A “weather bomb" causes ocean swells that trigger faint but detectable tremors into the oceanic crust. | Kiwamu Nishida and Ryota Takagi
An uncommon "weather bomb" in the North Atlantic Ocean produced seismic waves that can be used to "x-ray" the Earth's deep hidden structure, scientists conclude in a study published in the 25 August issue of Science.
The weather bomb, a powerful cyclone which formed between Greenland and Iceland in 2014, is a rare weather event. During such a cyclone, the stormy churning waters slosh against the sea floor, unleashing seismic energy called microseisms that radiate through the Earth's surface and interior.
By studying the microseismic waves created by the weather bomb — and possibly other strong storms like it — scientists can amass a collection of data that "may open a new and different perspective from which to explore the Earth's deep interior in the absence of seismic stations and earthquakes," write authors Kiwamu Nishida and Ryota Takagi, professors at the Earthquake Research Institute at the University of Tokyo.
Microseisms are faint but prevalent deep-Earth tremors, detectable as surface and body waves. Microseismic surface waves usually cannot be observed beyond the coast, but body waves, divided into two types called P- and S-waves, can penetrate through the deep Earth and be detected by distant seismic stations on land. (Nishida offers an interactive, kinetic explanation of P- and S-waves on his website.)
Fundamentally, P-waves are those that animals can feel before an earthquake, and S-waves are those that humans feel during earthquakes. While P-waves are dominant in microseismic activity, S-waves, further categorized into SV- and SH-waves, are rarely detected by seismic stations.
Nishida's study is one of the first to detect microseismic S-waves in addition to P-waves. To achieve this, Nishida and Tagaki traced the pathway and direction of the waves coming from the Atlantic "weather bomb" by using 202 wave detection stations, each with "Hi-net" arrays, operated by the National Research Institute for Earth Science and Disaster Prevention in Japan's Chugoku district.
"Hi-net" arrays are highly sensitive seismic observation networks normally used to monitor earthquakes, but they are also able to record microseismic waves. Hi-net stations draw information from seismometers placed on the bottom of deep boreholes to track the "noise" created by the movement of the waves through different layers of sediment and convert that noise into subtle electronic signals that can be charted as data.
Among the resonating body waves, "SH-wave signals are weaker, so the chance of their detection is less," said Peter Bromirski, a geophysical oceanographer at the Scripps Institution of Oceanography, and co-author of a related Perspective on the study.
Nishida said the researchers were excited to detect these enigmatic waves, which he attributes to a fine-tuned method and recently-developed instrument corrections.
Furthermore, "the recent observation of microseismic S-waves gives seismologists a new tool with which to study Earth's deeper structure," write Bromirski and his Perspective co-author Peter Gerstoft, also from Scripps.
This is especially evident considering microseismic S-waves have certain properties that help scientists visualize the Earth's interior. For example, as S-waves are more sensitive to liquids, charting their pathways through the Earth can help mark transition zones, or areas where solid crust transforms into liquid magma.
Transition zones are just some of the dark patches of the Earth's interior structure that microseismic waves can help illuminate — all thanks to one storm.
"This [study] demonstrates the connection of the solid Earth to the atmosphere and ocean climate system," said Bromirski. "New discoveries of any kind are always exciting, particularly when multiple fields of study are involved."