What Are "Weather Bombs," And Why Can They Help Us Peek Inside Our Planet?
Weather bombs. They are earthly echoes of intense storms, and by listening to them, scientists just detected a rare type of seismic rumbling inside the Earth for the first time.
These rare, deep-Earth tremors are called S-wave microseisms. Now the discoverers, a duo of Japanese seismologists led by Kiwamu Nishida at the University of Tokyo, are hoping to use these S-waves to probe into the dark unknown of our Earth's core. The earthquake researchers have outlined their discovery today in the journal Science.
These S-waves could help us X-ray our own planet.
This S-wave discovery "gives seismologists a new tool with which to study Earth's deeper structure," write Peter Gerstoft and Peter Bromirski, two independent earthquake researchers at University of California, San Diego, in an essay accompanying the paper.
Trembling Earth
Although we may go a lifetime without ever noticing it, the earth below our feet is rumbling in myriad different ways. Even the most obvious movements, earthquakes, are in fact complex combinations of different types and frequencies of jiggling earth. So, to understand where S-waves fit in, let's quickly dissect a quake.
When our planet's tectonic plates slip and smash to cause an earthquake, most of the destruction is caused by a class of seismic waves called surface waves. These are the various undulations of the Earth's crust itself, the way dirt, concrete, and rock roll like an ocean wave across the surface. But even before these dangerous surface waves hit, quakes also send out a smaller, faster vanguards.
These are called body waves, and instead of slowly crashing across the surface, they move directly through the Earth's core in more or less straight lines. These body waves are much fainter than surface waves, and usually go undetected by humans. Fun fact, though: it's these smaller-frequency pulses that alert animals like dogs and horses to earthquakes seconds before they hit.
Body waves come in two flavors. The first are a bit like sound-waves, which stretch and squinch the Earth as they move forward. These are called P-waves, for preliminary wave, because they're usually the first to arrive. But the second type are our S-waves. The name stands for secondary waves, and they undulate up and down and side to side a lot like ocean waves as they move through the earth. S-waves are so hard to detect because they're often so faint (for earthquakes at least) that they're drowned out by larger rumbles, specifically P waves.
But they could be incredibly important.
Planetary Fingerprint
S-waves contain unique fingerprints of the material they travel through. Because they travel through the Earth into the unexplored depths of our planet, these S-waves could help us X-ray our own planet, in a sense, creating more detailed maps of the Earth's core.
The key is finding more of them. Nishida and his colleague didn't detect these waves by listening in on the tectonic crashes that cause earthquakes. Rather, they listened into the faint rumbles caused by storms known as weather-bombs. These are intense storms that are born rapidly in the open ocean. The seismologists found these storms to cause thudding waves that thump against the ocean's floor. S-waves caused by that drum-like beating could be detected with the right calibrations and equipment.
Now that they've found these S-waves, the Japanese researchers are hoping they can start to create a large catalog of them. By comparing all these S-waves together, "such a catalog may open a different perspective from which to explore Earth's deep interior beneath a storm," and tease out new information about the makeup of our planet, writes Nishida.
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