Seismic Swarm S20190624.1: Earthquake Sequence Near Abepura, Indonesia

A seismic swarm designated S20190624.1 was recorded 236 km west of Abepura, Indonesia, between 01:45 UTC on 24 June 2019 and 01:03 UTC on 28 June 2019. Over 95 hours and 18 minutes, the sequence comprised 54 earthquakes. The events clustered in a compact area, with the majority occurring at shallow depths of 10 km. Magnitudes ranged from 3.7 to a peak of 5.2, reflecting a typical swarm pattern of numerous moderate shocks without a single dominant mainshock.

Geological Setting

The swarm location lies within the tectonically complex New Guinea region, where the Australian and Pacific plates converge. This boundary features oblique subduction along the New Guinea Trench, combined with strike-slip faulting and crustal shortening. Shallow seismicity predominates due to active fault systems accommodating rapid plate motion. The area experiences frequent earthquake swarms linked to fluid migration, stress redistribution, and volcanic or hydrothermal influences in the overriding plate.

Swarm Characteristics and Timeline

The sequence began with a 4.8 magnitude event at 01:45 UTC on 24 June, followed by rapid aftershocks including multiple 4.4–4.7 events within the first 24 hours. Activity remained elevated through 25 June, with events such as the 4.7 at 11:38 UTC. The largest shock, magnitude 5.2, occurred at 12:48 UTC on 27 June, accompanied by several 4.7 events. Depths stayed predominantly shallow (10–16 km), consistent with upper-crustal faulting. By 28 June, activity declined sharply, ending with a 3.8 magnitude event.

This temporal distribution shows an initial burst, sustained moderate activity, and abrupt termination—hallmarks of swarm behavior rather than a classic foreshock-mainshock-aftershock sequence.

Regional Seismic History

Since 2000, only one prior swarm has been documented in this immediate vicinity, occurring in 2019. A notable strong earthquake struck nearby on 27 July 2015: a magnitude 7.0 event centered 234 km west of Abepura, just 13 km from the 2019 swarm epicenter. That quake highlighted the region’s capacity for large-magnitude thrust and strike-slip events along the plate boundary.

Insights from the 2019 Swarm

The 2019 sequence illustrates how moderate-magnitude swarms can relieve accumulated strain without producing destructive shaking. Most events remained below magnitude 5.0, limiting potential damage despite proximity to population centers. The shallow focal depths amplified ground motion locally but confined energy release to a small volume. Such swarms provide valuable data on fault interactions and may serve as indicators of evolving stress fields ahead of larger events.

Ongoing monitoring in this high-strain zone remains essential for understanding long-term seismic hazards along the Australia-Pacific plate boundary.

References

SeismoSight internal swarm classification S20190624.1
USGS earthquake catalog (regional tectonics and 2015 M7.0 event)