A marsquake recorded by NASA’s InSight lander in 2019 appears to have triggered a concentrated series of boulder falls in Cerberus Fossae, providing new evidence that seismic activity can reshape the modern Martian surface. Researchers identified 27 fresh boulder-fall ejecta tracks in orbital images taken after the S0235b marsquake, with the tracks appearing close to the quake’s most probable source region.
The study, published in npj Space Exploration, combines high-resolution orbital imagery, InSight seismic measurements, statistical analysis and estimates of ground shaking. Taken together, the findings support a connection between the S0235b seismic sequence and the sudden movement of unstable boulders along the steep walls of Cerberus Fossae.
Fresh Boulder Tracks Appeared After the Marsquake
S0235b was detected on 25 July 2019 by the Seismic Experiment for Interior Structure, or SEIS, aboard NASA’s InSight lander. The event was a high-quality, low-frequency marsquake with an estimated moment magnitude of about 3.6. Previous analysis indicates a tectonic source compatible with the extensional faulting of Cerberus Fossae and an estimated focal depth of roughly 17 to 32 kilometres.
To investigate whether the quake produced visible surface changes, the researchers compared Mars Reconnaissance Orbiter HiRISE images acquired before and after the event. A June 2019 image provided the pre-quake view, while an image from December 2020 showed the same terrain after the seismic event.
The comparison revealed 27 previously absent boulder-fall tracks in the sector closest to the inferred S0235b source, which the researchers designated Zone 1. Together, the new tracks extend for approximately 2.75 kilometres.
The tracks occur at seven separate locations along the northern wall of the graben. Five locations contain individual tracks, while two contain groups of eight and 13 tracks. All seven locations fall within a roughly 4-kilometre section of the wall, and about 96% of the newly detected tracks are concentrated within an even narrower 2-kilometre stretch.
The surrounding terrain was already rich in boulders and older tracks, showing that rockfalls are not unique to the period following S0235b. The researchers mapped thousands of boulders in the Zone 1 area, including many larger than one metre. Such terrain provides a supply of blocks that could have been progressively weakened before seismic shaking finally caused some of them to move.
The Post-Quake Increase Was Far Above the Background Rate
The team also tested whether 27 new tracks could reasonably have formed through the region’s normal background level of boulder-fall activity.
Between 2012 and 2019, researchers identified 12 tracks forming within the same search area, equivalent to an average rate of about 1.7 tracks per year. If that rate had continued unchanged, only around 2.6 tracks would have been expected during the approximately 1.5-year interval covered by the post-S0235b observations.
Instead, 27 appeared.
Using a constant-rate Poisson model, the researchers calculated that the probability of observing 27 or more tracks during that interval under the earlier background rate was less than 10−4. The test therefore indicates a greater than tenfold increase in track formation following the marsquake.
The researchers caution that this statistical result is not a direct probability that every individual rockfall was caused by S0235b. Some tracks within dense clusters may not represent fully independent events, while image coverage and the preservation of tracks can also affect how many are detected. The statistical anomaly nevertheless strengthens the broader evidence when considered alongside the timing and geographic concentration of the new tracks.
Researchers Refined the Probable Marsquake Location
Determining the source of a marsquake is difficult because InSight effectively operated as a single seismic station rather than part of a planet-wide network. The team therefore treated the location of S0235b probabilistically rather than as a precisely determined epicentre.
Using 10,000 Monte Carlo samples incorporating uncertainties in back-azimuth and epicentral distance, followed by two-dimensional probability analysis, the researchers placed the highest-probability source region near 9.36° N, 162.42° E. Their resulting uncertainty region was about 30% smaller than earlier estimates.
Importantly, the Zone 1 boulder-fall cluster lies within the high-probability portion of that estimated source region. The closest mapped wall segment is approximately 19 kilometres from the inferred epicentre.
S0235b was also followed by two smaller seismic events, S0235c and S0235e, approximately 35 and 45 minutes later. Their estimated directions overlap with that of S0235b, suggesting that nearby slopes may have experienced repeated shaking during a localized seismic sequence rather than only a single disturbance.
Was the Shaking Strong Enough to Move Martian Boulders?
The researchers estimated the ground motion that could have reached the Zone 1 slopes by projecting the acceleration recorded at InSight back toward the inferred source region while accounting for geometric spreading and frequency-dependent attenuation.
The most probable peak ground acceleration at the boulder-fall sites was estimated at approximately 1.5 × 10−3 metres per second squared. The study gives a one-standard-deviation range of roughly 3 × 10−4 to 10−2 metres per second squared.
Those values are below many slope-failure thresholds derived from Earth or the Moon. However, the researchers argue that the shaking is physically compatible with disturbing weathered blocks that were already close to failure under Mars’ lower gravity.
This does not mean that the calculated acceleration represents a precise reconstruction of conditions at every rockfall site. Near-source seismic properties on Mars remain poorly constrained, and the attenuation calculations rely on simplified assumptions. The authors therefore describe the values as order-of-magnitude plausibility estimates rather than exact predictions of local ground motion.
Why Did the Boulder Falls Occur on Only One Wall?
One of the study’s notable findings is that all 27 new tracks occur on the northern wall of the graben. No equivalent set appeared on the opposing southern wall.
The difference cannot simply be attributed to a shortage of boulders or gentler terrain on the southern side. The southern wall actually contains more mapped boulders within the investigated segment and has a slightly higher average slope angle. HiRISE coverage and illumination were also sufficiently similar that the researchers found no clear observational bias capable of explaining the difference.
Instead, the response appears to depend strongly on local geological conditions. The northern wall is generally more planar to convex, while the southern wall is more concave. Differences in fractures, regolith, erosion, block attachment and slope structure could determine which boulders were close enough to instability to respond to shaking.
The result suggests that a marsquake alone may not determine where rockfalls happen. Long-term weathering and structural weakening may gradually prepare individual blocks for failure, with seismic shaking supplying the final disturbance.
Other Causes Cannot Be Completely Excluded
Cerberus Fossae is an active surface environment where wind, thermal cycling, dust movement, granular flows and gradual structural degradation can also alter slopes. The researchers therefore considered non-seismic explanations for the newly observed tracks.
These processes may contribute to weakening boulders over time, but the study argues that they do not readily explain the abrupt concentration of 27 new tracks within a short section of the northern wall during the post-S0235b observation interval. If ordinary weathering or wind were the dominant immediate cause, similarly concentrated activity might be expected across other comparably exposed sections of the graben.
The team also examined other seismic events associated with Cerberus Fossae during the relevant period. Several had lower magnitudes or substantially less certain locations. S0235b remained the strongest candidate because it combines relatively large magnitude, close spatial association and the correct temporal relationship with the newly detected tracks.
The evidence therefore supports seismic triggering, although the available orbital images do not show the precise moment at which each boulder moved. The study establishes an interval between the pre-event and post-event images rather than an exact rockfall date for every track.
Mars’ Surface Is Still Being Reshaped
The findings provide a clearer physical connection between marsquakes detected by InSight and coarse-scale changes visible from orbit. Earlier work had connected Martian seismic events with dust avalanches and other surface disturbances, but associating recorded marsquakes with the movement of metre-scale boulders has been more difficult.
The study also shows that the geological record visible in orbital images is incomplete. Boulder tracks can be created and later obscured by dust or additional mass wasting over relatively short periods, meaning the tracks visible today represent only part of recent activity in Cerberus Fossae.
Although the evidence does not allow every individual boulder fall to be assigned definitively to S0235b, the combination of timing, spatial clustering, the sharply elevated formation rate and physically plausible ground shaking makes the 2019 seismic sequence the most consistent explanation identified by the researchers. The result adds another indication that Mars is not a geologically static world: tectonic activity detected beneath its surface can still produce observable changes in the landscape above.