! Classic. We hope you enjoy reading it!
For over 60 years, this natural phenomenon has puzzled researchers all over the world: Every 26 seconds, an impulse can be measured that is imperceptible to humans but is recorded by sensitive seismological devices around the globe. The cause is still unclear.
The seismological phenomenon was first studied on a larger scale in the early 1960s. In 1962, the American geologist John Ertle "Jack" Oliver published the article "A Worldwide Storm of Microseisms With Periods Of About 27 Seconds" (PDF) in the seismological journal Bulletin of the Seismological Society of America (BSSA), in which he presented the results of his research into microearthquakes.
Microearthquakes at 26-second intervals
The quakes occurred at intervals of between 26 and 27 seconds on average, and Oliver located the epicenter in the Gulf of Guinea. In total, the quakes were investigated in the period between June 6 and 7, 1961 and measured at 16 of 18 research stations around the globe. He compared the strongest shock with the explosion of around 600 tons of TNT. The strength fluctuated over time; Oliver also found that the periodic pulse was strongest in the winter months in the Southern Hemisphere. The pulses can still be measured today.
Microearthquakes are nothing special at first. In Germany, the earth shakes up to 8,000 times a day without anyone outside seismological research being aware of it. The quakes are so weak that they are imperceptible to humans.
In 2020, the corona crisis even improved research, as the lockdown restrictions reduced the everyday sources of interference that otherwise easily overlay these quakes.
Earthquakes as a monitor of soil properties
The findings are helpful for geological research – in moist rock, liquids and gases can be responsible for the stress conditions. The measured microquakes therefore provide more information about the composition of the rock.
The difference between these irregular quakes and the phenomenon off West Africa is the exact periodicity. And that is what makes it so difficult for researchers to localize the source of this phenomenon. And Jack Oliver was limited by the technical possibilities offered by his time; he could therefore only speculate.
Hypotheses between wave action and magnetic activity
The main hypothesis was that the microquakes were generated by waves hitting the coast in the Gulf of Guinea. A second hypothesis was that magnetic activity under the South Atlantic caused the quakes.
(Bild: CC BY SA 3.0)
In August 2006, an article was published in the journal Geophysical Research Letters [PDF], in which the researchers narrowed down the origin of the periodic pulse (0.038 Hz). The microquakes were detected both in the Bay of Bonny, in the Gulf of Guinea and in the antipodal Pacific region east of Papua New Guinea.
In 2013, a Chinese research team published another article (PDF) on the phenomenon in the Geophysical Journal International. They discovered a second periodic pulse (0.036 Hz). While they suspect the Sao Tome volcano to be the source of the known pulse due to its proximity, the possible cause of the newly discovered source remains unclear. The researchers also suspect volcanic activity there.
New findings in research
Dr. Charlotte Bruland and Prof. Céline Hadziioannou from Universität Hamburg's Center for Earth System Research and Sustainability (CEN) investigated the 26-second tones in 2023 (PDF). For them, it sounds as if a 10-kilometer clarinet is producing the tones, which are buried deep in the earth. New questions have arisen: The researchers found another tone, for example, which appears irregularly and rises linearly and whose origin is also unexplained. It might be the same pulse that the Chinese researchers discovered. Hadziioannou told the university's website that he suspects the cause of the 26-second phenomenon to be "that gas regularly escaping from the Earth's interior is forced through a kind of volcanic passage and then through smaller cracks. This could cause the signal."
Be that as it may, the origin of this fascinating natural phenomenon is not certain. So the research question remains exciting; whereby – another original theory is provided by geek comic author Randall Munroe in his xkcd comic "26-Second Pulse". Microquakes as the pulse of a giant – the research remains exciting! (mawi)
