Seismic Swarm S20151209.2: Analysis of Earthquake Activity Southeast of Amahai, Indonesia

An earthquake swarm designated S20151209.2 was recorded 103 km southeast of Amahai on Seram Island, Indonesia. The sequence began at 08:55 on 9 December 2015 and concluded at 13:02 on 11 December 2015, spanning 52 hours and 7 minutes. During this interval, 38 earthquakes were registered, with magnitudes ranging from 4.0 to 4.9 and focal depths between 6 km and 30 km.

The events clustered primarily on 9 December, accounting for the majority of activity, followed by a sharp decline on 10 and 11 December. Notable higher-magnitude shocks included a 4.9 event at 10 km depth on 9 December at 10:38:55 and another 4.9 at 10 km depth on 9 December at 14:16:34. Multiple events at shallow depths near 10 km suggest possible involvement of crustal fluid migration or minor fault slip along pre-existing structures. The absence of a single dominant mainshock followed by clear aftershocks is characteristic of swarm behavior rather than a typical mainshock-aftershock sequence.

Seram Island lies within the Banda Arc, a tectonically complex zone formed by the convergence of the Australian, Pacific, and Eurasian plates. Subduction along the Seram Trough and associated thrust faulting generates frequent moderate seismicity. The arc’s geology features imbricated sedimentary and metamorphic rocks overlying a basement of continental fragments, with active deformation concentrated along north-dipping thrust systems. Historical records document destructive earthquakes in the region, including events in 1899 and 1938 that produced significant ground shaking and local tsunamis.

The 2015 swarm occurred at intermediate crustal depths consistent with the regional stress regime driven by oblique convergence. No volcanic centers lie immediately adjacent to the swarm location, supporting a tectonic rather than magmatic origin. Similar swarms have been documented elsewhere along the Banda Arc, often linked to episodic slip on segmented faults.

Post-2015 monitoring by regional networks has recorded continued low-level seismicity in the broader Seram area, underscoring the persistent seismic hazard. The S20151209.2 sequence illustrates how short-lived clusters can release strain without producing a large mainshock, providing useful data for refining local seismic hazard models.

References

USGS Earthquake Catalog
Global CMT Project
Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) annual seismicity reports