EXAMINATION OF SLOW AND LATE MOMENT RELEASE OF THE 1988 SPITAK AND 1991 RACHA, CAUCASUS, EARTHQUAKES

Two large shallow earthquakes occurred in the Caucasus, part of the co llisional belt caused by the northward movement of the Arabian plate w ith respect to the Eurasian plate: the 1988 December 7 Spitak (Armenia n) (M(s)6.7) and 1991 April 29 Racha (M(s)6.9) earthquakes. In the pre sent study, slow and late moment releases of the earthquakes, which we re previously proposed based on the body-wave analyses, are examined u sing 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 utiliz ing the long-period surface waves, which are optimized to detect slow and late moment release, were employed, no evidence of slow and late m oment release was found for either earthquake. For the Racha earthquak e, the obtained source duration is consistent with the result from bod y-wave analyses. Within the resolution of the body-wave data, the simp le 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 sour ce duration obtained from the surface waves is about 30 s shorter than the source model suggested in the body-wave analysis. This discrepanc y 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 arriva ls of P waves as the result of source complexity. Numerical experiment s predict that Rayleigh-wave radiation is sensitive to the radiation f rom 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, bec ause 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 th an source complexity, for example complex local structure near the ear thquake source in the continental collisional belt. For the Spitak ear thquake, a large non-double-couple component is observed in the moment tensor solution obtained at long periods. If the non-double-couple co mponent is due to source complexity, it should be produced within the 20 s rupture process.