Morphotectonic analysis and branching for Mygdonia active fault system (Macedonia, N. Greece)

The Mygdonia basin is considered to be of a rather moderate seismicity rate area, with strong earthquakes occasionally occurring and affecting the northern Greek mainland. It corresponds to a complicated extensional setting bounded from normal faults that reveal a characteristic S – shape development. According to previous studies, the central part of the basin which mainly consists of faults trending E – W, are active structures that played a basic role in the formation of the basin. Both active faults and earthquakes appear in populations, characterized by certain spatial properties. A composite examination is attempted in order to investigate both earthquake and fault population properties taking into account all the available information that can be extracted from the correlation of seismicity and topographic data of the broader Mygdonia domain. It is known that the establishment of a dense seismological network contributes to the detailed analysis of the majority of the active structures since the distribution of the earthquake foci reveal the presence and particular properties of the active seismogenic zones. All earthquakes with magnitude M ≥ 1.0 which were recorded during the time period 2007 to 2009 from the National Greek Seismological Network are thoroughly examined. For this reason, arrival times of well recorded events that occurred in the basin were taken into account. The Wadati method was applied, to compute the Vp/Vs ratio and the origin times of the earthquakes with adequate data. Using the origin times derived from the best fitting data, travel times of the P waves were constructed to define the crustal structure in the area. In addition, time residuals were calculated in order to take into account the lateral variations of velocities. According to the results, all earthquakes that occurred in the area were relocated and their focal properties were determined again. Hypocentre determination was improved with the use of the VELEST algorithm. Cross sections perpendicular to the fault zones were plotted in order to approximate their depth. It is also known that innovative advanced tools lately applied in geosciences, provide a versatile approach in studying active fault systems. For this reason, high quality topographic maps along with any available tectonic data regarding active faulting were also used in order to investigate the properties of the faults population that dominates in the study area. Fault outcrops with a wide range of sizes are depicted as tectonic lineaments and GIS methodology is used for their analysis. Accurate digital elevation models (DEMs) of the area were constructed, while, cross sections and topographic profiles were produced mainly where seismicity is clustered. Similarities extracted from both methods, give combined interpretation about the fault possible segmentation or linkage either at the surface or at depth. The combined results from such an investigation provide important contribution to fault interaction, fault segmentation, seismotectonic zoning and seismic hazard assessment.