Water covers some 70% of Earth’s surface, and interactions of the ocean with solid ground and the atmosphere cause most of the planet’s seismic activity. The strongest of those vibrations are known as secondary microseisms and are formed from interactions between ocean waves. Most of the observed secondary microseisms are Rayleigh waves that oscillate in an up–down direction as they propagate across a surface. But some of the seismic energy takes the form of Love waves that oscillate horizontally.
Despite about a century of observations, the origin of those Love waves has puzzled seismologists. Now Lucia Gualtieri of Stanford University and her colleagues at Princeton University have uncovered their mysterious source. Using high-frequency numerical simulations, the researchers show that secondary-microseism Love waves form when seismic energy traveling through Earth’s subsurface scatters haphazardly because of three-dimensional changes in temperature, density, and rock and mineral composition.
When researchers first started modeling seismic activity in the 1970s with computers, the lowest minimum period that could be simulated was about 20 seconds. But Gualtieri and her colleagues used a state-of-the-art Earth model with higher-frequency resolution to simulate secondary microseisms, which have a 4- to 10-second period. They tuned their model with and without bathymetry and 3D heterogeneities, the features that could potentially generate secondary microseism Love waves.
The top panel of the figure below shows results of a simulation that incorporates realistic bathymetry and a simple, layered Earth structure, and secondary microseism Love waves are recorded in just a few seismic stations in Europe. The bottom panel, however, includes a 3D structure and shows a distribution of Love waves that agree more closely with the observations. The background color indicates the intensity of the pressure sources, computed using an ocean wave model. The polar histograms (pink bars) show the arrival direction of the Love waves, and the length of each bar represents the amount of energy estimated to arrive at each station.
Gualtieri and her colleagues found that 3D variations in Earth’s structure were necessary for the model to produce a realistic picture. The new findings may improve efforts to image the subsurface. Love waves are recorded in the horizontal component of seismic activity. But previous imaging studies either did not use the horizontal components or assumed they originated at the surface. (L. Gualtieri et al., Proc. Natl. Acad. Sci. USA 117, 29504, 2020.)
