A new type of rare deep-Earth tremor, created by fast-developing ocean storms, has been detected by researchers from Japan. The signals from this kind of faint Earth tremor – known as an “S-wave microseism” – may provide geophysicists with a new tool to study not only oceanic storms but also the Earth’s interior.
First observed in the 1940s, microseisms are faint Earth tremors generated by the sloshing of ocean waves on the sea floor during storm events. The strongest microseisms are generated by the interaction of directly opposing wave systems. These create pressure excitations that travel almost unattenuated to the sea floor because of the nonlinear effects of the fluid – unlike with normal ocean surface waves, which decay with depth.
Seismic signals
Microseisms – which are observed globally – may either travel across the surface of the Earth, or through its interior as body waves. Like the seismic waves generated by earthquakes, these body waves may either be compressional (P-waves) or transverse (S-waves) – although until now, because of their larger amplitude, only the former had been observed – and, unlike surface-wave microseisms, can be tracked back to their point of origin.
In their study, Kiwamu Nishida of the University of Tokyo and Ryota Takagi of Tohoku University looked at a special kind of small, fast-developing extra-tropical cyclone colloquial dubbed a “weather bomb”. Seismic signals from one of these storm events – which developed in the North Atlantic between Iceland and Greenland in the December of 2014 – were recorded on the Japanese High Sensitivity Seismograph Network. The researchers found signals not only of P-wave but also S-wave microseisms, both in vertically (SV) and horizontally (SH) polarized forms. While modelling had only predicted the creation of SV waves, the researchers believe that the SH waves may be generated by the repeated reverberation of shear waves in poorly layered shallow sediments on the ocean floor.
The discovery of S-wave microseisms may offer a new way of understanding the nature of these couplings between the atmosphere and deep Earth. “The energy ratio between P- and S-wave microseisms is crucial for inferring the excitation mechanism,” says Nishida. For example, he explains, “excitation sources on the sea surface excite P-waves dominantly, whereas excitation sources on the seaf loor excite larger S-waves.”
Crustal analyses
At the same time, the shorter wavelengths of S-wave in comparison to P-wave microseisms make them more sensitive to temperature, pressure and resultant composition changes within the Earth – potentially allowing for more detailed analyses of crust and upper-mantle structures.
“The excitation of transverse surface waves – Love waves – had been established before, but because of the small amplitude of body waves compared to surface waves, the observations of Nishida and Takagi are a surprise,” says Roel Snieder, a geophysicist at the Colorado School of Mines, who was not involved in this study. “Since array techniques are in general quite robust, their detection of SH waves is convincing.”
With their initial study complete, the researchers are now looking to develop new methods to use body-wave microseisms to explore the nature of the Earth’s interior beneath oceanic storms. To this end, they are presently compiling a catalogue of storm events, similar to the weather bomb already examined, for use in such studies.
The research is described in the journal Science.
