Seismic Swarm in the Flores Sea: Analysis of the December 2021 Event

On 14 December 2021, a seismic swarm was recorded approximately 128 km north-northwest of Ende, Indonesia, in the Flores Sea. The sequence began at 03:20 UTC and concluded at 08:31 UTC, encompassing seven earthquakes over a span of five hours and eleven minutes. The largest event reached magnitude 7.3 at a depth of 14 km, followed by events of magnitudes 5.7, 5.8, 4.6, 5.1, 5.0, and 5.6, most occurring at depths of 10 km.

This swarm occurred within the tectonically active Flores Sea region, part of the Sunda Arc system. Here, the Indo-Australian Plate converges with the Eurasian Plate at rates exceeding 70 mm per year, producing frequent thrust-fault earthquakes. The area's geology features a complex transition from oceanic subduction to continental collision, with the Flores Thrust serving as a primary structure accommodating strain. Historical records indicate recurrent moderate-to-large events driven by this plate boundary dynamics.

Notable prior activity includes a magnitude 7.0 earthquake on 27 February 2015, centered 150 km northeast of Maumere and 62 km from the 2021 swarm epicenter. The December 2021 mainshock itself registered magnitude 7.3 in the Flores Sea, 20 km from the swarm centroid, consistent with regional patterns of clustered seismicity along thrust faults.

Such swarms typically reflect stress redistribution following a primary rupture, often involving afterslip or fluid migration at shallow depths. In this instance, the tight temporal clustering and uniform depths underscore localized fault segment activation without widespread rupture propagation.

• References
• USGS Earthquake Catalog (earthquake.usgs.gov)
• Global CMT Project (globalcmt.org)
• Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports