Earthquake Swarm S20240322.1 in the Java Sea: Seismic Activity and Regional Context

An earthquake swarm designated S20240322.1 was recorded in the Java Sea, beginning at 04:22 on 22 March 2024 and concluding at 11:39 on 24 March 2024. Over this 55-hour and 16-minute period, a total of 51 earthquakes were detected. The sequence featured predominantly shallow events, with depths ranging from 6 km to 14 km and magnitudes between 3.0 and 6.4.

The swarm initiated with a magnitude 5.6 event at 04:22:45 on 22 March, followed rapidly by additional shocks including a 5.2 at 05:31:12. Activity intensified midday, culminating in the largest event of magnitude 6.4 at 08:52:58 on the same day at a depth of 9 km. Subsequent events included multiple shocks above magnitude 4.0, such as 4.7 at 09:19:27, 4.6 at 09:55:46, and 4.5 at 21:42:01. On 23 March, notable activity comprised a 4.2 at 07:04:18 (depth 6 km), 4.4 at 23:14:58 (depth 12 km), and several magnitude 3.5–3.8 events. The final recorded event was a magnitude 3.9 at 11:39:12 on 24 March.

This swarm reflects clustered seismic energy release typical of the Java Sea tectonic setting. The region forms part of the tectonically complex Sunda Shelf, where the Indo-Australian Plate converges with the Eurasian Plate. Subduction along the Sunda Trench south of Java drives regional deformation, producing both thrust and strike-slip faulting that can extend northward into the Java Sea basin. Shallow crustal earthquakes in this area often occur along secondary faults accommodating strain from the main subduction zone.

Historically, the Java Sea has experienced episodic seismic sequences linked to the broader Indonesian archipelago's position on the Pacific Ring of Fire. Major historical events in adjacent areas, such as the 1994 Java earthquake and earlier 19th-century shocks, underscore the persistent seismic hazard arising from plate boundary interactions. The March 2024 swarm aligns with patterns of distributed aftershock-like activity following larger regional releases, though no single mainshock preceded this particular sequence.

The concentration of events at depths near 10 km suggests activation within the upper crust, potentially indicating fluid migration or stress transfer along pre-existing fractures. With 51 events registered, the swarm represents a significant short-term increase in local seismicity, providing data for refining fault models in this under-monitored offshore zone.

References

SeismoSight internal classification data for swarm S20240322.1.
USGS Earthquake Catalog (general tectonic framework of the Sunda region).
Global Seismographic Network reports on Indonesian plate tectonics.