We are constantly learning more about earthquake stimuli, but there is also much more to learn about how these seismic shifts work. Now geologists think they have identified the key mechanism behind some of the largest earthquakes on the planet.
Megatrust earthquakes occur in subduction zones, where one tectonic plate is pushed beneath another. They are especially common around the Pacific and Indian Oceans, and can lead to giant tsunamis.
New research suggests that gradual, slow sliding deep below the subduction zone could be key to understanding how megastrust earthquakes trigger and could potentially improve forecasting models to better predict them in the future.
These slow-sliding (SSE) events do not occur in every subduction zone, but they can affect how the pressure increases underground, the researchers say. It is crucial that they move energy in different directions to megatrust earthquakes and do not necessarily follow the movement of the plates themselves.
“Usually, when an earthquake happens, we find that the movement is in the opposite direction to the movement of the plates, accumulating that sliding deficit,” says geoscientist Kevin Furlong of Pennsylvania State University.
“For these slow-moving earthquakes, the direction of motion is directly downward in the direction of gravity, instead of in the directions of motion of the plate.”
Using high-resolution GPS station data, Furlong and colleagues analyzed movement along the Cascadia subduction zone (which stretches from Vancouver Island in Canada to northern California) over several years.
An earthquake measuring 9 on the Richter scale occurred in Cascadia in 1700 and since then SSEs have occurred far below the subduction zone, moving short distances slowly. They are like a “swarm of events” as the researchers say, and the pattern matches a similar record of data from New Zealand.
Although collectors occur many kilometers below the surface, their movement can affect the timing and behavior of megatrust earthquakes, the team suggests. These smaller events happen every year or two, but they can trigger something much more serious.
“There are subduction zones that don’t have these slow slides, so we don’t have direct measurements of how the deeper part of the subductive plate moves,” says Furlong.
SSEs were first discovered by geologists about 20 years ago, and only recently have GPS instruments been sensitive enough to record their movements in detail – in this case 35 kilometers underground.
The findings of the new study, which the researchers described as “quite unexpected”, will help inform future earthquake models. It is possible, for example, that the collectors deep underground release some stress from the movement of the plate in the subduction zones.
Moreover, for planning, it is crucial for them to know the direction of the forces that will be released by future earthquakes. These natural disasters can be very unpredictable, so any information that can be gathered ahead of time is invaluable.
“Even more thoroughly, we don’t know what caused the big earthquake in this situation,” says geoscientist Kirsty McKenzie of Pennsylvania State University. “Every time we add new data on the physics of a problem, it becomes an important component.”
The research was published in Geochemistry, Geophysics, Geosystems.