The Gilbert Islands (Republic of Kiribati) Earthquake Swarm of 1981—1983
284 Physics ofthe Earth and Planetary Interiors, 33 (1983) 284—303 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands The Gilbert Islands (Republic of Kiribati) earthquake swarm of 1981—1983 Thorne Lay Seismological Laboratory, California Institute of Technology, Pasadena, CA (U.S.A.) Emile Okal * Department ofGeology and Geophysics, Yale University, New Haven, CT (U.S.A.) (Received July 28, 1983; accepted August 24, 1983) Lay, T. and Okal, E., 1983. The Gilbert Islands (Republic of Kiribati) earthquake swarm of 1981—1983. Phys. Earth Planet. Inter., 33: 284—303. 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.
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