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Several faults near the epicenter of Haiti’s magnitude 7.2 earthquake continued slipping for weeks after the main rupture, even though strong shaking had ended.
That prolonged motion affects how scientists estimate where stress lingers after earthquakes, with direct implications for hazard assessment in damaged regions.
A detailed analysis of the 2021 Haiti earthquake shows that multiple nearby faults remained active well beyond the main rupture, producing measurable slip long after the initial ground motion subsided.
Large earthquakes can tear a main fault and also crack nearby rocks, creating new paths for motion.
The Haiti secondary-fault analysis was led by Bryan Raimbault, Ph.D., at École Normale Supérieure (ENS) in Paris.
His research focuses on how earthquakes and slow slip reshape stress across fault zones, helping teams interpret hazards after big events.
Small faults keep slipping
Small fractures near the rupture can become secondary faults, smaller breaks near a main rupture, and they may slide quietly afterward.
Ground-motion records from GPS stations let researchers spot this motion, because the land keeps moving by fractions of an inch.
Aftershock patterns do not always match this motion, so scientists need other clues when tracing active breaks.
Satellite and GPS measurements can detect motion down to hundredths of an inch, capturing geodesy, measuring Earth’s surface changes over time.
Researchers match these signals to fault traces on the ground, then estimate how much sliding continues after a quake.
Clear patterns in that lingering motion can reveal which breaks stay locked and which ones slip with little push.
Fault zone under Haiti
Southern Haiti lies along the Enriquillo-Plantain Garden fault zone, a plate boundary, the line where large plates push together.
Over centuries, plate motion loads this region with sideways strain, so many faults share the job of slipping.
That complexity makes it hard to know which breaks will move in the next event, especially near cities.
Stress and friction control slipping
When a main fault breaks, shear stress, a sideways force that encourages sliding, changes across nearby faults in seconds.
Even small nudges in that stress can speed up or slow down failure, because friction holds faults near their limit.
The same push can also drive gentle sliding without a quake, so nearby movement does not always mean aftershocks.
Fault motion after shaking stops depends on friction, roughness, and pressure along the rock surface.
Damage from the main rupture can crush rock and open cracks, letting fluids raise pressure and lower friction.
Weak patches can then slip under tiny stress changes, which challenges the idea that shallow faults always stay locked.
When faults slip backwards
Some nearby faults in Haiti moved opposite the direction expected from long-term tectonic loading, slow stress buildup from moving plates.
Stress changes from the main rupture can briefly flip the balance, so sliding follows the new forces instead.
That odd direction does not rewrite plate motion, but it does show how complex the stress field becomes after damage.
High-resolution maps in the analysis showed secondary faults slipping under small stress changes, meaning very little force was needed.
“We model this time-dependent behavior as frictional slip in response to shear stress changes imparted by the earthquake,” wrote Dr. Raimbault.
That simple friction model links the postquake motion to stress change, but it cannot pinpoint every hidden fault trace.
Many activated breaks lay in the hanging wall, the rock above the main rupture surface, rather than on the main fault trace.
Rocks there can crack as the block lifts and twists, so a web of small faults becomes ready to slide.
Ignoring that zone can leave blind spots in hazard maps, because surface clues may be subtle between big earthquakes.
Between major earthquakes, weak secondary faults may take up some plate-driven strain in the long run.
That slow creep spreads the work across more faults, because each small slip releases a bit of built-up stress.
Less stress may then pile up near the surface, yet deeper parts of the system could still lock and break later.
Where hazards may hide
Small fault motion can warp roads, pipelines, and canals, even when people no longer feel shaking on the surface.
Engineers often plan for strong, fast shaking, but slow ground movement can also strain structures as joints and foundations creep.
Recognizing these quiet movements could change postquake inspections, especially in places where repair crews must prioritize limited resources.
Limits of the evidence
Measurements still miss some motion, because instruments are spaced far apart and clouds can block some satellite views.
Modeling also relies on assumptions about friction and rock strength, so different choices can change the estimated weakness.
Those limits mean scientists can rank which areas look more mobile, but they cannot forecast the next rupture on its own.
Lessons from fault slips
Denser GPS networks and more frequent satellite passes could catch small fault motion sooner, especially right after a main quake.
Field teams can then check fresh cracks and offsets, linking the remote data to real breaks in the landscape.
Better maps could also guide land-use decisions, although they still cannot remove risk in heavily settled coastal regions.
Taken together, the analysis shows that postquake stress can drive quiet slip on many small breaks.
Future hazard planning will need better monitoring and realistic friction models, while accepting that some faults will stay unmapped.
The study is published in Geophysical Research Letters.
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