Earthquake Swarm PS20241021.1 in Halmahera, Indonesia: Seismic Analysis and Regional Context

On 21 October 2024, an earthquake swarm designated PS20241021.1 occurred in Halmahera, Indonesia, registering seven events over a period of five hours and ten minutes. The sequence began at 03:24 local time and concluded at 08:35, with magnitudes ranging from 3.2 to 5.8 and focal depths between 10 and 30 km. The largest event reached magnitude 5.8 at a depth of 16 km, followed by additional shocks of 5.6, 5.1, and multiple 5.0 events clustered at shallower depths around 10 km.

This swarm represents a short-lived cluster of seismic activity typical of regions experiencing rapid stress adjustments. The events were tightly grouped in both space and time, consistent with swarm behavior rather than a classic foreshock-mainshock-aftershock pattern. Historical records since 2000 indicate only two prior swarms in the area, occurring in 2007 and 2019, underscoring the relative infrequency of such sequences in this sector of Halmahera.

Halmahera occupies a tectonically complex zone within the Molucca Sea region of eastern Indonesia, where the Philippine Sea Plate interacts with the Australian and Sunda plates. The island arc is shaped by double subduction, with the Halmahera Arc featuring westward subduction of the Philippine Sea Plate and opposing dynamics along the adjacent Sangihe Arc. This configuration generates frequent intermediate-depth seismicity and accommodates significant crustal deformation. The swarm's location aligns with the broader Halmahera seismic belt, where shallow crustal events often reflect faulting within the overriding plate or along active thrust systems.

A notable prior event was the magnitude 7.2 earthquake on 14 July 2019, located 155 km south-southeast of Sofifi and approximately 97 km from the 2024 swarm center. That quake highlighted the potential for larger ruptures along regional faults, though the recent swarm remained moderate in scale. Halmahera's geology also includes active volcanic centers influenced by the same subduction processes, which can contribute to episodic seismic swarms through fluid migration or magma-related pressure changes.

Insights from the 2024 sequence suggest localized stress release along shallow structures, with the majority of events nucleating near 10 km depth. Such patterns may indicate permeability changes or afterslip on nearby faults without escalation to a larger mainshock. Continued monitoring remains essential given the region's history of both swarm activity and strong isolated events.

References: SeismoSight internal classification data for swarm PS20241021.1 USGS Earthquake Catalog for regional historical events since 2000