Valles Marineris is the kind of feature that makes a planet look dramatic from orbit. It is about 3,900 kilometers long, up to 600 kilometers wide and more than 9 kilometers deep, and it cuts across Mars like a scar. It has long been read as a relic, a record of what Mars did billions of years ago. A new study suggests that reading leaves something out: the crust beneath the canyon is still moving.
The evidence is a single marsquake. NASA's InSight lander recorded it on Aug. 25, 2021, and it is catalogued as S0976a. In a paper published Oct. 15 in Nature Communications, researchers place the event in the deep crust under Valles Marineris. They report that it was caused by strike-slip faulting on faults that formed long ago and are now being reactivated by stresses acting on Mars today.
One quake from the far side
InSight's seismometer recorded many events during its mission, but S0976a stands out. Universe Today reports that it is the only marsquake detected in the planet's western hemisphere. The paper calls it a "farside" event, meaning it happened on the opposite side of Mars from the lander.
That location made it hard to study and also unusually useful. According to EarthSky, the quake's seismic waves traveled through Mars's core before reaching InSight. Its epicenter had earlier been placed only somewhere in the western Valles Marineris region, and its exact location and cause remained unclear until the new analysis.
The team was led by Jung-Hun Song of Imperial College London, working with researchers at Columbia University and the University of Illinois Urbana-Champaign. They placed the epicenter roughly 20 to 40 kilometers (12 to 25 miles) beneath Valles Marineris. Columbia and Imperial announced the result on Oct. 19, and Doyeon Kim, a senior author of the study, discussed the findings in EarthSky's coverage.
What kind of fault moved?
The location alone would be notable, but the study goes further and identifies the type of fault motion. The authors conclude that S0976a was a strike-slip event, in which the two sides of a fault slide past each other horizontally instead of one side rising or dropping relative to the other.
The source mechanism also tells researchers about the stresses that produced the quake. According to the paper's abstract, the stress orientation implied by S0976a fits a picture in which pre-existing faults are being reactivated, not new ones forming. The authors attribute today's stress field to three sources:
- Planetary cooling. Mars continues to lose heat, which stresses its crust over time.
- Tharsis loading. The enormous volcanic Tharsis region places a large load on the surrounding lithosphere.
- Erosional unloading. Surface erosion removes material and changes the local stresses in the crust below.
The faults themselves are old. The present-day stresses acting on them are what matter, and according to the study they are oriented to push those ancient faults to slip again.
A planet without plates, but not without faults
This result helps clarify how a planet like Mars deforms. Earth's surface is broken into moving plates, and most of its large earthquakes happen where those plates meet. Mars has no such system. As Universe Today notes, it is a single-plate "stagnant lid" planet, with an outer shell that is not divided into separately moving pieces.
A stagnant lid does not mean a quiet one, though. Without plate boundaries to absorb stress, strain can build up in the existing crust and be released along weaknesses that are already there. The study's central claim follows from that: inherited faults can remain favorably oriented for reactivation over billion-year timescales. A fault that formed early in Mars's history can stay positioned to slip again whenever the stresses of the day line up with it.
S0976a gives direct seismic evidence that such reactivation is happening at Valles Marineris now. It also adds to other regions where InSight detected evidence of active faulting, including the Cerberus Fossae fissures and the southern part of Elysium Planitia, according to Universe Today. Those findings suggest that stresses in the Martian crust do not follow a single planet-wide pattern.
Why It Matters
Planetary scientists have mostly reconstructed Mars's tectonic history from surface geology, by reading faults, scarps and canyon walls the way a geologist reads layered rock. That approach shows what happened but cannot say whether any of it is still going on. A marsquake with a known location and mechanism can. S0976a gives seismic evidence that the faults beneath Valles Marineris are active today, not just preserved.
The finding also links that activity to specific causes. The stress field the authors infer combines cooling, the weight of Tharsis and the removal of material by erosion. Each of these processes has been shaping Mars for a very long time, and the stress pattern recorded by this quake is consistent with all three still contributing.
The study also has clear limits. It rests on one event recorded by one seismometer, from the far side of the planet. That makes S0976a valuable precisely because it is unique: as the only western-hemisphere marsquake in InSight's record, it shows what a single well-analyzed event can reveal. InSight stopped operating in 2022 after dust built up on its solar panels, according to EarthSky, and no active lander is currently recording marsquakes, so the seismic record remains sparse.
For now, the conclusion is narrow but notable. Under the solar system's largest canyon, faults that formed in Mars's distant past are still slipping, and InSight recorded one of them doing so.
