The origin of seismic Love waves induced by ocean waves – one of the four types of energy waves that are also produced by earthquakes – has been a mystery since they were discovered in 1911. That mystery has been solved by scientists at the Dublin Institute of Advanced Studies (DIAS) in findings reported in Nature Communications.
Seismology is the science of studying earthquakes. The field has a long history in Ireland going back to Robert Mallet, the Dubliner who, in 1849, proved that energy waves arising from rock movements were what triggered earthquakes. This revelation, demonstrated by an experiment on Killiney beach, saw the birth of seismology.
During an earthquake, the energy that is released after rock cracks, breaks, and shifts underground, and then travels through the Earth is called p waves and s waves. The p waves arrive first after an earthquake, followed by the s waves.
Surface waves, which only travel along the surface of the Earth, arrive after the p and s waves, causing more destruction. There are two types; Rayleigh waves and Love waves, named after the scientists who discovered them, Lord William Rayleigh and Augustus Love.
This new study looked into how the same Love waves can also originate from ocean waves. Being muchless energetic than the ones linked to earthquakes, they do however contribute continuously to the seismic background noise recorded on seismograms.
"If we monitor carefully all types of earth vibrations, even those that were considered mainly to be 'noise' until recently, we can use this information to better monitor the Earth, including large geological faults and volcanoes," explains Dr Florian Le Pape, lead author of the research conducted with Prof Chris Bean and Dr David Craig, all of DIAS and the Irish Centre for Research in Applied Geosciences (iCRAG).
“The study of seismic noise has multiple applications through high-resolution imaging of what lies beneath us, and through the continuous monitoring of large geological faults and volcanoes,” says Le Pape.
Most seismic 'noise', says Le Pape, is generated by oceans, with about 50 per cent of it recorded at seismic stations on land, in the form of Raleigh waves. "Our research sought to better understand the rest of the seismic noise recorded on land as Love waves and its origins," Le Pape explains.
The researchers solved a more than 100-year-old mystery when they found – using data from seismic monitoring stations, seismic computer simulations and ocean wave models – that Love waves were being produced along the subsea slopes of the deep ocean, notably around sedimentary basins.
Le Pape says this research, as well as solving a scientific mystery can be useful in understanding the link between ocean storms and continuous background earth vibrations.
“Ireland is directly exposed to storms and wave activity in the North Atlantic so it is in the front line in the study of those vibrations in detail and what they can tell us about how the Earth, the ocean and the atmosphere are linked,” Le Pape adds.
Love and Rayleigh waves generated at sea from ocean waves and recorded on land can, for example, provide scientists with information about ocean wave heights. “I’m a surfer, and when I’m surfing and waiting for a set in the water, I cannot help now but think about how every ocean wave set and the storm at their origin has been shaking the Earth,” he points out.
“The origin of ocean-generated Love waves has been unknown, though people have looked for a century,” Bean notes. “Our research suggests that Love waves originating in the deep ocean can be used to better understand our changing oceans, and how the pattern of storms is changing with time.
“The next time you’re by the coast and looking way out into the deep ocean,” he suggests, “think how information about the changing ocean and its seabed is zipping right past you, beneath your feet; as very subtle ground vibrations.”