Monday January 8, 2018 Q&A

According to a recent study, the number of earthquakes in 2018 may increase owing to:

Réponses

1 the slowing of the earth’s rotation

2 a shift in the geographic North Pole

3 the drifting of the Eurasian tectonic plate

Correct answer: 1, the slowing of the earth’s rotation.

We still cannot predict when or where earthquakes will take place or how strong they will be. But two US geophysicists recently published a study in Geophysical Research Letters that purports to identify which years will experience greater earthquake activity planet-wide. According to their model, 2018 may well be one of them.

The researchers analyzed a hundred years of data and identified a correlation between how fast the earth is rotating and the total number of earthquakes. They show that the speed at which the planet spins is now gradually slowing down as part of a regular 30-year cycle, and that a higher number of strong earthquakes are systematically recorded as soon as five or six years after a cycle begins. The current cycle is thought to have begun in 2011, which means conditions are now ripe.

The causal link between the earth’s rotation and the number of earthquakes has to do with the fact that the planet is not a rigid orb. The earth’s surface is made up of some 15 main tectonic plates and around 50 secondary ones, all of which shift among themselves, together with large quantities of fluids – oceans, seas, rivers and atmospheric gases. This gives the asthenosphere, the earth's upper mantle, viscoelasticity. As a result, even a small change in the earth’s rotational kinetic energy will upset the balance between these solid and fluid substances and the distribution of underground energy.

A promising topic of study

Many researchers cautiously believe this study is worth developing further. According to François Passelègue, a seismologist at EPFL’s Laboratory of Experimental Rock Mechanics (LEMR): “A correlation between the earth’s rotation and earthquakes has already been established – but the other way around: the energy released by a major earthquake can cause the earth to spin slightly faster.” Marie Violay, who runs the LEMR, concurs, adding that “Small disturbances in the states of stress in rocks are sometimes all it takes to trigger an earthquake. A correlation with the tides has also been established.” Slowing rotation can also play a role through its impact on these states of stress.

In the EPFL researchers’ view, at this point it is impossible to say for sure whether there will be more earthquakes in 2018. The research first needs to be taken much further and tested against additional data.

They both emphasize that earthquakes are extremely complex. “This field of study overlaps many others, such as geology, physics, statistics, and fracture and frictional mechanics,” says Violay. Each fault and every situation is different and brings with it a huge dataset – the size of the tectonic plates, the type of friction between them, the various materials involved, etc. What’s more, some important information – such as the condition of rocks below the surface – remain unknown. “The deepest hole ever drilled goes down only 8km,” says Passelègue. “What will we find 20km down?”

Given this state of affairs, it is extremely difficult to make predictions. “Even under controlled laboratory conditions, we cannot say with any certitude when an earthquake will take place,” says Passelègue. “We can already detect and predict certain aspects, but it is very difficult to know what the consequences will be.”

Low risk in Switzerland

Regardless of whether 2018 experiences a large number of earthquakes, Switzerland is not at any high risk. Even if a significant event cannot be completely ruled out, only around 20 out of every 1,000 earthquakes recorded in the country exceed a magnitude of 2.5 on the Richter scale.

 

Article in Geophysical Research Letters: http://onlinelibrary.wiley.com/doi/10.1002/2017GL074934/abstract

Conference at the annual meeting of The Geological Society of America, Seattle, Washington, USA, October 2017:

https://gsa.confex.com/gsa/2017AM/webprogram/Paper300667.html