• On November 11, seismologists began puzzling over a weird low-frequency rumble that rang the entire planet.
  • The wave coming from somewhere was weirdly simple and tied to no known events.
  • More comprehensive study of an uncharted area of the ocean floor could provide an explanation of the mystery.

Someone who tracks earthquakes for fun noticed it first. On November 11, 2018, a Twitter user going by @matarikipax spotted a weird signal on the U.S Geological Survey’s live seismogram page. The signal had been captured by equipment in Kilimambogo, Kenya. Matarikpax posted an image of it with the message, “This is a most odd and unusual seismic signal.” Then he saw it in data from Zambia and Ethiopia before looking farther away and finding it in Spain, and then eventually in his own corner of the world, Wellington, New Zealand. Other seismic-savvy people soon joined in as the mysterious low-frequency rumble circled the globe for about 20 minutes. It was also detected in Chile, Canada, and Hawaii. Eventually, its source was determined to be about 15 miles off of French archipelago Mayotte, located off Africa between Mozambique and the northern end of Madagascar.

Speaking to National Geographic, Columbia University seismologist Göran Ekström, a specialist in strange earthquakes, says that, “I don’t think I’ve seen anything like it.” Even so, he cautions. “It doesn’t mean that, in the end, the cause… is that exotic.” Even so, it’s got seismologists baffled.

What’s so weird about the mystery rumble?

While Mayotte has experienced hundreds of tremors since last May, the strongest, a 5.8 quake, occurred on May 15 and since then they’ve been tapering off in recent months. And there’s been no seismic activity that corresponds to the November wave. Still, the seismology community suspects it’s somehow related to the recent activity off Mayotte.

Normally, earthquakes produce “wave trains” comprised of high-frequency P (for “Primary”) waves that travel in pulses, as well as mid-frequency S (for “Secondary”) waves that wiggle side-to-side. Slow, low-frequency waves such as the mystery rumble are generally produced at the tail end of intense earthquakes, but again, there hasn’t been one anywhere in the right time frame that we know of.

Also, and just as “odd,” is that the wave is monochromatic. Most waves contain a cluster of waves at different speeds, or frequencies, that make for a fuzzy, complicated burst of a waveshape on monitoring equipment. The November wave was comprised of just a single frequency, and appeared as an unusually simple, clean zig-zag of about 17 seconds in length. Helen Robinson at the University of Glasgow, mischievously suggests to National Geographic, “They’re too nice; they’re too perfect to be nature.” It could be surrounding rock is filtering out other waves. Supporting this possibility is that, when the lowest frequencies are filtered out of the waveform, noise appears that could be faint P and S signals are seen. Independent seismologist Anthony Lomax tweeted the following image.

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