Evaluation of the seismogenic (M6+) nodes for the territory of Bulgaria and adjacent areas

In this study seismogenic nodes capable to generate earthquakes with magnitudes M ≥ 6 are identified for the territory of Bulgaria and adjacent areas. Definition of nodes is based on a morphostructural zonation. Pattern recognition algorithm Cora-3 is applied to identify the seismogenic nodes, characterized by specific geological and geophysical data. The pattern recognition method is trained on information for 30 seismic events with M ≥ 6 for the period 29 BC–2020, selected from historical and instrumental Bulgarian earthquake catalogues. As a result, 56 seismogenic nodes are recognized, most of them in southwestern Bulgaria.

Neo-deterministic seismic hazard assessment of Corsica-Sardinia block

The Corsica-Sardinia lithospheric block is commonly considered as a region of very low seismicity and the scarce reported seismicity for the area has till now precluded the reliable assessment of its seismic hazard. The time-honored assumption has been recently questioned and the historical seismicity of Sardinia has been reevaluated. Even more, several seismogenic nodes capable of M5 + have been recognized in the Corsica-Sardinia block exploiting the morphostructural zonation technique, calibrated to earlier results obtained for the Iberian peninsula, which has structural lithospheric affinities with the Corsica-Sardinia block. All this allows now for the computation of reliable earthquake hazard maps at bedrock conditions exploiting the power of Neo Deterministic Seismic Hazard Assessment (NDSHA) evaluation. NDSHA relies upon the fundamental physics of wave generation and propagation in complex geologic structures and generates realistic time series from which several earthquake ground motion parameters can be readily extracted. NDSHA exploits in an optimized way all the available knowledge about lithospheric mechanical parameters, seismic history, seismogenic zones and nodes. In accordance with continuum mechanics, the tensor nature of earthquake ground motion is preserved computing realistic signals using structural models obtained by tomographic inversion and earthquake source information readily available in literature. The way to this approach has been open by studies focused on continental Italy and Sicily, where the agreement between hazard maps obtained using seismogenic zones, informed by earthquake catalog data, and the maps obtained using only seismogenic nodes are very good.

Seismogenic nodes as a viable alternative to seismogenic zones and observed seismicity for the definition of seismic hazard at regional scale

A fixed increment of magnitude is equivalent to multiply the seismic moment by a factor γEM related to the partial factor γq acting on the seismic moment representing the fault. A comparison is made between the hazard maps obtained with the Neo-Deterministic Seismic Hazard Assessment (NDSHA), using two different approaches: one based on the events magnitude, listed in parametric earthquake catalogues compiled for the study areas, with sources located within the seismogenic zones; the other uses the seismogenic nodes identified by means of pattern recognition techniques applied to morphostructural zonation (MSZ), and increases the reference magnitude by a constant amount tuned by the safety factor γEM.Using γEM=2.0, in most of the territory the two approaches produce totally independent, comparable hazard maps, based on the quite long Italian catalogue. This represents a validation of the seismogenic nodes method and a tuning of the safety factor γEM at about 2.

SEISMOGENIC NODES DEFINED BY PATTERN RECOGNITION IN THE CENTRAL PART OF THE ALPINEHIMALAYAN BELT

Information on the areas prone to the strongest earthquakes in a region is very important for knowledgeable seismic hazard and risk assessment. We consider the central part of the Alpine-Himalayan Belt (Caucasus–Kopet Dagh-Alburz) for the recognition of seismogenic nodes capable of M7+ earthquakes. The nodes formed around the intersections of the fault zones are viewed as objects of recognition which have been described by the common set of geologic, geomorphologic and geophysical parameters. 150 nodes out of 510 ones delineated in the Caucasus Kopet Dagh- Alburz region were recognized prone for earthquakes M7+. We have recognized a number of capable nodes where earthquakes M7+ have not yet been recorded.

Recognition of Earthquake-Prone Areas in the Himalaya: Validity of the Results

In 1992 seismogenic nodes prone for earthquakes6.5+have been recognized for the Himalayan arc using the pattern recognition approach. Since then four earthquakes of the target magnitudes occurred in the region. The paper discusses the correlation of the events occurred in the region after 1992 with nodes previously defined as having potential for the occurrence of earthquakesM6.5+. The analysis performed has shown that three out of four earthquakesM6.5+occurred at recognized seismogenic nodes capable ofM6.5+.