TY - JOUR
T1 - The Gilbert Islands (Republic of Kiribati) earthquake swarm of 1981-1983
AU - Lay, Thorne
AU - Okal, Emile
N1 - Funding Information:
We thank Hiroo Kanamori and Terry Wallace for discussions about this sequence, and Anny Cazenave for accessing and interpreting the SEASAT data. Cindy Arvesen typed the manuscript. This research was supported by the National Science Foundation under Grants EAR 81-15236 (Caltech) and EAR 81-06106 (Yale), and the Office of Naval Research under Contract N00014-79-C-0292. Contribution number 3914, Division of Geological and Planetary Sciences,
PY - 1983/12
Y1 - 1983/12
N2 - 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.
AB - 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.
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U2 - 10.1016/0031-9201(83)90046-8
DO - 10.1016/0031-9201(83)90046-8
M3 - Article
AN - SCOPUS:0020869449
SN - 0031-9201
VL - 33
SP - 284
EP - 303
JO - Physics of the Earth and Planetary Interiors
JF - Physics of the Earth and Planetary Interiors
IS - 4
ER -