Seismic Swarm S20210814.1 in the Haiti Region: Analysis and Context

A seismic swarm designated S20210814.1 was recorded in the Haiti region between 13:14 UTC on 14 August 2021 and 04:57 UTC on 17 August 2021. Over 63 hours and 43 minutes, 89 earthquakes were detected, with magnitudes ranging from 2.3 to 4.9 and focal depths predominantly between 1 km and 23 km. The swarm initiated shortly after the M7.2 Nippes earthquake of 14 August 2021, whose epicenter lay approximately 8 km from the swarm centroid.

Haiti occupies a tectonically active segment of the northern Caribbean plate boundary. The island sits at the junction between the Caribbean and North American plates, where left-lateral strike-slip motion occurs along the Enriquillo–Plantain Garden fault zone (EPGFZ) and the Septentrional–Oriente fault system. These structures accommodate roughly 2 cm per year of relative plate motion. The EPGFZ, in particular, has produced several destructive events, including the 2010 M7.0 Léogâne earthquake and the 2021 M7.2 Nippes mainshock. Both quakes ruptured segments of the same fault system, releasing accumulated strain in shallow crustal volumes.

The August 2021 swarm exhibited classic swarm characteristics: a rapid onset of moderate-magnitude events without a single dominant mainshock, followed by a gradual decay in frequency and size. Depths clustered between 3 km and 12 km, consistent with brittle failure within the upper crust along the EPGFZ. The largest event (M4.9 at 9 km depth) occurred within the first four hours, after which activity migrated slightly along strike while remaining spatially compact.

Historical records maintained since 1 January 2000 document only two prior swarms in the same region: one in 2010 comprising a single cluster and the present 2021 sequence. This low recurrence suggests that swarm-type behavior is infrequent relative to the more typical aftershock sequences that follow large mainshocks on the EPGFZ.

The 2021 swarm is interpreted as a secondary response to the stress perturbation created by the M7.2 mainshock. Post-seismic creep and fluid migration along the fault plane likely contributed to the sustained, diffuse seismicity observed over the subsequent three days. No surface rupture or significant infrastructure damage has been attributed directly to the swarm events themselves.

Continued monitoring of the EPGFZ remains essential. The fault system retains the potential for future large earthquakes, and swarms such as S20210814.1 provide valuable data on fault-zone heterogeneity and post-seismic processes.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
SeismoSight internal swarm classification database