A seismic swarm designated S20211104.1 occurred in the Seram region of Indonesia between 02:42 on 4 November 2021 and 23:52 on 5 November 2021. Over 45 hours and 9 minutes, 45 earthquakes were recorded. The sequence initiated with a magnitude 5.7 event at 18 km depth, followed by numerous smaller events predominantly at depths of 10–16 km, with magnitudes ranging from 2.5 to 4.2. This pattern reflects clustered seismicity without a single dominant mainshock-aftershock decay, typical of swarm behavior driven by fluid migration or stress redistribution along local faults. Seram lies within the Banda Arc, a tectonically complex zone resulting from the convergence of the Australian and Eurasian plates. The island experiences oblique subduction along the Seram Trough, combined with strike-slip faulting and back-arc thrusting. These structures accommodate regional shortening and produce frequent shallow to intermediate-depth earthquakes. Updated geological mapping confirms active fault systems trending east-west across central Seram, consistent with the observed hypocentral depths in the swarm. Historically, the Seram region has hosted destructive events, including the 1899 magnitude 7.8 earthquake and multiple magnitude 6+ shocks in the twentieth century linked to the same plate-boundary segments. Instrumental records from the past two decades show elevated background seismicity, with swarms occasionally occurring near the 2021 sequence location. Such episodes highlight the persistent seismic hazard in the Maluku Islands. Analysis of the swarm timeline reveals an initial energetic phase on 4 November, featuring the magnitude 5.7 event and several magnitude 3+ aftershocks within the first 12 hours. Subsequent activity on 5 November consisted of lower-magnitude events distributed throughout the day, tapering by late evening. Depths remained shallow, indicating rupture within the upper crust where fault zones are most responsive to pore-pressure changes. This swarm underscores the value of dense seismic monitoring for distinguishing transient clusters from foreshock sequences that might precede larger events. Continued observation of the Seram Trough remains essential given the region’s history of producing earthquakes capable of generating local tsunamis.
USGS Earthquake Catalog (accessed via public API for regional events 2000–2024).
Geological Survey of Indonesia (PVMBG) tectonic maps of the Banda Arc, 2022 update.
Hamilton, W. Tectonics of the Indonesian Region, USGS Professional Paper (updated synthesis).