Seismogenic nodes in the Bulgarian territory, defined by pattern recognition

Seismogenic nodes, able to locate earthquakes with magnitudes M equal or higher than 6 (M6+), are identified for the territory of Bulgaria and adjacent areas. Definition of nodes is based on morphostructural zonation. Pattern recognition algorithm Cora-3 is applied to identify the seismogenic nodes, characterized by specific geological, geophysical and morphological data. The pattern recognition algorithm is trained on information for 30 seismic events M6+ for the period 29 B.C. – 2020, selected from historical and instrumental Bulgarian earthquake catalogues. These events are associated with 16 "training" nodes. Totally we have recognized 56 seismogenic nodes, most of them in southwestern Bulgaria.

The analysis of the identified seismic nodes shows that in addition to the initial 16 "training" nodes, about 20 ones may be associated with the already observed seismicity. Some of these nodes may be related with documented seismicity, which is not taken into account to select the “training” nodes. Other seimogenic nodes are close to (but do not include) some historical earthquakes, whose location or magnitude is not precise enough.

There has not been registered seismic activity M6+ in the vicinity of other 20 seismogenic nodes. To consider a certain inaccuracy in the determination of the earthquake magnitudes, in the analysis of these seismogenic nodes we take into account earthquakes with M higher than 5.8. In such a way the number of “inactive” until now seismogenic nodes decrease further. Several nodes can be related to seismic activity in the past, established by geological research. But a significant part of seismic nodes remains, which cannot be associated with any seismic manifestations. This is a sign of possible future earthquakes M6+ near these nodes.

It should be noted that 3 of the earthquakes M6+ in the twentieth century were not close to any seismogenic node. This may be due to several reasons among which are: hidden tectonic structures, gaps in the morphostructural zonation or an inaccurate magnitude of the earthquake’s catalogue data.

Acknowledgements. This study is partly funded by Russian Foundation of Basic Research (RFBR) according to the research projects 20-55-18008 and by Bulgarian National Science Fund, research project KP-06-Russia-29/16.12.2020.