Earthquake multiplets in the southeastern Solomon Islands

Abstract

The 350-km stretch of the Solomon Islands trench that lies adjacent to the islands of Guadalcanal and San Cristobal has been the site of 10 large shallow earthquakes since 1966: June 15, 1966 doublet (M_s = 7.7,7.3), May 20, 21, 1977 quadruplet (M_s = 6.7,7.5,7.5,7.5), November 4,5, 1978 doublet (M_s = 6.9,7.1), and two single events on October 23, 1979 (M_s = 7.1) and February 7, 1984 (M_s = 7.7). Analyses of P-wave first-motions, aftershock distributions, P-wave-forms recorded at epicentral distances of 30° to 90° on the WWSSN, and 256 s-period Rayleigh-wave spectra obtained from the IDA and GDSN networks are used to examine the source characteristics of the events. The close temporal association and large size of earthquakes precluded collection of useful seismograms for the latter of the two 1966 events and the second and third events of the 1977 sequence. P-waveforms analyzed show smooth traces that can be modeled with simple trapezoidal source-time functions located at shallow (15–45 km) depths. Focal mechanisms are consistent with oblique subduction of the Indian plate beneath the Pacific plate at about N73°E. Estimates of seismic moment for individual events range from 0.2 to 2.3 × 10^(27) dyne-cm. Average stress drops of the sequences are estimated to range between 10 and 40 bars. This section of island arc may have ruptured in a similar sequence of earthquakes during the 1930's. Conversion of the cumulative seismic moment to displacement and averaging over 40 years suggests a convergence rate of about 4.5 cm year^(−1), about one half of the rates that have been assessed on the basis of seafloor magnetic lineations.

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