Peculiarities of aftershock process of the East Black Sea Mw=5.7 earthquake of 2012

Authors:
Baranov, S.V. 1; Gabsatarova, I.P. 1 - 1 Geophysical Survey of RAS, Kola Branch, Russian Federation

Speaker: I.P. Gabsatarova

Abstract:
Strong Mw = 5.7 earthquake occurred on 23.12.2012 at 13:31:42.46 in the Black Sea (Abkhazia) at the distance of 37 km from the coast, 38 km from Ochamchire, 51 km from Sukhumi, 46 km from Anaklia, Georgia, and 158 km from Sochi, Russia. To locate the event we used data of 37 seismic stations from Russia ad Georgia. The final estimation of the hypocenter coordinates are 42.510N ± 1.9km, 41.047E ± 1.6km; the depth is 10 km. The epicenter is connected to a regional fault. The source is a strike slip fault.

The earthquake of 2012 has a great interest for the regional seismology as it is the strongest one in the history of seismic monitoring since 1900 in that part of the Black Sea and only two earthquakes had occurred in sea before 2012 had the magnitude greater than the last one. The earthquake of 2012 was followed by aftershocks which have been continuing till now. According to Russian Geophysical Survey there were 274 aftershocks with ML > 2 from 23.12.2012 till 31.12.2015. The aftershock epicenters agree with the regional fault system. An intensity of the aftershock process reached 44 events with ML > 2 per day in the first 24 hours after the mainshock.
In 57.2 hours after the mainshock an event with Mw=5.4, which initiated a secondary aftershock sequence, occurred. We studied the two aftershock sequences as a whole by relaxation LPL model and trigger ETAS model. To compare the aftershock process produced by the earthquake of 2012 we fitted LPL and ETAS models for three aftershocks sequences occurred in the Northern Caucasus (1st and 2nd Racha- Dzhava earthquakes of 29.04.1991 and 15.06.1991 and Oni earthquake of 07.09.2009 as well). Bayes informational criterion (BIC) was used to choose the best fitted model.

The aftershock processes produced by the earthquake occurred before 2012 had a relaxation type (LPL model gives lower BIC values than the ETAS). Aftershock process by the earthquake of 2012 had trigger type (ETAS model gives more than 30% lower BIC value than LPL one). Thus the aftershock process of the earthquake in the East Black Sea could not be described only by stress relaxation in the fault zone. To explore this non-relaxation behavior we fit the ETAS model in moving time window. We find a systematic variation in the external forcing strength which is incompatible by time with the large aftershocks driven by tectonic. We conclude that the non-relaxation behavior of aftershock process of the earthquake of 2012 was not controlled by tectonic deformation but fluid effect on the local stress field in the fault zone.

The research was supported by Russian Foundation for Basic Research (Project 16-05-00263).