A Systematic Test on Precursory Seismic Quiescence in Armenia

A systematic test on seismic quiescence occurring before large earthquakes is conducted. For a fixed geographical location, the degree of clustering in space and time is analysed and the results are tested against randomized earthquake catalogs. A gridding technique allows to investigate the entire spatial volume covered by a certain earthquake catalog. The result is a significance K( x, t) for seismic quiescence as a function of space and time. A point ( x, t) is considered as quiet, if K( x, t) exceeds a threshold value K(99) such that the null hypothesis is rejected with a probability of p ≥ 99%. Because earthquake clusters, like aftershocks and swarm events, generate erroneous quiescence, declustered catalogs are also investigated and the influence of the clusters is discussed. Applying this method to an earthquake catalog from Armenia, several cases of seismic quiescence before mainshocks are obtained. These quiescence periods occur in the original data as well as in the declustered data. Using alarm conditions, it is found that quiescence periods and mainshocks are correlated ‘better-than-chance’. Thus, the results support the claim that seismic quiescence makes a contribution to the improvement of seismic hazard assessment.