A quite energetic seismic excitation consisting of one main and three additional distinctive earthquake clusters that occurred in the transition area between the Kefalonia Transform Fault Zone (KTFZ) and the continental collision between the Adriatic and Aegean microplates is thoroughly studied after the high-precision aftershocks’ relocation. The activated fault segments are in an area where historical and instrumental data have never claimed the occurrence of a catastrophic (M ≥ 6.0) earthquake. The relocated seismicity initially defines an activated structure extending from the northern segment of the Lefkada branch of KTFZ with the same NNE–SSW orientation and dextral strike slip faulting, and then keeping the same sense of motion, its strike becomes NE–SW and its dip direction NW. This provides unprecedented information on the link between the KTFZ and the collision front and sheds more light on the regional geodynamics. The earthquake catalog, which was especially compiled for this study, starts one year before the occurrence of the M_w5.4 main shock, and adequately provides the proper data source for investigating the temporal variation in the b value, which might be used for discriminating foreshock and aftershock behavior.

The empirical Gutenberg–Richter (GR) relation corresponds to an exponential model of magnitude distribution, the most widely used in the probabilistic assessments of seismic hazard and related risk. However, due to the complexity of seismic processes induced by technological activities, this model may not be applicable to anthropogenic seismicity (AS). Applying statistical hypotheses testing procedures, we investigate 63 AS catalogs resulting from various anthropogenic activities such as reservoir impoundment, underground mining, conventional and unconventional hydrocarbon extraction, geothermal energy production, and underground gas storage. In 30 cases (47.6%) the exponential model for magnitude is rejected. Furthermore, in 16 out of these cases, the magnitude probability density functions are complex, having either modes or bumps or both. We discuss possible reasons for the discovered statistically significant deviations of the actual magnitude distributions from the exponential distribution and hence from the GR relation. We demonstrate that using the exponential distribution may lead to unacceptable inaccuracy of seismic hazard estimates in AS. As a remedy, we recommend the use of kernel nonparametric estimators of magnitude distribution.