Seismic quiescence precursor to the 1983 Kaoiki (MS = 6.6), Hawaii, earthquake

Abstract

A remarkable seismicity rate decrease of 65 per cent occurred in most of the aftershock volume 2.4 yr before the 1983 Kaoiki MS = 6.6 main shock. If the background seismicity rate (1978 to mid-1981) is extrapolated to the time of the main shock, more than 300 earthquakes of ML ≧ 1.8 are missing because of the quiescence. Subvolumes measuring 5.5 km on a side, and located within the aftershock zone, showed seismicity rate decreases ranging from 0 to 90 per cent. The volume of no change had dimensions of approximately 10 × 3 km. It contained the main shock hypocenter, and was located near the center of the 10 km radius aftershock area. The seismic quiescence in different subvolumes of the aftershock volume surrounding this central nonquiet zone varied somewhat in starting time, amount of rate decrease, and statistical significance. According to the standard deviate z-test, the reported rate decrease is the most pronounced example of precursory quiescence defined to date. The seismicity pattern before the 1983 Kaoiki shock conformed to the quiescence hypothesis proposed on the basis of the precursors to the 1975 Hawaii main shock: no quiescence was observed within the immediate surrounding of the hypocenter, while strong rate decreases occurred in most of the rest of the main shock source volume. It is thus hypothesized that major asperities which contain high stress levels, and from which main ruptures can emanate, may be recognizable as volumes of constant seismicity rate within surrounding volumes of quiescence, provided that decreases of seismicity rate as a function of time can be defined quantitatively.