A major swarm of intraplate earthquakes at the southeastern end of the Gilbert Islands Chain (3.5°S, 177.5°E) commenced in December 1981 and lasted through March 1983. No seismicity had been reported in the historical record in this region prior to 1981, but during the swarm 217 events with mb ≥ 4.0 were located by the NEIS, with 86 events having mb ≥ 5.0. The source region is quite remote, and the uniform detection level for the NEIS is for mb ≥ 4.8. A b-value of 1.35 is found for the swarm using the maximum likelihood method. Four events in the sequence were large enough (mb = 5.6-5.9) to determine focal mechanisms teleseismically using body- and surface-wave analysis. These events are found to have a range of mechanisms, from predominantly thrust with a significant oblique component, to purely strike-slip. The compression axes are consistent for all four events, with horizontal orientation trending NNE-SSW. This orientation is perpendicular to the direction of plate motion. The events are located at depths between 15 and 20 km placing them deep in the oceanic crust or in the upper mantle. No obvious bathymetric feature can be related to the fault plane orientations, though there is an offset in the island chain near the epicenters. While some characteristics of the swarm suggest a magmatic origin, the nature of the focal mechanisms, the location of the swarm, and the large accumulated moment release of the sequence favor a tectonic origin.
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Physics and Astronomy (miscellaneous)
- Space and Planetary Science