Examination of slow and late moment release of the 1988 Spitak and 1991 Racha, Caucasus, earthquakes

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SUMMARY Two large shallow earthquakes occurred in the Caucasus, part of the collisional belt caused by the northward movement of the Arabian plate with respect to the Eurasian plate: the 1988 December 7 Spitak (Armenian) (M,6.7) and 1991 April 29 Racha (M,6.9) earthquakes. In the present study, slow and late moment releases of the earthquakes, which were previously proposed based on the body-wave analyses, are examined using long-period surface waves. Results from spectral inversions using fundamental-mode surface waves show that the source durations of both earthquakes are about 20 s. Although additional investigations utilizing the long-period surface waves, which are optimized to detect slow and late moment release, were employed, no evidence of slow and late moment release was found for either earthquake. For the Racha earthquake, the obtained source duration is consistent with the result from body-wave analyses. Within the resolution of the body-wave data, the simple P and SH waveforms are well modelled by a source time history model with a duration of 20 s. However, for the Spitak earthquake, the source duration obtained from the surface waves is about 30 s shorter than the source model suggested in the body-wave analysis. This discrepancy is attributed to a slow and late thrust-fault slip in the body-wave source model, which was proposed in order to interpret the late arrivals of P waves as the result of source complexity. Numerical experiments predict that Rayleigh-wave radiation is sensitive to the radiation from the proposed slow and late slip. A comparison between the observed and predicted Rayleigh-wave radiation patterns, however, provides no indication of the surface-wave radiation from the slow and late slip. It is unlikely that the discrepancy in the source model is only due to errors in modelling the propagation of long-period surface waves, because for the Racha earthquake the source models obtained from body and surface waves are consistent. The present results thus indicate that the late arrivals of P waves are probably caused by something other than source complexity, for example complex local structure near the earthquake source in the continental collisional belt. For the Spitak earthquake, a large non-double-couple component is observed in the moment tensor solution obtained at long periods. If the non-double-couple component is due to source complexity, it should be produced within the 20 s rupture process.