Thursday, November 21, 2019
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    Museum of the City of Skopje

    The Seismological Observatory of the Faculty of Natural Sciences and Mathematics in Skopje contributed in the exhibition,

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Tectonics and observed seismicity

Seismicity of Republic of Macedonia

Tectonics and observed seismicity

The territory of the Republic of Macedonia is in the central and southeastern parts of the Balkan Peninsula. As known, this peninsula is in the internal part of the Euro-Asian tectonic plate, and it has a complex geological structure, which is characterized with an intensive dynamics and geophysical fields’ anomalies.

More precisely, the Balkan Peninsula is in the Mediterranean part of the Alpine-Himalayan Orogenic Belt. The neotectonic history of this belt started between the end of the Paleogene and the beginning of the Neogene, i.e. about 23 millions years ago. However, the major, first order tectonic units got developed much earlier, before the Cambrian, i.e. earlier than 550 millions years ago (Fig. 1).

4_Rubrika_4_SEIZMICNOST NA MAKEDONIJA_Sl_1_Del_Balkan_MAK

Fig.1. Tectonic map of the central and southeastern Balkans (UNDP/UNESCOSurvey of the Seismicity of the Balkan Region,Skopje, UNESCO, 1974). The first order tectonic units are denoted by thick lines, while those of higher order – by thinner lines. More detailed explanation is in the text.

Thus, in the neotectonic times, the central and southeastern parts of the Balkan Peninsula are areas of confrontation of few tectonic structures of first order, such as the northern and southern branches of the Mediterranean Alpine Orogenic Belt, and the Median Massifs, all spreading along NW–SE direction. The northern branch of the Mediterranean Alpine Orogenic Belt is presented by parts of the Carpathian massif (symbol   in Fig. 1) and the Balkan massif. The southern branch of this belt is presented by parts of the massifs Dinarides (symbols   ,   ) and Albanides (symbols   ,  ). The Median Massifs are presented by a part of the Pelagonia zone (symbol   ), the Vardar zone(symbol   ) and the Serbo-Macedonian massif (symbol   ).

The neotectonic activity in the central and southeastern Balkans has been characterized by permanent uplifting of the terrain. It has passed through two phases, the first one expressed into a flattening of the terrain, due to erosion and denudations of the mountains’ rocks, and the second, orogenic one, expressed into differential, predominantly vertical movements of the first and second order tectonic structures. The second, still lasting phase has been resulting into new faultings, ruptures which walls exhibit relative sliding. These new faultings were oriented approximately along the E–W direction, nearly perpendicularly to the previous faultings, oriented along the NW–SE direction. That had caused a formation of tectonic structures of higher order, as smaller uplifting units and sinking depressions. In fact, the entering of the different relative movements of these higher order units into dynamical phases are the main causes of earthquakes. Beside, the strongest earthquakes occur at the crossing places of the approximately E–W oriented and NW–SE oriented active faults. The present situation for the territory of the Republic of Macedonia is presented in Figs. 2, 3 and 4.

Fig. 2. Neotectonic map of the territory of the Republic of Macedonia (M. Arsovski, 1997).

Fig. 3.  Neotectonic faults on the territory of the Republic of Macedonia (M. Arsovski, 1997).

Three main neotectonic regions spread in the territory of the Republic of Macedonia and neighboring regions, namely, Vardar zone, West Macedonia and East Macedonia(M.Arsovski, 1997), which developed within major, regional tectonic units (Fig.1). These three regions are permanently uplifting with different intensities. The intensity of uplifting of West Macedonia is the greatest, while that of the Vardar zone is the smallest. However, the intensities of uplifting of the three regions are of order of few millimeters per a year.

Those three regions show differences in the seismic activity too, which is the reason to assume them as separate seismic zones, named with the same names. According to the investigations of the Seismological Observatory of the Faculty of Natural Sciences and Mathematics in Skopje, except some collapse earthquakes, all observed historic and contemporary earthquakes in these zones are tectonic. The seismic source for the greatest number of earthquakes is a dynamical faulting (a dynamical relative sliding of walls of a fault). The strongest earthquakes (Table 1) occur at the places of crossing of the NW–SE oriented and approximately E–W oriented faults, Fig. 4.

It should be noticed again that the present seismic activity and the occurrence of separate epicentral areas within the three seismic zones are due to the different intensities of permanent movements of the higher order tectonic units within the seismic zones themselves. Namely, the latter movements cause accumulation of stresses and strain, which, in certain moments, leads to dynamical movements of the units. The intensities of the permanent uplifting of the mountains in the Republic of Macedonia are found to be in the interval from 2 mm to 4 mm per a year, while the intensities of sinking (subsiding) of some valleys within the Vardar zone are about 2 mm per a year. (The whole amplitude of the different vertical movements reached during the neotectonic phase is about 4,000 m.)

The West Macedonia seismic zone spreads along the valleys of the Crn Drim and Bel Drim rivers, as well as northern of them, including in that way epicentral areas from Republic of Albania, Republic of Macedonia and Republic of Greece.

The Vardar seismic zone stretches along the Valley of Vardar River, as well as northern of it, up to the city of Belgrade in the Republic of Serbia. Thus, it includes epicentral areas from Republic of Macedonia, Republic of Greece and Republic of Serbia.

The East Macedonia seismic zone spreads along the valley of Struma River, as well as northern of it, thus including epicentral areas from Republic of Serbia, Republic of Bulgaria, Republic of Macedonia and Republic of Greece.

The distribution of the epicentral areas within the three seismic zones in the territory of the Republic of Macedonia and the neighboring regions is as follows.

West Macedonia seismic zone: epicentral areas Tetovo – Gostivar, Debar, Kičevo, Kruševo, Bogomila (near the town of Veles), Debarca – Plaken Mountain, Peštani – Ohrid – Struga, Prespa (crossing the borders with the Republic of Albania and Republic of Greece), Bitola, Bitola – Lerin (crossing the border with the Republic of Greece), Peškopija (crossing the border with the Republic of Albania) and Southern part of the Ohrid lake (crossing the border with the Republic of Albania).

Vardar seismic zone: epicentral areas Skopje, Kumanovo, Veles, Sv. Nikole – Štip, Štip – Radoviš, Gradsko – Kavadarci – Negotino), Demir Kapija, Mrežičko (near the town of Kavadarci), Valandovo, Gevgelija – Gumendža, Dojran – Kukuš (crossing the border with the Republic of Greece) and Uroševac (Kačanik – Vitina – Gnjilane), which belongs entirely to Kosovo.

East Macedonia seismic zone: epicentral areas Zletovo, Kočani, Delčevo – Berovo, Strumica and Pehčevo – Kresna (crossing the border with the Republic of Bulgaria).

NASLOVNA i Rubrika 4_Sl_Br_2_Epicentralna karta na Makedonija_1901_2012_ENGL

Fig. 4. Epicentral map of the territory of the Republic of Macedonia and neighboring regions for the period 1900–2012. (Source: Seismological Observatory at the Faculty of Natural Sciences and Mathematics in Skopje.)

Table 1. The most significant historic and recent earthquakes occurred in the territory of the Republic of Macedonia and neighboring areas; <![if !vml]><![endif]> – hypocentral time, <![if !vml]><![endif]> and <![if !vml]><![endif]> – the epicentre latitude and longitude, <![if !vml]><![endif]> – hypocentral depth, <![if !vml]><![endif]> – magnitude, <![if !vml]><![endif]> – epicenral intensity in degrees of the EMS-1998 macroseismic scale; the magnitudes <![if !vml]><![endif]> for the historic earthquakes (prior to 1900) are obtained by the macroseismic method. (Source: Seismological Observatory at the Faculty of Natural Sciences and Mathematics in Skopje.)

Date

<![if !vml]><![endif]> (UTC)

<![if !vml]><![endif]>

<![if !vml]><![endif]>

<![if !vml]><![endif]>

<![if !vml]><![endif]>

<![if !vml]><![endif]>

Epicentral area

Year

M.

D.

h

min

s

<![if !vml]><![endif]>

<![if !vml]><![endif]>

(km)

(EMS-1998)

Ancient times

41.25

22.50

6.1

IX

Valandovo (village of Marvinci), R. Macedonia

(400)

41.50

22.00

6.1

IX

Gradsko (Stobi), R. Macedonia

518

42.10

21.40

6.1

IX

Skopje (Skupi), R. Macedonia

527

41.10

20.80

6.1

IX

Ohrid Lake

896

9

4

41.70

23.00

6.1

IX

Pehčevo - Kresna (R. Macedonia – R. Bulgaria)

1555

42.00

21.50

6.1

IX

Skopje (ancient town of Skupi), R. Macedonia

1755

2

26

42.50

21.90

6.1

IX

Uroševac (Kačanik), Kosovo

1904

4

4

10

2

38.1

41.78

22.93

25

7.3

IX

Pehčevo - Kresna (R. Macedonia – R. Bulgaria)

1904

4

4

10

25

50.0

41.71

23.08

30

7.8

X

Pehčevo - Kresna (R. Macedonia – R. Bulgaria)

1905

10

8

7

28

51.4

41.80

23.10

30

6.5

VIII

Pehčevo - Kresna (R. Macedonia – R.Bulgaria)

1906

9

28

2

30

00.0

40.88

20.66

25

6.0

VIII

Ohrid Lake

1911

2

18

21

35

18.0

40.86

20.71

25

6.7

IX

Ohrid Lake

1912

2

13

8

4

00.0

40.86

20.75

25

6.0

VIII

Ohrid Lake.

1921

8

10

14

10

40.0

42.30

21.40

20

6.1

IX

Uroševac (Vitina), Kosovo

1931

3

7

0

16

44.8

41.28

22.50

25

6.0

VIII

Valandovo, R. Macedonia

1931

3

8

1

50

24.0

41.28

22.50

10

6.6

X

Valandovo, R. Macedonia

1942

8

27

6

14

15.6

41.62

20.47

15

6.0

IX

Peškopia, R. Albania

1963

7

26

4

17

11.7

42.02

21.42

5

6.1

IX

Skopje, R. Macedonia

1967

11

30

7

23

49.9

41.42

20.43

20

6.5

IX

Debar region (R. Macedonia – R. Albania)

The seimic activity of the territory of the Republic of Macedonia and neighboring regions is one of the strongest on the land part of the Balkan Peninsula. The hypocentres of the earthquakesare shallow, mostly located within the Earth's crust, which lower boundary in the Vardar seismic zone is at a depth of 32 km to 35 km, and in the other two zones down to 45 km and deeper. The range of the Richter local magnitudes of the earthquakes is from 0 to 7.8, and of the maximum observed intensities is from II to X degrees of the European macroseismic scale from 1998 (EMS-1998). Thus, the earthquake with the highest observed Richter local magnitude of the Balkan Peninsula, valued 7.8, occurred in the epicentral area Pehčevo – Kresna, in 1904.

The epicentral areas with the weakest seismic activity are the Zletovo and Kočani epicentral areas (parts of the East Macedonia seismic zone), the Kumanovo epicentral area (a part f the Vardar seismic zone), as well as the areas within the Pelagonia Anticlinorium, the latter being is a part of the West Macedonia seismic zone that includes the uplifting structures numbered as 14, 15 and 16 in Fig. 2. As known (Arsovski, 1997), the reason for the weak seismic activity of this anticlinorium is that it has been being a consolidated block with only oscillatory movement since the Precambrian times (probably since about 800 to 1,000 millions years ago), and still keeps the structure consisted of relicts of the Precambrian Earth’s crust. This structure differs very much in comparison with the neighboring areas.

In all the epicentral areas that show strong or moderate seismic activity, the periods of reoccurrence of earthquakes with Richter local magnitudes between 5.0 and 6.0 are from 2-3 decades to about 100 years, and of the earthquakes with local Richter magnitudes between 4.0 and 4.9 - from about one to five decades. The weaker earthquakes in these epicentral areas occur frequently.

The epicentral areas with the strongest earthquakes (Richter local magnitudes greater than 6.0 and maximum observed intensities greater than VIII degrees of the EMS‑1998scale) are given below, in Table 1. The observed periods of reoccurrence of thestrongest earthquakes are from about 500 to about 2000 years.

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