Seismic Swarm PS20161208.1: Earthquake Sequence Near Kirakira, Solomon Islands

The seismic swarm designated PS20161208.1 occurred 41 km south of Kirakira in the Solomon Islands. It began at 17:38 on 8 December 2016 and concluded at 21:39 on 11 December 2016, spanning 76 hours and producing 57 recorded earthquakes.

The sequence initiated with a magnitude 7.8 event at 40 km depth. Subsequent activity included multiple events exceeding magnitude 5.0, with notable shocks such as a magnitude 6.5 at 12 km depth and a magnitude 6.9 at 19 km depth. Depths throughout the swarm ranged from 7 km to 61 km, indicating activity across both shallow crustal and intermediate levels. The events clustered tightly in both space and time, characteristic of swarm behavior rather than a classic mainshock-aftershock pattern.

The Solomon Islands occupy a tectonically active segment of the Pacific Ring of Fire. The region lies at the convergent boundary between the Pacific Plate and the Australian Plate, where oblique subduction drives frequent seismicity and volcanism. The San Cristobal Trench to the south accommodates much of this convergence, generating thrust and strike-slip faulting. Historical records show recurrent large earthquakes, consistent with the high strain accumulation along this plate margin.

Since 2000, five prior swarms have been documented in the immediate area: two in 2002, and one each in 2004, 2005, and 2014. Strong earthquakes recorded since 2000 include the magnitude 7.8 event of 8 December 2016 itself (69 km WSW of Kirakira), a magnitude 7.4 on 13 April 2014 (112 km south of Kirakira), a magnitude 7.6 on 12 April 2014 (93 km SSE of Kirakira), and a magnitude 7.3 on 20 January 2003 (126 km west of Kirakira). These events cluster within 100 km of the 2016 swarm center, underscoring persistent seismic productivity.

Analysis of the 2016 swarm reveals a rapid onset followed by sustained moderate-magnitude activity over three days. The largest events occurred within the first 48 hours, after which magnitudes declined while smaller events continued. Depth distribution shows a concentration between 10 km and 40 km, with occasional deeper or shallower outliers. This pattern aligns with fluid migration or stress triggering along the subduction interface and overlying crustal faults.

The Solomon Islands experience elevated seismic hazard due to their location on the plate boundary. Monitoring by regional and global networks continues to improve detection and characterization of such swarms, aiding in the assessment of short-term hazard.

References
USGS Earthquake Catalog
Global CMT Project
SeismoSight internal swarm classification PS20161208.1