Seismic Swarm PS20070401.2: Analysis of the April 2007 Event near Gizo, Solomon Islands

The seismic swarm designated PS20070401.2 occurred approximately 67 km west of Gizo in the Solomon Islands. It began at 20:47 on 1 April 2007 and concluded at 09:46 on 4 April 2007, spanning 60 hours and 58 minutes. During this period, 62 earthquakes were recorded, with the majority occurring at shallow depths around 10 km and several reaching magnitudes above 5.0. The largest event measured 6.9 at 21:11 on 1 April, followed closely by a 6.6 at the swarm's onset and additional events of 6.1 on subsequent days.

This sequence exhibited typical swarm characteristics, featuring a rapid onset of multiple moderate-magnitude shocks without a single dominant mainshock. Magnitudes ranged from 4.8 to 6.9, with most events clustered in the first 24 hours. Depths remained predominantly crustal, though a few reached 30–39 km, indicating activity across varying segments of the fault system. The temporal distribution showed sustained activity through 3 April before tapering, consistent with stress redistribution in a tectonically active zone.

The Solomon Islands region forms part of the Pacific Ring of Fire, situated at the convergent boundary between the Pacific Plate and the Australian Plate, with involvement of the Solomon Sea Plate. Subduction along the New Britain–San Cristobal Trench drives frequent seismicity, producing both shallow crustal events and deeper Wadati-Benioff zone activity. The area's geology includes volcanic arcs, coral atolls, and uplifted sedimentary sequences shaped by ongoing convergence rates of several centimeters per year. Historical records indicate elevated earthquake frequency, with swarms often linked to fluid migration or aseismic slip along the plate interface.

Since 2000, four swarms have been documented in the vicinity, occurring in 2000 (one swarm), 2001 (two swarms), and 2007 (this event). Stronger regional earthquakes since 2000 include an M7.9 event 35 km WNW of Panguna, Papua New Guinea, in January 2017; an M7.1 shock 143 km SW of Panguna in May 2015; an M7.5 event 70 km SW of Panguna in April 2014; and an M7.1 quake 56 km WSW of Panguna in April 2014. These occurrences, located 33–70 km from the swarm center, underscore the persistent seismic hazard along the plate boundary.

The 2007 swarm aligns with the broader tectonic framework of rapid subduction and complex fault interactions that characterize the western Solomon Islands. Such sequences contribute to understanding precursory patterns and aftershock productivity in subduction settings.

References
USGS Earthquake Catalog
Global CMT Catalog
Pacific Tsunami Warning Center historical bulletins