Time Variations in Earthquake Focal Mechanisms of the Racha-Dzhava Seismic Zone

Abstract

In this publication we consider how local seismicity in the Racha-Dzhava seismic zone of the Caucasus changed in time under the influence of large earthquakes that occurred in 1971–2011. In order to divide the whole studied period into time intervals within which the total energy released by local earthquakes was summed up, we use the averaged focal mechanisms (obtained from the first P-waves arrivals) of earthquakes presented in the Seismological Bulletins of the Caucasus. The averaged focal mechanisms were built according to the method of K. Aki, in the terms of which a set of weak local earthquakes with a close distribution of the first P-waves arrivals are considered one large earthquake. The change in averaged focal mechanisms with time is revealed; this is associated with a change in tectonic stresses within the Racha-Dzhava zone. We also compare the changes in directions of tectonic stresses and the influence of strong earthquakes. It is shown that one of the factors of an abrupt and short-term change in a stress state and in a local seismicity level is an impact of distant strong and catastrophic earthquakes. The results can be interesting for understanding some aspects of local seismic activity and the causes of changes in the stress field and the activity of seismic process in the Racha-Dzhava seismic zone.