Energy Characteristics of Some Epicentral Areas in the Vardar Zone
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343 Geologica Macedonica, Vol. 32, No. 2, pp. 165–169 (2018) GEOME 2 IISSN 0352 – 1206 Manuscript received: May 15, 2018 e-ISSN 1857 – 8586 Accepted: November 17, 2018 UDC: 550.348.436.098.3(497.71) Short communication ENERGY CHARACTERISTICS OF SOME EPICENTRAL AREAS IN THE VARDAR ZONE Blagica Doneva, Todor Delipetrov, Marjan Delipetrev, Gјorgi Dimov, Krsto Blažev Faculty of Natural and Technical Sciences, Institute of Geology, “Goce Delčev” University in Štip, Blvd. Goce Delčev 89, 2000 Štip, Republic of Macedonia [email protected] A b s t r a c t: Seismological phenomena are a natural phenomenon associated with the development of the Earth. They inflict huge damage and cause a large number of human casualties. The territory of the Republic of Macedonia is, seismically, very active part of the Mediterranean area where a large number of earthquakes have occurred. When studying these phenomena, the determination of the energy released during earthquakes is of great importance. This paper deals with the seismic energy of the Vardar zone through the empirical formula for the dependence of the magnitude of the earthquake M and the activated geotectonic surface. Seismicity in Vardar zone is particularly pronounced in the areas of crossing of the reactivated old faults from the Vardar direction with neotectonic faults that predominantly stretch in the transverse direction. Vardar zone is defined as the most labile tectonic unit on the Balkan Peninsula with very expressed seismicity. This paper presents the energetic characteristics of the epicentral areas separated in the Vardar zone: Kumanovo, Skopje, Veles–Sveti Nikole, Štip–Radoviš, Kavadarci and Valandovo. Key words: energy; characteristics; magnitude; Vardar zone; epicentral areas INTRODUCTION Earthquake is a natural occurrence that origin Energy characterizes the essence of occurrence of in the deep parts of the Earth with rapid release of earthquakes and is one of the main physical cha- energy in the focus area. Such released energy spre- racteristics of the focus. The seismic energy, spre- ad through the ground and on the Earth’s surface is ading in space, excites particles and they begin to manifested as earthquake. Related to the depth of oscillate. The energy released in the focus is greater, the focus, earthquakes are divided in: depth earth- and the oscillation of the ground is more intense quakes – their focus is deeper from the limit of the (Delipetrov, T. et al., 2014). earth's crust (depth to 800 km) and earthquakes Vardar zone is defined as the most labile tec- whose focuses are in the earth's crust, so called tonic unit on the Balkan Peninsula where Alpine tectonic earthquakes. Depth earthquakes are formed orogeny processes were so expressed and continued in the subduction zones of the tectonic plates, in neotectonics stage. mixing the subduction material leads to complex physical-chemical processes that generate large Seismicity in this zone is particularly pro- quantity of energy which is reason for this type of nounced in the areas of crossing of the reactivated earthquakes (Blažev and Arsovski, 2003). old faults from the Vardar direction with neotec- From physical point of view, released seismic tonic faults that predominantly stretch in the trans- energy is directly related to the volume of activated verse direction. earthquake focus. Looking at the spatial distribution of the earth- At the origin of earthquakes, the potential quakes so far (Figure 1) in the Vardar zone, the fol- energy of strain is quickly released and trans- lowing epicentral areas can be separated: Kumano- formed in part of the kinetic energy of seismic vo, Skopje, Veles–Sveti Nikole, Štip–Radoviš, (elastic) waves, and some other types of energy. Kavadarci and Valandovo. 166 B. Doneva, T. Delipetrov, M. Delipetrev, G. Dimov, K. Blažev Fig. 1. Map of the earthquake’s epicenters in the Repubic of Macedonia and surrounding recorded between 1900–2000 for 4.0 < M < 8.0 (Donevam 2014) ENERGY CHARACTERISTICS OF THE EPICENTRAL AREAS IN VARDAR ZONE One of the most important parameters in seis- years and the calculated total released seismic ener- micity is seismic energy released during earth- gy is Е = 87.19·10–11 (Ј) that corresponds to earth- quakes. Using the empirical formula of Gutenberg- quake with magnitude М = 5.43. Richter for calculation the relation between the Knowing that in this area was occurred earth- energy and magnitude (Storcheus, 2011): quake with magnitude М = 5.3, it’s clear that the largest part of the energy released within 75 years is logE =1.5M +4.8 emitted through the strongest earthquake. calculation is made for released seismic energy of Average annual released seismic energy of the 11 every earthquake occurred in certain epicentral area, Kumanovo epicentral area is Е = 1.16·10 Ј and and then that energy is summarized for all earthqua- corresponds to magnitude М = 4.18. kes and obtained is the total released seismic energy In the total released seismic energy in Vardar of the area. zone, this epicentral area (focus) contributes with Also, calculated is percentage contribution of 0.47 %. every epicentral area in the total released energy in Vardar zone. Calculations are made for a period of Epicentral area Skopje. This epicentral area 75 years (Delipetrov et al., 2014). is connected to the tectonic activity of Skopje depre- ssion which represents neotectonics depression. Epicentral area Kumanovo. In this epicentral In Skopje epicentral area is very expressed area are analyzed several earthquakes with magni- concentration of earthquake focuses in almost a tudes from 2.7 to 5.3, occurred in a period of 75 local terrain on depth of 2–12 km. Geologica Macedonica, 32 (2), 165–169 (2018) Energy characteristics of some epicentral areas in the Vardar zone 167 Small depth of the focus of the main earth- This epicentral area contributes with 0.09 % in quake in 1963 mainly led to such a large macro- the total released seismic energy in Vardar zone. seismic manifestation on the surface (the maximum surface effect in the central parts of the city was up Epicentral area Kavadarci. This area is to IX degrees on the MCS scale). After this catast- located in the south-western part of Varadr zone rophic earthquake, detailed seismic and seismo- where in Pliocene–Quaternary was manifested tectonic investigations were carried out in this area. volcanism (Alšar – Kožuf) and the Mrežicko fault It can be said that the Skopje seismogene area is the was reactivated. most explored region not only in the Vardar zone, This area, similarly to the previous, is charac- but also in Macedonia. terized with relatively weak earthquakes. For determination of the released seismic In a period of 75 years, from the analyzed 10 energy are taken 27 occurred earthquakes which earthquakes with magnitude M = 2.8 – 5.1, the total according the Catalogue of occurred earthquakes 11 released seismic energy is E = 151.44·10 J that belong to Skopje epicentral area. corresponds to an earthquake with magnitude M = The total released seismic energy, calculated 5.6. for this area, is Е = 935.40·1011 Ј for a period of 75 Average annual released seismic energy in years and corresponds to an earthquake with epicentral area Kavadarci is Е = 2.02·1011 Ј and magnitude М = 6.2. corresponds to earthquake with magnitude М = 4.3. Average annual released energy is Е = 12.47· Only 0.82% of the total released seismic 1011 Ј, corresponding to earthquake with magnitude М = 4.10. energy in Vardar zone belongs to this focus. On this active seismogenic focus belongs 5,05% of the total released seismic energy in the Epicentral area Valandovo. Seismicity in Vardar zone. this area is conditioned with the neotectonics acti- vity of the Valandovo fault that pass along the sout- Epicentral area Veles–Sveti Nikole. For the hern edge of the Valandovo depression. It is crossed time being, this region has been characterized by a with north-northwest oriented fault system that relatively constant occurrence of only weak earth- created complex tectonic node. quakes (magnitude M = 3.3 to 4.8). In this epicentral area occurred the strongest The total released seismic energy in this area is earthquake in Vardar zone till now with magnitude Е = 69.62·1011 Ј which corresponds to earthquake M = 6.7. This major earthquake was preceded by a with magnitude М = 5.4. foreshock activity in a wider area, approaching Average released annual seismic energy is Е = gradually to the epicenter of the main earthquake 0.93·1011 (Ј), corresponding to earthquake with occurred on 8. III. 1931. magnitude М = 4.1. From the impact of this earthquake the most The contribution of this epicentral area in the affected was the village of Pirava, near Valandovo. total released seismic energy in Vardar zone, in Beside Pirava and Valandovo, another 40 settle- period of 75 years, is 0.38 %. ments from Gevgelija to Strumica were destroyed. After this strong earthquake, similarly to the Epicentral area Štip – Radoviš. This focal foreshock activity, occurred aftershock activity and zone, tectonically is closely related with the neo- the region covered with this action was very large. tectonics movements, or formation of the Lakavica This showed the size of the Valandovo focus and ridge. In this area, as in the previously described, so complex tectonic structure of this area. far only weak earthquakes have appeared. From the For a period of 75 years were analyzed 28 recent seismic activity in this area or analyzed 22 earthquakes occurred in this epicentral area. The earthquakes with magnitude from 2.7 to 4.6, in a total released seismic energy amounts to E = period of 75 years, the total released seismic energy 7816.92·1011 Ј and corresponds to an earthquake 11 is Е = 16.28·10 Ј which corresponds to an earth- with magnitude M = 6.73.