Seismic Swarm S20180506.1: Analysis of El Salvador's May 2018 Earthquake Sequence

Seismic swarm S20180506.1 was recorded in El Salvador between 04:04 on 6 May 2018 and 04:23 on 8 May 2018. Over this 48-hour and 19-minute period, a total of 35 earthquakes were registered. The sequence began with a magnitude 4.4 event at 10 km depth, followed rapidly by smaller shocks at shallow depths between 2 km and 5 km. Peak activity included two magnitude 5.2 earthquakes on 6 May at 12:37 and 12:44, both at 10 km depth. Subsequent events ranged from magnitude 3.5 to 4.8, with depths predominantly between 2 km and 10 km, reflecting shallow crustal processes.

The swarm exhibited a typical pattern of clustered, moderate-magnitude events without a single dominant mainshock. Early activity on 6 May featured rapid succession of magnitude 3.5–4.4 shocks, transitioning to larger events midday before tapering into smaller aftershocks through 7 May and concluding with a final magnitude 3.5 event on 8 May. Depths remained consistently shallow, suggesting involvement of upper crustal faults rather than deeper subduction interfaces.

El Salvador lies within the tectonically active Central American volcanic arc, formed by the subduction of the Cocos Plate beneath the Caribbean Plate at rates of approximately 60–70 mm per year. This convergence drives frequent seismicity, including both interplate thrust earthquakes and shallower crustal events along volcanic alignments. The country hosts numerous Quaternary volcanoes, with historical eruptions often accompanied by seismic swarms linked to magma movement or hydrothermal activity.

Seismic swarms in subduction settings like El Salvador commonly arise from fluid migration, stress transfer along fault networks, or volcanic unrest. The 2018 sequence aligns with these mechanisms, given its short duration, lack of a clear foreshock-mainshock-aftershock progression, and concentration at depths under 10 km. Historical records indicate limited swarm occurrences in the instrumental era, with the prior documented swarm in 2001. Since 2000, only this single earlier event has been classified under similar internal criteria, underscoring the relative rarity of such clustered activity compared to isolated large earthquakes.

Geological monitoring in the region benefits from networks operated by national agencies, which provide data on depth distributions and magnitude-frequency relationships. The shallow focus of the 2018 swarm events suggests potential interaction with local fault systems near volcanic edifices, though no surface rupture or significant damage was associated. Such sequences contribute to understanding stress accumulation along the plate boundary and inform probabilistic seismic hazard assessments for Central America.

References

• SeismoSight internal swarm classification database (S20180506.1 parameters).
• USGS Earthquake Catalog for regional tectonics and subduction parameters.
• Global Volcanism Program, Smithsonian Institution, for volcanic-seismic correlations in El Salvador.