Valles Marineris Is Still Cracking: A Marsquake Traced to a Fault Under the Great Canyon

The largest canyon system in the solar system is still active, according to scientists at Columbia University and Imperial College London. New research presents direct seismic evidence that a marsquake detected by NASA's InSight lander came from a fault deep beneath Valles Marineris, billions of years after the canyon first formed. The study was published in Nature Communications.

Valles Marineris stretches nearly 3,900 kilometers (2,400 miles) across Mars, reaches up to 600 kilometers (370 miles) wide and is more than 9 kilometers (5.6 miles) deep. Compared with the Grand Canyon at their maximum dimensions, it is nearly nine times longer, more than 20 times wider and about five times deeper.

The team studied a seismic event labeled S0976a, which InSight recorded on Aug. 25, 2021. It was unusual: the signal came from the opposite side of the planet from the lander, and it was the only marsquake of the mission that could be located in Mars's western hemisphere. Earlier work placed it near western Valles Marineris, but its cause was unclear. A seismic signal on Mars does not necessarily mean a fault has broken, because meteoroid impacts, landslides and other processes also shake the ground.

To find the cause, the researchers analyzed how different seismic waves from the event arrived at InSight, using their amplitudes and directions to reconstruct the motion at the source. The best explanation was a strike-slip marsquake, in which blocks of crust slide horizontally past each other along a steep, deeply rooted fault. The rupture happened roughly 20 to 40 kilometers (12 to 25 miles) below the surface.

The team tested alternatives, including meteoroid impacts and landslides, and none could reproduce the observed waves. They also checked satellite images of the likely source region taken after the event and found no fresh crater or large new landslide. "What makes the result compelling is that several completely different types of evidence point in the same direction," said Jung-Hun Song, the lead author from Imperial's Department of Earth Science and Engineering. "The seismic waves tell us that there was a sideways sliding motion on a fault well below the surface, while the geology seen from orbit shows a region crossed by long-lived fault systems."

Mars has no moving tectonic plates, so its outer shell behaves more like a single, slowly cooling lid. Folarin Kolawole, a geologist at Columbia's Lamont-Doherty Earth Observatory and a co-author, said the result shows that on a planet without plate tectonics, structures created billions of years ago can still control where the crust breaks. The orientation of the faulting matches what would be expected if old structures were slipping again under stresses from the planet's long-term cooling, the nearby Tharsis volcanic region and local effects around the canyon.

The finding adds Valles Marineris to a small set of regions where InSight data have revealed active faulting, alongside the Cerberus Fossae fissures and the southern Elysium Planitia plain. Different styles of faulting in each place suggest stresses in the Martian crust vary from region to region instead of following one planet-wide pattern.

"Our study opens up exciting possibilities for discovering ongoing seismic activity on other planetary bodies that we might otherwise consider geologically inactive," Song said. Senior author Doyeon Kim, an assistant professor at Imperial, noted that the waves from S0976a also traveled through the planet's core. InSight's mission ended in 2022, but its recordings are still yielding discoveries.