Ireland and Great Britain are two neighboring islands that should be equally boring from a seismological point of view. Both are not volcanic hotspots and lie thousands of kilometers from the nearest plate boundary. However, there are far fewer earthquakes on the Emerald Isle than in neighboring Britain. New research has shown that the thickness of the lithosphere may be responsible for the surprisingly small number of earthquakes in Ireland.
According to a paper published in the Geophysical Journal International, the lithosphere-the Earth’s outermost rocky shell, which includes the crust and the solid upper mantle-is thicker and colder under Ireland than under Britain. The cool, thick lithosphere is mechanically stronger than the warm, thin lithosphere, which may explain the surprisingly low number of earthquakes on the Emerald Isle.
About 90% of all earthquakes start at the boundaries between tectonic plates, but the plates or continental interior tend to be seismically quiet. The rarity of earthquakes within plates makes them difficult for scientists to understand. And it is often unclear why some places deep within continents have more earthquakes than others.
Such is the case in the British Isles, where the sharp seismic contrast between Britain and Ireland is a centuries-old mystery. The geological structure of the two islands shares a common history, both being assembled from pieces of an ancient continent called Laurentia and a scrap of crust called Avalonia during the Caledonian orogeny. However, Ireland is characterized by significantly fewer earthquakes than Great Britain.
Sergei Lebedev, a seismologist at the University of Cambridge and lead author of the new study, described the situation, quoting British Geological Survey seismologist Roger M. W. Musson: “The search to understand how the distribution of seismicity [in Britain and Ireland] is related to geological structure has been long and fruitless.
The key question is why there are so few earthquakes in Ireland. Errors are not the answer, Lebedev said, because many of Britain’s major faults intersect with Ireland as well. Neither is the type of crust present on the two islands. The blocks of crust that make up the islands extend from Britain to Ireland and have nothing to do with the distribution of seismicity.
It is interesting to note that there have been cases of missing earthquakes in Ireland. In 2017, there was a 2.6 magnitude earthquake in the Kerry area that was recorded at seismological stations but was not felt on the surface. However, in 2018, an even stronger magnitude 4.4 earthquake struck the same area, causing panic among local residents.
Although earthquakes in the Emerald Isle are rare, they are still possible and can be dangerous. Scientists continue to investigate the causes of low seismic activity in Ireland and hope that new data will help them solve the mystery.