Weather bombs could help us see deep inside Earth | New Scientist
Severe storm clouds have an unexpected silver lining: they may help us visualise parts of our planet’s interior that are otherwise hidden.
Much of what we know about Earth’s interior comes from studying how seismic waves created by earthquakes travel through it. These “body waves” come in two basic forms, called P-waves and S-waves.
Such waves sweep through Earth in great arcs before returning to the surface, where geologists can detect and measure them. When they meet significant boundaries in Earth’s chemical or physical structure – at the boundary between the core and mantle, for instance – they are reflected, refracted, or even stopped in their tracks.
This means that studying the distribution of P-waves and S-waves after an earthquake helps illuminate our planet’s internal structure.
There’s just one problem with the approach. The image resolution is lower in regions where earthquakes are rare, far away from the margins of tectonic plates, says Kiwamu Nishida at the University of Tokyo, Japan.
This is where severe oceanic storms can help, he says. Atmospheric pressure can drop rapidly during these events, generating oceanic waves so strong that a small fraction of their energy makes it all the way down to the sea floor and generates faint P-waves and S-waves in the rocks – as if a very weak earthquake has occurred.
Geologists have previously detected P-waves generated in this way. Now, for the first time, Nishida and his colleague Ryota Takagi at Tohoku University, Japan, have detected the far fainter S-waves too.
The pair focused on a particularly severe storm known as a “weather bomb” that occurred in the North Atlantic near Greenland in December 2014. Their network of seismometers in Japan detected the P-waves from the event, as expected – and also an S-wave signal.
The researchers were able to pick up this signal because their detection network contained a large number of seismometers – 200 – in a relatively small area of Japan’s Chugoku region. “The important factor for the detection is station density,” says Nishida.
Recording both P-waves and S-waves generated by the weather bomb offers the chance to create a high-resolution image of Earth’s structure immediately below the Atlantic storm system. This should then help Nishida and Takagi calculate the precise depth of the boundary layer between the upper and lower mantle in the region.
It’s possible that this sort of work might ultimately help reveal undulations in such important boundary layers, which could have an effect on our understanding of convection in the mantle and the movement of tectonic plates – although Nishida stresses this is just speculation at the moment.
Peter Gerstoft at the University of California, San Diego, says generating images using the weak seismic waves from storms should be possible. “Having both P-waves and S-waves gives more information,” he says. “Because S-waves have shorter wavelengths than P-waves, smaller-scale vertical and lateral variations in Earth’s structure can potentially be imaged.”
Strong weather-bomb-style storms are no more common than earthquakes, says Nishida – but unlike quakes, they have the potential to occur almost anywhere in the ocean. This means they could provide a complementary source for imaging Earth’s interior, he says.
Countries including Japan and the US are beginning to install and use the dense seismometer networks required to detect weak waves, such as those generated by these storms, he adds.
Gerstoft points out that researchers are already finding ways to use ambient seismic noise – for example, the vibrations in Earth generated by ocean waves or even heavy cars – to construct better images of Earth’s structure. “Geophysicists are always looking for more data,” he says.