Seismic quiescence before the M 7, 1988, Spitak earthquake, Armenia

Abstract

A detailed analysis of the 35 yr of seismicity between 1962 and 1997 using a gridding technique shows that the M7, Spitak earthquake of 1988 December 7 was preceded by a quiescence anomaly that started at approximately 19840.5, and lasted about 50.5 yr, up to the main shock. This quiescence anomaly had a radius of about 203 km, estimated from circular areas with 75 per cent rate decrease, centred at the point of maximum significance of the anomaly. The quiescence was clearly present in the aftershock volume during the 5 yr before the 1988 main shock, but its statistically strongest expression was located 30 km NW of the epicentre. This anomaly fulfills the association rules between precursory quiescence anomalies and main shocks, even for a tight definition, and is therefore proposed as a case of precursory quiescence. The largest value of the standard deviate Z, found by random selection of samples by gridding, was Z=14 for a time window of T w =3 yr, using a sample size of N=300 events. This makes this anomaly the strongest observed so far, and it is the first documented in an environment of continental collision. There are no false alarms exceeding in significance the precursor. The Armenian earthquake catalogue used for this study had 4600 earthquakes with MM min =2.2 in the area bounded by 39.5to 42N/42.5to 47E. From the point of view of homogeneous reporting this is the best catalogue we have analysed so far. The limits of the data used and the density of the grid are dictated by the data, and have no influence on the results. The choice of free parameters does not influence the results significantly within the following limits: 100N500, 2T w 7, 2.2M min 2.8.