Source characteristics of the Kunashiri strait earthquake of December 6, 1978 as deduced from strain seismograms

Abstract

A great intermediate-depth earthquake (M JMA = 7.7) occurred on December 6, 1978 beneath the Kunashiri strait in the southern part of the Kurile arc. Strain seismograms recorded by the three components of a fused-quartz extensometer at the Erimo Geophysical Observatory are used to study the source process of this earthquake. The seismograms show a notable sequence of pulses with a short-period of several seconds. The sequence follows the first P-wave arrival and lasts for ∼ 60 s after the first S-wave arrival. Furthermore, a long-period (∼ 60 s) wave arriving at the first P time appears on each seismogram. From the short-period pulse sequence, at least 18 distinct events which have a total seismic moment of 2.5 × 10 27 dyne · cm can be identified during ∼ 60 s by comparing the synthetic seismograms to the observed ones. However, the amplitude of the observed long-period wave cannot be explained enough by the sequence of short-period events. From this fact it may be concluded that this earthquake contains, in addition to short-period events, a main long-period event with a source process time (rupture time) of ∼ 60 s and a seismic moment of 4.5 × 10 27 dyne · cm. The observed strain steps, in both magnitude and shape, are also explained well by the complex multiple shock sequence.