Belinda Smith
Belinda Smith is a science and technology journalist in Melbourne, Australia.
Hurricanes don’t just wreak havoc on land and sea, they also shake the Earth to its core – literally. And now, a pair of seismologists detected a rare, faint, deep-Earth tremor in Japan which was evoked by a distant storm in the North Atlantic.
Kiwamu Nishida from the University of Tokyo and Tohoku University’s Ryota Takagi observed an “S wave microseism” for the first time – one, they say, originated from a special, fast-developing storm called a weather bomb. The work, published in Science, could give geologists a new tool to study the planet’s deep structure which we know little about.
When chunks of Earth’s crust shift, grind and slip, two types of wave course through the planet. Primary waves, or P waves, push and pull solid rock and fluids – including the hot, liquid layers of the Earth – as well as molecules in the air (which animals can sense). For this reason, P waves are often called compression waves.
Secondary waves, or S waves, travel much slower. Rather than compress matter, S waves propagate up and down, or side to side.
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They only move through solid rock, and it’s by monitoring S waves that seismologists have been able to deduce that the Earth’s outer core is liquid.
So while earthquakes have helped scientists effectively see through the Earth’s layers, it’s incredibly difficult to predict when and where the next quake will strike. Was there another way to generate these seismic waves – particularly S waves?
If anything, weather bombs – small but intense storms in which the central pressure drops rapidly over the course of 24 hours – were an excellent candidate.
The massive pressure drop generates exceptionally strong winds which whip up the ocean’s surface into gravity wave systems (not to be confused with gravitational waves, ripples in space-time thanks to cosmic cataclysmic crashes).
Energy from these gravity waves propagates to the sea floor and through the Earth in what are called microseisms.
Weather bomb-generated P wave microseisms have been studied since the 1940s as they’re fairly easily picked up by seismic stations. But S wave microseisms are an order of magnitude smaller, so are much harder to detect.
So Nishida and Takagi turned to the high-sensitivity seismograph network, or Hi-net – some 202 sensors embedded 100 metres below the surface in Japan’s Chukugu district.
The entire network of around 600 stations was implemented after the 1995 Hyogoken-nanbu earthquake to monitor the country’s fault system. But it also picks up rumblings from other parts of the world.
And when they monitored a weather bomb which formed between Greenland and Iceland on 9-11 December 2014, they saw the telltale signature of P waves – followed by the much fainter S waves.
The pair traced the timing and direction of the waves to their origin in the North Atlantic.
Using similar detectors to listen for thumps from future weather bombs, write Peter Gerstoft and Peter Bromirski from the University of California, San Diego, can give seismologists valuable information of the Earth’s crust between the source and detectors, and “add to our understanding of the deeper crust and upper mantle structure”.
