Seismic Swarm PS20210814.1: Analysis of the 2021 Nippes Sequence in Haiti

Haiti occupies a tectonically active zone at the boundary between the Caribbean and North American plates. The island’s southern peninsula is traversed by the Enriquillo-Plantain Garden fault system, a left-lateral strike-slip structure responsible for repeated destructive earthquakes. Regional geology features folded sedimentary rocks, uplifted coral terraces, and active fault segments that accommodate oblique convergence and shear. Historical records document major events in 1751, 1770, and 2010, confirming the fault system’s capacity for magnitude-7+ ruptures.

Seismic swarm PS20210814.1 began at 12:29 UTC on 14 August 2021, centered 4 km southeast of Baradères in Haiti’s Nippes department. The sequence lasted 14 hours and 51 minutes and comprised eight events. The initiating shock reached magnitude 7.2 at a focal depth of 10 km. Subsequent events included two magnitude-5+ shocks within the first 20 minutes, followed by additional magnitude-4 to 5.8 earthquakes distributed between depths of 8 km and 15 km. The final recorded event, magnitude 5.8 at 03:20 UTC on 15 August, occurred at 8 km depth. All activity remained tightly clustered, consistent with aftershock triggering along the same fault segment.

This swarm illustrates post-mainshock stress redistribution. The magnitude-7.2 mainshock released accumulated strain along the Enriquillo-Plantain Garden fault, prompting immediate afterslip and secondary slip on adjacent fault patches. The rapid succession of moderate events within the first hours reflects dynamic triggering and pore-pressure changes at shallow crustal levels. Depths between 8 km and 15 km align with the seismogenic zone documented for this fault system.

Since 2000, only two swarms have been identified in the region; the earlier episode occurred in 2010 following the magnitude-7.0 Léogâne earthquake. The 2021 Nippes mainshock (magnitude 7.2) remains the sole magnitude-7+ event recorded in Haiti during the same period and lies 39 km from the swarm centroid. These statistics underscore the episodic nature of clustered seismicity along Haiti’s southern faults.

Continued monitoring is essential because residual stress and afterslip may sustain elevated seismicity for weeks to months. The 2021 sequence reinforces the need for updated hazard models that incorporate both mainshock rupture and swarm-type clustering in the Enriquillo-Plantain Garden fault zone.

References
SeismoSight internal catalog, swarm PS20210814.1 parameters
USGS Earthquake Catalog, Nippes, Haiti event of 14 August 2021
Global Earthquake Model, Caribbean tectonic framework (2023 update)