Abstract
A singular sequence of three episodes of 3.5, 4.4 and 3.6 mainshock-aftershock occurred in the West Bohemia/Vogtland earthquake-swarm region during 2014. We analysed this activity using the WEBNET data and compared it with the swarms of 1997, 2000, 2008 and 2011 from the perspective of cumulative seismic moment, statistical characteristics, space-time distribution of events, and prevailing focal mechanisms.
For this purpose, we improved the scaling relation between seismic moment and local magnitude by WEBNET. The total seismic moment released during 2014 episodes ( Nm) corresponded to a single event and was comparable to of the swarms of 2000, 2008 and 2011.
We inferred that the earthquake is the maximum expected event in NovA 1/2 Kostel (NK), the main focal zone. Despite the different character of the 2014 sequence and the earthquake swarms, the magnitude-frequency distributions (MFDs) show the b-values 1 and probability density functions (PDFs) of the interevent times indicate the similar event rate of the individual swarms and 2014 activity.
Only the a-value (event-productivity) in the MFD of the 2014 sequence is significantly lower than those of the swarms. A notable finding is a significant acceleration of the seismic moment release in each subsequent activity starting from the 2000 swarm to the 2014 sequence, which may indicate an alteration from the swarm-like to the mainshocks-aftershock character of the seismicity.
The three mainshocks are located on a newly activated fault segment/asperity (D in out notation) of the NK zone situated in the transition area among fault segments A, B, C, which hosted the 2000, 2008 and 2011 swarms. The segment D appears to be predisposed to an oblique-thrust faulting while strike-slip faulting is typical of segments A, B and C.
In conclusion, we propose a basic segment scheme of the NK zone which should be improved gradually.