Seismic Swarm PS20070404.1 Near Gizo, Solomon Islands

On 4 April 2007, a seismic swarm designated PS20070404.1 occurred approximately 107 km west-northwest of Gizo in the Solomon Islands. The sequence began at 00:35 local time and concluded at 10:42 the same day, encompassing five earthquakes within a span of roughly ten hours.

The events unfolded as follows. The initial shock registered magnitude 5.1 at a depth of 10 km. Four minutes later, a magnitude 6.0 event struck at the same depth. Activity resumed in the morning with a magnitude 6.4 earthquake at 06:34, located at 17 km depth. Two additional shocks followed: a magnitude 5.0 at 09:16 (10 km depth) and a final magnitude 4.7 at 10:42 (10 km depth).

This swarm forms part of the elevated seismicity recorded in the Solomon Islands since 2000. Six swarms have been documented in total, with two occurring in 2001, one in 2004, and three in 2007. The April 2007 sequence took place shortly after the region’s major Mw 8.1 earthquake of 2 April, consistent with patterns of clustered aftershock activity along active plate boundaries.

The Solomon Islands occupy a tectonically complex segment of the Pacific Ring of Fire. Convergence between the Pacific Plate and the Australian Plate drives subduction along the New Britain–San Cristobal trench system. The western Solomon Islands, including the area around Gizo, lie near the junction of multiple microplates, resulting in frequent moderate-to-large earthquakes at shallow to intermediate depths. Crustal deformation is accommodated by both subduction-related thrust faulting and strike-slip motion along transform segments.

Historical records show recurrent large events in the region. Notable examples include the 1977 Mw 7.9 and 1991 Mw 7.5 earthquakes, each generating local tsunamis. The 2007 Mw 8.1 mainshock and its immediate aftershock sequence, including swarm PS20070404.1, underscore the persistent seismic hazard. Depths between 10 km and 17 km, as observed in the swarm, align with the shallow seismogenic zone typical of this subduction margin.

Seismic swarms such as PS20070404.1 provide insight into stress redistribution following major ruptures. The rapid succession of five events within ten hours, with magnitudes ranging from 4.7 to 6.4, illustrates how aftershock sequences can cluster both temporally and spatially in this environment. Continued monitoring remains essential for refining hazard assessments in the western Solomon Islands.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Pacific Tsunami Warning Center historical bulletins