Low Mag Earthquake Damage a Mystery
Walls began trembling, dinner plates and glasses crashed to the ground and a booming sound reverberated throughout the small French town of Le Teil.
A moderate temblor clocking in at a magnitude of 4.8 damaged numerous buildings and injured four people on November 11, 2019, mystifying citizens and scientists alike.
For one, while France is no stranger to temblors, they are often quite small, explained seismologist Jean-Paul Ampuero of the Université Côte d’Azur in France.
Monday’s event was only of moderate intensity by global measures, but it was a “very large one for French standards,” he says.
Even more surprising the temblor cut clean to the surface, cracking Earth’s crust like an eggshell.
Such breaks are common for hefty earthquakes, such as the 7.2 magnitude Landers earthquake that struck California in 1992.
This outcome left researchers scratching their heads prompting a hunt for the quake’s source.
By studying the region’s past and current geodynamics, scientists hope to find clues as to what sparked this event, why it unfolded so unexpectedly, and what it can reveal broadly about the mechanics of our quaking planet.
France has no San Andreas Fault so while large earthquakes are possible here; mega-temblors like those in California are rare.
The country sits on the Eurasian tectonic plate, which borders the African plate to the south, but the boundary between the two is complex, and it includes a number of smaller plate fragments known as microplates.
Therefore, the varying movements when these blocks of Earth collide, squishes France from multiple directions.
What’s more, moderate ruptures in any location do not usually fracture the earth’s surface.
Scientists studied radar data from satellites active during the quake where they identified minute movements of the ground’s surface.
The result, rainbow colored maps that showed buckling in the landscape caused by the quake.
A clear signal emerged, revealing signs of not only land deformation, but also a surface rupture.
Part of the reason was the quake’s very shallow depth, suggesting this temblor unzipped the earth’s crust at a mere mile or so underground.
For more clues, researchers headed to the field on November 13, using the satellite analyses to help pinpoint the crack as it jutted across roads.
Now, they’re working to study the site in many ways, including installing seismometers to track the fault’s activity and recording any additional signs of deformation.
Other scientists are now studying older satellite images of the region to find any past deformation that might offer more clues to the temblor’s source.
One intriguing possibility has already emerged; the mystery quake might be due to quarry activity nearby, Robin Lacassin of the Institute de Physique du Globe de Paris notes on Twitter.
The fault plane likely extends below the quarry, so the significant removal of rock could have reduced the normal tectonic stress in this area and forced the surface to readjust.
Ampuero and his colleagues are investigating this possibility.
