Ancient Mars Faults Still Rupture Today, Seismic Data Reveal

Mars has long been dismissed as a geologically dead world, a frozen relic whose tectonic engines sputtered out billions of years ago. A new study published in Nature Communications now delivers the strongest seismic evidence yet that this picture is wrong. By carefully decoding the faint ripples recorded by NASA’s InSight lander from a marsquake on the opposite side of the planet, researchers have shown that the colossal canyon system of Valles Marineris is still tectonically active, with ancient faults slipping under stresses that persist to the present day. The finding transforms our understanding of how rocky planets without plate tectonics release strain over billions of years.

The event in question, catalogued as S0976a, was recorded on Sol 976 of the InSight mission, corresponding to 25 August 2021. It is the only marsquake ever detected in the Martian western hemisphere, originating roughly 146 degrees away from the lander near Ius Chasma in western Valles Marineris. Because InSight operated a single seismic station rather than a network, determining where and how the quake occurred demanded unusual ingenuity. The team, led by Jung-Hun Song of Imperial College London, exploited the fact that seismic waves from such a distant source bounce off the surface before reaching the station, producing reflected phases known as PP and SS that carry information about the quake’s location, depth and faulting style.

By jointly inverting the polarities and relative amplitudes of these reflected phases, the researchers recovered a focal mechanism, the seismic fingerprint that reveals the orientation and slip direction of the rupturing fault. The result was unambiguous: S0976a was a strike-slip event, the kind of sideways shearing motion familiar from California’s San Andreas Fault, occurring on steeply dipping fault planes within the middle to lower crust. The centroid depth lies between roughly 20 and 40 kilometres, well above the estimated Moho boundary of 55 to 65 kilometres beneath western Valles Marineris. Across five different models of the Martian interior, the preferred depths remained within this range, and more than 82 percent of all acceptable solutions belonged to the strike-slip family, with rakes clustering near pure lateral slip.

Ruling out alternatives was as important as identifying the mechanism. Meteoroid impacts on Mars produce distinctive high-frequency seismic signatures, but S0976a is a low-frequency event, and force-based models designed to mimic landslides or other mass-wasting processes could not reproduce the observed phase polarities, arrival times or the strong horizontal ground motion. The team also scoured post-event images from the Context Camera on the Mars Reconnaissance Orbiter, covering roughly half of the candidate source region, and found no fresh landslide deposits or new impact craters. An energy balance calculation showed that a landslide large enough to generate a magnitude 3.6 event would need to move at least ten million cubic metres of rock, leaving a collapse scar several hundred metres to a kilometre across that would be plainly visible in orbital imagery. Nothing of the sort exists.

With a tectonic origin established, the structural geology of the source region came into focus. Digitising normal faults, grabens and wrinkle ridges from high-resolution orbital images across western Valles Marineris revealed a bimodal population of grabens with dominant strikes of about 15 and 115 degrees, defining an orthogonal fault network. Near the inferred epicentre, on the plateaus between Ius, Tithonium and Candor Chasmata, these orthogonal sets intersect in dense, well-developed patterns. Critically, the compressional axis inferred from the marsquake runs subparallel to the canyon system and to the dominant WNW-ESE fault set, while the tensional axis lies roughly perpendicular to it, aligning with the secondary NNE-SSW structures. The seismic source therefore appears to sit within a structurally complex transfer zone where inherited fabrics are favourably oriented for renewed shear.

The stress geometry carries a deeper message about Martian history. Kinematic analyses of the canyon’s Hesperian-era development, more than three billion years ago, suggest a maximum compressive axis oriented roughly east-west, between 090 and 100 degrees. The present-day compressive axis inferred from S0976a sits at about 95 degrees, an almost perfect match. In other words, the stress field that shaped Valles Marineris during its violent youth appears to have survived essentially intact, and the faults it created remain mechanically favourable for reactivation today. This directly contradicts models invoking gravitational collapse of overthickened crust, which predict dominantly vertical loading and axisymmetric extension, and instead supports a transtensional origin for the canyon system involving significant oblique slip.

What keeps these stresses alive on a planet that has lost most of its internal heat? The study points to a combination of forces. Long-term planetary cooling imposes a background horizontal compression as the interior shrinks. The immense volcanic load of the Tharsis rise, home to the largest volcanoes in the solar system, flexes the lithosphere and modifies the orientation and magnitude of horizontal stresses, with recent gravity-topography models suggesting a deep buoyant anomaly contributes about a fifth of the present Tharsis uplift. Meanwhile, the deep incision of the canyon itself acts as an erosional unloading that reduces vertical stress and promotes horizontal compression. At depths of 20 to 40 kilometres, the basaltic crust experiences confining pressures of several hundred megapascals, conditions that suppress tensile opening and favour brittle shear failure, exactly the strike-slip regime observed.

The absence of aftershocks, initially puzzling, turned out to be an expected consequence of Martian geometry. Simulations of aftershock sequences, generating a thousand synthetic catalogues conditioned on S0976a as the mainshock, showed that ground displacements from events in western Valles Marineris would rarely exceed the ambient noise level at the InSight landing site. Core shadowing, in which the planet’s core blocks direct seismic paths, and attenuation by the deep interior conspire to make distant source regions nearly invisible. The probability of detecting even a single sizeable aftershock within 50 days was calculated at just 0.1 percent. The quiet record after S0976a therefore says little about the true level of activity in the canyon and much about the limits of single-station seismology on another planet.

Placed alongside earlier InSight results, the new findings sketch a planet whose deformation is strikingly partitioned. Normal faulting dominates beneath Cerberus Fossae, compressional faulting occurred near the crustal dichotomy in southern Elysium Planitia during the mission’s largest recorded quake, and now strike-slip faulting has been documented beneath Valles Marineris. Three tectonically distinct regions, three contrasting failure styles, all within a lithosphere that lacks plate boundaries. This partitioning implies that broad stresses act on a mechanically heterogeneous shell, where long-lived structural and rheological variations dictate where strain is released. Limited viscous relaxation allows these stress heterogeneities to persist, so that ancient weaknesses keep localising deformation rather than new faults forming from scratch.

The broader implication extends beyond Mars. Plate tectonics appears to be rare among rocky worlds, with most terrestrial planets and large moons inferred to operate in a stagnant-lid regime, where the exterior shell does not recycle into the interior. The Valles Marineris results suggest that in such worlds, seismicity preserves a mechanical imprint of ancient tectonics: slow fault healing and slowly evolving stress fields jointly control where activity continues long after the structures themselves were first established. A fault system born during the Tharsis-building upheavals of the Hesperian can still rupture today, four billion years later, releasing strain in modest, MPa-scale stress drops on inherited fault patches. For planetary scientists, the message is that a planet’s tectonic past is never truly past; it sleeps in the crust, waiting for the right stress to wake it.

Subject of Research: Seismic evidence of present-day strike-slip fault reactivation on inherited crustal structures in Valles Marineris, Mars

Article Title: Active crustal straining and fault reactivation at Valles Marineris

Article References: Song, J.-H., Kim, D., Kolawole, F., Davis, J. M., Parry, R., & Maguire, R. (2026). Active crustal straining and fault reactivation at Valles Marineris. Nature Communications, 17(1), Article 10530. https://doi.org/10.1038/s41467-026-78193-6

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78193-6

Keywords: Mars, Valles Marineris, marsquake, InSight, seismology, fault reactivation, strike-slip faulting, tectonics, stagnant-lid planets, Tharsis, crustal deformation, Nature Communications