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Seismicity of South-Western Bulgaria and border regions
J. Geodynamics Vol. 26, No. 24, pp. 309-325, 1998 c 1998 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain 026&3707/98 $19.00+0.00 PII: SO264-3707(97)0007S-6
Pergamon
SEISMICITY OF SOUTH-WESTERN BORDER REGIONS
S. DINEVA,”
D. SOKEROVA
BULGARIA
AND
and D. MICHAILOV
Acad. G. Bonchev St. block 3, Geophysical Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (Received 27 December 1996; recked 10 August; accepted 14 August 1997)
Abstract-Earthquakes that occurred between 1985-1991 in SW Bulgaria and border regions with Greece and the Former Yugoslav Republic of Macedonia (bounded by 22.O”E22.4”E and 40.8”N-42.4”N) are located to define the characteristics of the present seismicity.
The hypocenters of 2657 earthquakes with magnitudes 0.3 < M, < 5.1 are determined using the program HYPOLOC (modification of the HYP071 program) and a velocity model is obtained for the local area of SW Bulgaria. Earthquakes tend to cluster in seven areas. The seismic activity in the clusters changes with time and correlates mainly with the occurrence of the strongest events in each cluster. The clusters are classified by the density of earthquake epicenters. Following this classification, those with the ‘highest’ seismicity are characterised as the Krupnik cluster, and those with the ‘lowest’ seismicity, the Butkovsko (Kerkini) Lake cluster. The location and shape of the cluster areas are related to boundaries between areas of subsidence (grabens, depressions) and areas of uplift (horsts, swells). A few areas without seismicity, which are related to horst structures are outlined. Historical data exists for each of the obtained cluster areas (from 52) for earthquakes of magnitude larger than 5.0. 0 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION The most significant
event of the recent seismicity
of Bulgaria
and of the central part of the
Balkan peninsula is the strong earthquake of magnitude 7.8 (Krupnik earthquake) which occurred in 1904 in SW Bulgaria near the border with the former Yugoslav Republic of Macedonia. This earthquake is related to very active neotectonic fault structure (Brezani fault zone, Krupnik fault) (Grigorova and Palieva, 1968). In recent years the fault zone of the strongest Krupnik earthquake is characterised by low seismicity. The main purpose of this paper is to precisely define the seismicity in the region of the
*Author acad.bg.
to whom
all correspondence
should
be addressed:
309
Fax: (+ 359-2) 700226; E-mail:
dgm-sid@geophys.
Krupnik earthquake and adjacent area bounded by 22.0 Em-22.4 E and 40.8 N 42.4’N and to outline the source zones during the period I985 199 1. An attempt is also made to relate the present low and moderate seismicity to the strong earthquakes (M > 6) and to the neotectonic structures. The present paper is a summary and development of previous papers concerning the small seismicity in SW Bulgaria and border regions as well as the strong Krupnik earthquake in I904 (Sokerova et ul.. 1989a. l989b, 1990, 1995; Dineva and Sokerova. 1992).
NEOTEC'TONICINFOKMATION The investigated area is a part of the huge Macedono-Rhodope (Morava-Rhodope) Zone, formed as a result of early- to mid-Cretaceous collisions (Dabovski Ct al., 1989. Zagorcev, 1992a). It can be divided into the following neotectonic units: the Struma (Strimon, Kraistid) Zone. the Vardar (Axios) Zone, Serbo-Macedonian Swell (Massif) and the Rila-Rhodope Swell (Massif) ( Fig. I ). In extentional environments (Ritsema. 1974; McKenzie. 1972. 1978; Pavlides et u/.. 1990). as a result of disintegration due to differential vertical motion, a complex mosaic structure of horst and graben structures is formed in the whole area. The graben structures, as neotectonic units. are usually bounded by normal faults crossing the old structures. Both Struma and Vardar Lineaments present first-order lineaments tending NNW-SSE. The Struma Lineament is a fault system 40&60 km wide. It consists of numerous individual faults which form the fault zones. Principal faults strike NNW-SSE (Struma Lineament), SSW-NNE to SW-NE and WNW-ESE to W-E. The grabens are elongated along the strike of the lineament (Blagoevgrad Graben and Seres Graben) but some of them follow the strike of oblique or transverse fault sets (Simitli Graben. Strumeshnica Graben) (Zagorcev. 1992a, 1992b). The Vardar Lineament covers a relatively narrow area along the Vardar valley. This is the most unstable part of eastern Dinarides with neotectonic movements which very often change direction (Arsovski, 1974). Along the Vardar Lineament, with a dominant tendency of subsidence, hors& and grabens are formed. There are also depressions. as the Valandovo one. which are formed during the Quaternary (Arsovski, 1974). The important neotectonic first-order structure. the Middle-Mesta fault zone (lineament). is traced at a distance of about 300 km in west-east direction. The Middle Mesta fault zone and the Belasica Horst present the northern palaeographic barrier between the general subsidence of the Aegean region and the uplift of the Serbo-Macedonian and Rhodope areas (Zagorcev, 1992b). The Serbo-Macedonian Swell is elongated in its central part in NNW-SSE direction and is limited between the Vardar and Struma lineaments. It is characterised by a dominant tendency to uplift since the Pleistocene and Holocene. In Serbo-Macedonian Swell the mountain massifs of Belasica, Ograzden, Osogovo, Vardenik are manifested as neotectonic structures, separated from each other by the graben depressions of Strumeshnica and Kochani, (Arsovski, 1974; Arsovski et cd.. 1975a). The Rila-Rhodope Swell is the largest fragment of the once-united Thracian Massif. It is elongated WNW-ESE and is limited between Marica, Struma and Middle-Mesta lineaments. It shows a stable uplift tendency during almost the entire Phanerozoic, slightly differentiated with marked relative subsidence in the area of Mesta (Nestos) valley
311
Seismicity of south-western Bulgaria and border regions
42.0
41.0
.._. I-‘.--1Miocene-Eady L
^__
23.0
Pliocene-Quatemsry and fViocene continental basins
Neofectonic swells
-
la
Faults and fault zones
Fig. 1. Neotectonic sketch of the investigated region (according to Zagorcev, 1992a, 1996; Pavlides, 1996; imd Bornovas et al., 1974). Neotectonic structures: Faults: EPF - East Pirin; MMF - Middle-Mesta fault zcIne; DG ~ Delcevo; DrG PrF - Predela; WPF - West Pirin; BrF - Brezani; Grabens: BG -Blagoevgrad; Drama; GG - Goce Delcev; KG - Kochani; SdG - Sandanski; SrG - Seres; StG - Strumesnica; VG Valandovo.
(Karagjuleva et al., 1974). The Rila and Pirin horsts in the Rila-Rhodope ondary swells (Zagorcev, 1992b).
Swell are sec-
Transversal fault zones striking SW-NE are most important for the development of the Vardar and Struma lineaments and play an important role in the transverse division of the lineaments and in the bounding horsts and grabens. Usually they are of normal character, sometimes of a left lateral strike-slip character. The intersection with the major faults of Struma and Vardar lineaments are characterised by the most intensive (3-4 km) vertical neotectonic movements and very intensive seismicity (e.g. Brezani fault zone) (Zagorcev, 1992b).
S. Dineva cf ul.
312
STRONG AND MODERATE
SEISMICITY
Few catalogues of historical earthquakes exist for the region under investigation. Earthquake data used in the present study are selected from the following sources: Catalogue of earthquakes in the Balkan region (Shebalin et al., 1974), Catalogue of earthquakes in Bulgaria and border regions (Christoskov er al., 1982; Grigorova et al., 1982), Catalogue of earthquakes in Greece and adjacent areas (Makropoulos and Burton, I98 I ; Papazachos and Papazahou, 1989) Dineva and Sokerova (1979). A map of the earthquakes with magnitudes larger than 4.5 occurring up to 1991 (Fig. 2) is compiled on the basis of all available data. On this map, the dimension of the circles are proportional to the magnitude M, (e.g. Kiratzi et al., 1985) and the year of events for M, 3 5.0 is also shown. (The magnitude values ML for 198551991 are from the present work.)
42.4
,
45
/I,_.
6
/C&L& i-_
j
-j
_$ y--b?:/
___<:i-,
7
kmi
Fig. 2. Map of earthquakes with Ms24.5 during the period 52.-1991. The circle dimensions are proportional the earthquake magnitude following the relation: log r=0.405Ms1.614 derived from the relation S=0.91M,-2.73 (Kiratzi ?I cl/.. 1985). The year of earthquakes with magnitude M,>5.0 is given.
to log
Seismicity of south-western Bulgaria and border regions
313
The strongest event in the region is the earthquake of April 4, 1904 (lo:25 GMT) with a magnitude MS = 7.8 followed by an aftershock sequence that lasted until the beginning of 1907 and was preceded by six-months of foreshock sequences. The earthquake of MS = 7.1 on April 4, 1904 (lo:02 GMT) and the one with MS = 7.8 present a couple of earthquakes (Sokerova et al., 1989b). The foreshock-aftershock sequence outlines an area bounded on average by 22.7”E-23.6”E and 41.6”N42.3”N. After 1907, the seismic activity in the area of the Krupnik earthquake decreased (Sokerova et al., 1989b). Following this sequence, only six earthquakes occurred to date with magnitudes larger than 5.0 (MS = 5.0-5.5), respectively in 1910, 1911, 1917, 1921, 1936 and 1939 and after 1939, only earthquakes with M, = 4.64.8 occurred. There is historical information however about earthquakes that occurred in the same area before 1904: in 896 (MS = 6.9) 1866 (Ms = 7.1), 1893 and 1894 (MS = 5.3) 1895 (MS = 5.1) and 1896 (M, = 5.0). The epicenters of other known earthquakes in the region under investigation can be grouped in a few areas: the Valandovo area, the Radovish area, Gevgelija (Goumenissa) area, the Drama area, the Thessaloniki area, the Mesta river area and the Velingrad area. In 1931 (on March 7th and 8th) two strong earthquakes occurred in the Valandovo area SW of the Krupnik earthquake with magnitudes 6.0 and 6.7. These earthquakes were followed by an aftershock sequence lasting until 1932. There is no information about other strong events in this area. In the Radovish area, located in the north of the Valandovo area an earthquake with MS = 5.0 occurred in 1911. Other strong events occurred in 597 near the Seres town. Further east towards the town of Drama earthquakes were felt in 52 (M, = 6.5), in 1867 (MS = 6.0) and in 1829 (MS = 7.3). The epicenter of the last earthquake is outside the investigated region. An earthquake occurred in the same area in 1985 with M, = 5.1. At the southern border of the investigated region a few strong earthquakes occurred in 700 and in 1902 (MS = 6.6), in 1931 (M, = 5.1) and in 1932 (MS = 6.4). The epicenters of these earthquakes are located NE of the town of Thessaloniki. In the present century, a few earthquakes with MS < 5.1 occurred in the Mesta river area: one in 1944 (Ms = 5.1) and a second one in 1972 with magnitude MS = 4.7, followed by an aftershock sequence. In the Greece-Bulgaria border region, two earthquakes have been recorded, in 1905 (MS = 5.8) and in 1974 (MS = 5.1). The last earthquake occurred near the Butkovsko (Kerkini) Lake. In the south-western part of the investigated area near the town of Goumenissa two earthquakes of magnitudes MS = 5.0 and ML = 5.1 occurred in 1955 and 1990. In the Velingrad area, an earthquake with MS = 5.3 occurred in 1977, followed by an aftershock sequence which lasted until 1978 (Dineva and Sokerova, 1979). In the same area an earthquake with magnitude Ms = 4.2 occurred in 1905. At the NW border of the investigated region an earthquake with M, = 6.9 occurred in 1641 near the town of Kjustendil. SEISMOLOGICAL DATA FOR THE PERIOD 1985-1991 The present study is based on a data set of 2657 earthquakes, occurring between 1985 and 1991 and located in the region between latitude 40.8”-42.3” N and longitude 22.0 -
314 24.2 E. For the determination of the focal parameters of the earthquakes and their magnitude, the data published in the bulletins of Bulgarian, Greek, the former Yugoslav Republic of Macedonian. Albanian. Turkey and Romanian seismic networks are used (monthly bulletins of Bulgarian National Seismological System, seismogram readings at Skopje. Ohrid and Valandovo stations, monthly bulletins of the Geophysical Laboratory of Aristotelian University of Thessaloniki, monthly bulletins of the National Observatory of Athens and the Earthquake Data Reports of the National Earthquake Information Service (NEIS)). The map of stations is shown on Fig. 3. The arrival times used for the location of the earthquakes are from short period seismograph records. The average number of stations used for an event with magnitude 2.0 is
I VRI ) ISR
I 20
Fig. 3. Map showing
22
the location
of seismological
24
6
stations used for determination (196 1991).
26
of hypocenter
parameters
Seismicity of south-western Bulgaria and border regions
315
9, for magnitude 3.0 is 20, and for magnitude 4.0 is 45. The number of used phases (Pg, P, Sg, S) for magnitude 2.0 is 15 on average, for magnitude 3.0 is 35 and for magnitude 4.0 is up to 80. Usually the stations used are at hypocentral distances up to 2”. Only for two strongest events (ML = 5.1) are used data from stations at distances up to 5”. Hypocenters are estimated with the computer program HYPOLOC [Solakov and Dobrev, 19871, which is a modification of HYP071 [Lee and Lahr, 19721. The velocity model obtained for the area of SW Bulgaria is used: d, = 30.0 km, vp = 5.90 km/set and vs = 3.44 km/set; d, = 20 km, vp = 6.70 km/set, vs = 3.90 km/set; half-space: d = CI, vp = 8.00 km/set and d, = 4.52 km/set (Velichkova and Sokerova, 1988, 1989; Vakarelski, 1990). The mean errors of hypocentral parameter estimations are: latitude error, 1.9 km, longitude error, 2.0 km, depth error, 3.4 km, origin time error, 0.38 s, and standard deviation of the hypocenter determination (RMS), 0.43 s. The duration magnitudes are estimated according to the formula: M,(SOF) = I .92 + 2.72 log z -0.026A
(1)
where 5 is the record duration in set and A is the epicentral distance in km (Christoskov and Samargjieva, 1983). Only record durations from Bulgarian short period (S-l 3) seismic stations are used for the determination of M,. The mean error of magnitude estimation of the earthquakes is 0.43 magnitude units. A comparison between (1) - ML (SOF) and duration magnitude values, published in the monthly bulletins of the University of Thessaloniki (M,(THE)) for the period 19901991 for 452 events in the magnitude range 0.70 d M, (SOF) 6 5.10 is obtained: M,(THE)
= 0.81 *M,(SOF) + 0.70
(2)
with a correlation coefficient 0.85. On average the magnitude values M, (SOF) are lower than the M, (THE) (Fig. 4a). The difference is the smallest around M, (SOF) = 3.10, only 0.13 magnitude units, but for ML (SOF) = 5.10 it reaches 1.06 magnitude units. In the range of magnitudes M, > 4.0 there are only 2 events and the correlation between both magnitude values is not reliable. A comparison is made between magnitudes of M, (SOF) and magnitudes from Valandovo station - ML (VAY) (local magnitude act. Hadzievski, 1975) for 479 events in the same period (199G-1991) for the magnitude range 1.OOd ML (SOF) < 4.00. The following relation between the two magnitude scales is obtained: M,(VAY) = 0.82M,(SOF)+0.41
(3)
with a correlation coefficient of 0.74 (Fig. 4b). For magnitude ML (SOF) = 2.50 the magnitude M, (VAY) is 2.42. At the edges of the magnitude interval the difference is bigger: for M, (SOF) = 1.00, ML (VAY) = 1.39 and for MI, (SOF) = 4.00, M, (VAY) = 4.35. The magnitude distribution (Fig. 5) shows that the earthquakes occurring in the investigated region during the period 1985-1991 are predominantly moderate (0.3 I M, < 5.1). There are only two events with magnitudes 5.1,4 events with magnitudes between 4.0 and 5.0 and 86 events with magnitudes between 3.0 and 4.0. There are other 244 events without magnitude values because of the lack of input data on the seismic wave durations. As it is obvious from Fig. 5, the earthquake catalogue is complete for a magnitude level above 1.6. The magnitude-depth distribution (Fig. 6) shows that most events are shallower than 30 km, mainly between depths of 10 and 20 km (37% of all events). A minor seismic activity in the upper part of crust (h < 10 km) is also observed. The strongest events (ML > 4.0) are
316
S. Dineva et al. (a)
6.0 I
/
I
i
50’
m/
I i I
I
0.0 10
0.0
20
40
3.0
5.0
6.0
M,_(SOF)
I /
40
5 e
1
3.0
;
)
E 2.0
I
‘0,
0.0
; 1.0
0.0
20
3.0
5.0
M,WF) Fig. 4. Relations between magnitude
scales: a) M,(THE)
=
60
4o O.XI
M,(SOF) +0.70,
R = 0.85; b) M, (VAY)
=
0.82M, (SOF)+ 0.40. R =0.74.
located at I5 to 30 km depths. There are only a small amount which are of magnitude between 1.O and 3.0.
SEISMICITY The The period cluster
DURING
THE PERIOD
of deep events
(> 30 km)
1985-1991
epicentral map of all 2657 earthquakes is shown on Fig. 7. main outstanding feature of the seismicity in the investigated region during the 19851991 is the grouping of events in a few clusters: a cluster near Krupnik (‘1’) a near Radovish (‘2’). a cluster near Valandovo (‘3’), a cluster near Goumenissa (‘4’).
Seismicity of south-western Bulgaria and border regions
ML(SOF)
Fig. 5. Magnitude
b/M
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8 2
histogram
22
of earthquakes
2.4 2.6 2.8 3
in 1985-1991.
3.2 3.4 3.6 3.8 4
4.2 4.4 4.6 4.8 5
5 2
M/h 0
i
0
2.5
2.5
5
5
1.5
75
10
;..
10
12.5
12.5 15
15
17.S
17.5
20
2a
22.5
X.5
25
2s
p
17 5
27.5 30
39
i..
12.5
32.5 35
35
37.5
17.5
48
48
42.5
12.5 45
45 H/M
0.2 0.4 0.6 0.8
Fig. 6. Magnitude-depth
I
1.2 1.4 1.6 1.8 2
distribution
2.2 2.4 2.6 2.8 3
of hypocenters
3.2 3.4 3.6 3.8 4
in 1985-1991.
4.2 4.4 4.6 4.8 5
(M - magnitude
5.2 MI
II
ML, H - depth in km).
S. Dineva ar d.
42.
23.0
24.0
Fig. 7. Epicentral map of earthquakes in IYXS 199 I. ‘Thecircle dunensions correspond to the magnitude M,. The numbers l-7 correspond to the cluster numbers. The magnitudes of strongest earthquakes in each cluster are given
near Butkovsko (Kerkini) lake (‘S’), a cluster near Drama (‘6’) and a cluster along the Mesta river (‘7’). The rest of the events have a diffused distribution and there are a few areas without any events. The clustering character of the seismicity in the investigated region is also noticed in Sokerova c’t ui. (I 989b) and Dineva and Sokerova (I 992). The cluster ‘1’ (Krupnik area) is well formed and is elongated in SW-NE direction. It includes more than 900 earthquakes (35% of all earthquakes). The earthquakes of this cluster are predominantly of low magnitudes. The strongest event, of magnitude 4.2, occurred on 15.05.1986 is located in the center ofthe cluster. There are 21 earthquakes with a cluster
Seismicity of south-western Bulgaria and border regions
319
magnitudes between 3.0 and 4.0 located mainly at the northern and south-western parts of the cluster. Cluster ‘2’ (Radovish area) is elongated in SSW-NNE direction. This cluster includes about 350 earthquakes. The earthquakes of this cluster are also of low magnitude. The strongest event has magnitude 4.6 (28.09.1985). There are 8 events with magnitudes between 3.0 and 4.0 located at the southern part of the cluster. Cluster ‘3’ (Valandovo area) is elongated in NNW-SSE direction and includes about 150 events. The strongest earthquakes in this cluster of magnitude M, = 4.3 occurred on 31 .01.1990 in the most northern part of the cluster. There are only 4 earthquakes with magnitudes between 3.0 and 4.0. Three of these events are in the most northern part of the cluster area also and the fourth one is in the most southern part. There is information (from the seismogram readings at Skopje) that a number of events in the northern part of the cluster are due to explosions. Cluster ‘4’ (Gevgelja or Goumenissa area) is formed during 1990-1991 after the earthquake of magnitude 5.1 (21.12.1990). The area of this cluster is elongated in NE-SW direction and includes more than 400 earthquakes. The epicenter of the strongest event is located in the south-eastern edge of the area cluster. The strongest aftershocks have magnitudes between 3.0 and 4.0 (12 events). Before 1990 there is only one event (13.05.1989) with magnitude 3.1 located in the northern part of the cluster. Cluster ‘5’ (Butkovsko or Kerkini lake area) is outlined along the Struma river between the Butkovsko (Kerkini) lake and the Greece-Bulgaria border region. This cluster area follows the direction of Struma river in this section (NE-SW). It includes more than 100 events. The earthquakes of this cluster are small. The strongest event is of magnitude 3.9 (25.10.1991). There are 7 events with magnitudes between 3.0 and 3.9. The epicenters of the stronger events are distributed mainly around the Butkovsko (Kerkini) lake. Cluster ‘6’ (Drama area) is formed due to the earthquake of magnitude 5.1 on 09.11.1985. This cluster includes only 60 events. Among the earthquakes there are 6 events of magnitude between 3.0 and 4.0. Cluster ‘7’ (Mesta river area) is outlined along the Mesta river, mainly in NE direction from it. The cluster trends almost in a NW-SE direction. The borders of this cluster are not very well defined and magnitudes of the earthquakes are less than 3.4 (30.09.1987). Around 350 events, or, more than 13% of the total are diffused and they do not form a group. Among these, there are a few with magnitudes between 3 and 4. Well defined areas without earthquakes are outlined to the south and NW of Krupnik cluster, as well as between areas of clusters ‘3’ and ‘5’ (along the Greece-Bulgaria border region). A better idea of the spatial distribution of the epicenters is obtained from the earthquake density map (earthquake numbers per 7 years in grid cells of area 0.05” x 0.05” or 23 km2) (Fig. 8). The isolines of this map outline the clusters and the earthquake density of each one of them. The ‘most active’ cluster area in the investigated region is the Krupnik, cluster ‘1’. For this cluster, the isolines outline an area consisting of two relatively separated parts: the first one with the biggest maximum in the whole region with more than 70 events (at the northern part of the cluster), and the second one with maximum 30 events. The next cluster with the higher density is cluster ‘4’ where one maximum with more than 50 events is observed. Next is cluster ‘7’ with a maximum of 30 events. Two clusters: ‘2’ and ‘3’ have maximum isolines of 20 events and at cluster ‘2’ they appear in two separated places. The cluster with the lower density is cluster ‘6’ with a small maximum of 10 events and last in this classification is the cluster ‘5’ with a maximum of only 5 events.
320
42.1
Fig. 8. Density map of earthquakes in 1985 I YY I. N IS the number of earthquakes mately 23 km’) for 7 years. On the map are shown the neotectonic faults according ( 1996) and Bornovas et al. C1974).
in area 0.05 x 0.05 (approxito: Zagorcev (1996). Pavlides
The seismicity is traced in time with a set of maps for each year between 1985 and 1991 (Fig. 9). Only the area of Krupnik cluster ‘1’ has an almost constant seismic regime during this period. The seismic activity in the other clusters changes with time and correlates mainly with the strongest events in each cluster and their aftershocks. Cluster ‘2’ was formed at the end of 1985 after the earthquake of magnitude 4.6 (28.09.1985). From 1985 to 1989 the seismic activity decreases, after which in 1990 some seismicity increase is observed without any connection to any strong earthquakes (earthquakes in 1990-1991 are of magnitude below 2.9). Cluster ‘3’ is formed from 198.5 to 1990 with some increases of seismicity in 1986. Magnitudes of earthquakes up to 1990 are below 2.9. In 1990, after the earthquake of magnitude 4.3, (31 .Ol. 1990) there is an increasing of seismic activity. More than one third of all events occurred in this year. Cluster ‘4’ formed in 1990 after the earthquake of magnitude 5.1 occurred on 2 1.12.1990 and after it, the seismicity decreases gradually. Only 13 events of this cluster are recorded before 1990. Cluster ‘5’ formed in 1988-1989 when four earthquakes of magnitude 3.1L3.6 occurred. The seismic activity is the strongest in 1991. During this year, one third of the cluster events were recorded, and one event of magnitude 3.9 (25.10.1991).
Seismicity
of south-western
Bulgaria and border regions 321
Fig. 9. A set of epi central maps for 1985-1991 .Vertical
arrows show the strongest
their magnitudes
are given.
earthquakes
in e ach cluster and
322
Cluster ‘6’ was formed there is minor seismicity Cluster ‘7’ originated nitude 3.0-3.4 occurred.
S. Dineva cr d
by the earthquake of magnitude 5. I (09.11.1985). Following that up until 1990. In 1991 some activation is observed. in 1986 and developed during 1987~ 1988 when 5 events of magThis cluster is seen in 1989--1990 as well.
The comparison between the maps of the strong earthquakes during the period 52--l 99 1 (Fig. 2) and all earthquakes in 1985 1991 (Fig. 7) shows, that in the areas of all clusters or close to them there are earthquakes of magnitude larger than 5.0. The comparison between observed seismicity in 1985%1991 (Fig. 7) and the density map with the neotectonic sketch of the region (Fig. 8) shows. that the most active seismic cluster area ‘I’ (Krupnik) follows the direction of Brezani fault zone (Krupnik fault) and crosses the Struma lineament from Serbo-Macedonian swell to Rila and Pirin horsts in a general WSW-ENE direction. The western edge of the cluster area ‘I’ reaches the Bregalnica river near the town of Berovo and the eastern edge reaches the boundary zone between Rila and Pirin horsts. The most active is the NE part of this cluster area. This coincides with the Simitli graben and the conJunction of Brezani and Predela faults. The maximum on the density map corresponds to the intersection of the fault structures at depth (Sokerova cz (I/.. 1995). The second smaller maximum, which is in the south-western part of the cluster area, coincides with the southern edge of the Delcevo graben. The prevailing earthquake fault type in this cluster area is normal with a fault strike ENE--WSW (Dineva and Sokerova, 1992). The orientation of the aftershock area of the earthquake in 1904 is in almost a NESW direction (Sokerova rt II/., 1989b), the same orientation as the cluster. Cluster ‘2’ (Radovish) develops in NED SW direction, crossing the Vardar lineament. at the boundary between a neotectonic depression and an area of uplifting (Arsovski et al.. 1975b). The fault plane solution of the strongest earthquake in this cluster in 1985-1991 (M, = 4.6) is of normal type with a fault plane in NE-~SW direction, e.g. it is the same as the cluster area direction (Dineva and Sokerova. 1992). Cluster ‘3’ Valandovo crosses the Vardar lineament also (direction NNW--SSE). It is developed at the western edge of Belasica horst and Strumeshnica graben (Zagorcev. 1992a). The maximum of this cluster coincides with a neotectonic fault with WNW-ESE direction, separating these two structures. There are only two fault plane solutions for the earthquakes from this cluster (Dineva and Sokerova. 1992). The fault strikes of these earthquakes are NE--SW, e.g. they do not coincide with the elongation of the cluster area. The first one of these solutions is of strikeslip type and the other of dip-slip type. Cluster ‘4’ (Goumenissa) which is developed in NESW direction follows the direction of a neotectonic fault, separating a quaternary basin and an area with uplift (Bornovas rt N/., 1974). The fault plane for the strongest earthquake in this cluster area is in the same direction, NE--SW, as the cluster area elongation. Cluster ‘5’ (Butkovsko (Kerkini) lake) is developed with N&SW direction in the contact area between the south-eastern edge of Belasica horst and the northern part of Seres graben. The maximums of the cluster area coincide with the conjunction of Middle Mesta fault and Struma lineament zone (Zagorcev, 1996). Three of the five fault plane solutions obtained for the earthquakes from this cluster are strike-slip (with strike NE-SW) and two are reverse type (strike N-S) (Dineva and Sokerova, 1992). Cluster ‘6’ which is with N-S direction has a maximum close to the fault bounding from the north the Drama graben (Zagorcev, 1992a). Fault plane solutions obtained for this cluster area (only for three events) have a
Seismicity of south-western Bulgaria and border regions
323
strike almost E-W, the strongest event (M, = 5.1) being of strike-slip type and two others, of dip-slip type. All the fault planes are almost vertical (Dineva and Sokerova, 1992). Cluster ‘7’ with a NW-SE direction coincides with the area between the eastern boundary of Pirin horst (East Pirin fault) and the western boundary of West-Rhodope swell (Zagorcev, 1992a). The elongation of this cluster is in the same direction as the East Pirin fault. The fault-plane solutions obtained for this area do not have a prevailing type. Areas without seismicity coincide with the horsts of West Osogovo, Rila, southern part of West-Rhodope, Ograzden, north Pirin, Belasica, Vertiskos-Krusla, Pangeon.
CONCLUSION The results of the present study are based on the catalogue of 2657 earthquakes in 1985199 1 with magnitudes smaller than 5.1. The depths of earthquakes are to 30 km with a few exceptions to 50 km. The depths of the strongest events range from 15 to 30 km. The seismicity in the region is characterised by clustering. Combining the epicentral map and the density map, the sites and the elongations of seven clusters are defined. Areas without seismicity are also defined. The seismicity time history of the period 1985-1991 is plotted. The seismic activity in the clusters changes with time and correlates mainly with the strongest events in each cluster. Only the Krupnik cluster is characterised by almost constant seismicity. A review of the strong and moderate earthquakes in the region during the period from 52 to 1991 shows that in each cluster, there is at least one earthquake with a magnitude larger than 5.0. An attempt is made to relate the observed clusters with the neotectonic structures. It is established that all clusters are related to boundaries between areas of subsiding (grabens, depressions) and areas of uplift (horsts and swells). Areas of small seismicity coincide with horst structures. Acknowledgements-This research was made possible by Bulgarian Ministry of Education, Science and Technologies contract N0.543/1995. We thank Prof. I. Zagorcev from the Geological Institute of the Bulgarian Academy of Sciences for the articles provided and discussions concerning the tectonics of the region. We also thank Dr. E. Botev from the Geophysical Institute of the Bulgarian Academy of Sciences for the help in collection of bulletin data.
REFERENCES
Arsovski, M. (1974) General characteristics
of neotectonic structure in SFR Yugoslavia,
Proceedings of the seminar on the seismotectonic map of Balkan region, Dubrovnik, 17-26 April, 1973, UNESCO, Skopje, 179-189.
Arsovski, M., Radulovic, V., Prelogovic, E., Sikosek, B. and Vidovic, M. (1975) Common neotectonic features of the territory of Yugoslavia. Acta Seismologica Zugoslauica 2-3, 23-28. (In Serbian). Arsovski, M., Petkovski, R., Miklavz, F., Prelogovic, E., Radulovic, V., Sikosek, B. and Vidovic, M. (1975) Neotectonic map of Yugolavia. Acta Seismologica Zugoslavica 2-3. Bonchev, E., Bune, V. I., Karagjuleva, J., Kostadinov, V., Reisner, G. I., Rizhikova, Sn., Sokerova, D., Christoskov, L., Shebalin, N. V. and Sholpo, V. N. (1983) A method for compilation of seismic zoning prognostic maps for the territory of Bulgaria. Evaluation of seismic hazard, Problems of engineering seismology 24, 5-35. (In Russian). Bornovas, J., Koroneou, V. and Karadasi, S. (1974) Neotectonic map of Greece. Pro-
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ceedings oj’the seminar on the seismotectonic map of’ Balkan region, Dubrovnik, 17-26 April, 1973, UNESCO, (application No 19). Christoskov, L., Sokerova, D. and Rizhikova Sn. (1982) Catalogue ojhistorical earthquakes in the territory of Bulgaria, Geophysical Institute of Bulgarian Academy of Sciences, Sofia (manuscript). Christoskov, L. and Samargjieva, E. (1983) Investigation on the seismic waves duration as energy characteristic of earthquakes. Bulgarian Geophysical Journal 9(l), 79-87. (In Bulgarian with English abstract). Dabovski. Ch., Boyanov, I., Harkovska, A., Tzankov, Tz., Zagorcev, I., Karagjuleva, J., Kostadinov, V. and Paskalev, M. (1989) Late Alpine collisional orogen on the Balkan Peninsula. E,xtended abstracts of’ the XIV Congress of’ the Curpatho-Balkan geological ussociation. Sofiu, 499-~502. Dineva, S. and Sokerova, D. (1979) An investigation of the Space-time and Energetic characteristics of the Velingrad earthquake sequence (November 1977, April 1978). Bulgarian Geophysical Journal V(3). 53-64. (In Bulgarian with English abstract). Dineva, S. and Sokerova, D. (1992) Seismic activity and structures in Kresna zone (SW Bulgaria) during the period I985- 1989. Proceedings of’the XXIII general Assembly of the ESC. Prague, 8-12 Sept., 1, 52-55. Grigorova. E. and Palieva, K. (1968) Macroseismic characteristics of the destructive earthquake from 4.4.1904. Bull. Geophys. Inst.. Sofia 12, 109-l 12. (In Bulgarian). Grigorova, E., Sokerova. D., Christoskov, L. and Rizhikova, Sn., Catalqque qf’earthquakes in XX century in the territory, of’Bulgariu, Geophysical Institute of Bulgarian Academy of Sciences, Sofia (manuscript). Hadzievski, D. (1975) Seismicity of the Territory of SR Macedonia, Seismological Obs. Univ. Kiril and Metodij, Skopje. 199 pp. (in Macedonian). Karagjuleva, J., Kostadinov. V. and Matova, M. ( 1974) Neotectonic map. Proceedings crj’ the Seminar on the Seismotectonic map of’ Balkan region, Dubrovnik, 17-26 April, 1973. UNESCO, 1033109. Kiratzi, A. A., Karakaisis, G. F., Papadimitriou, E. E. and Papazachos, B. C. (1985) Seismic source parameters relations for earthquakes in Greece. Pageoph 123, 2741. Lee, W.H.K. and Lahr, J.C. ( 1972) HYP07 I : A computer program for determining hypocenter, magnitude and first motion pattern of local earthquakes, U.S. Geol. Surv. OpenFile Report, 100 pp. Makropoulos, K. C. and Burton, P. W. (198 I) A catalogue of seismicity in Greece and adjacent areas. Geophys. J. R. ustr. Sock. 65, 741 762. McKenzie, D. (1972) Active tectonics of the Mediterranean region. Geoph_v.s. J. R. astr. Sot, 30(2), 1099185. McKenzie, D. (1978) Active tectonics of the Alpine-Himalayan belt: the Aegean Sea and surrounding regions. Geophys. J. R. astr. Sac. 55, 217 -2.54. Papazachos, B. and Papazachou. C. (1989) The earthquakes in Greece, Zete, Thessaloniki, 356 pp. (in Greek). Pavlides, S., Mountrakis, D., Killas, A. and Tranos, M. (1990) The role of strike-slip movements in the extensional area of Northern Aegean (Greece). A case of transtensional tectonics. Annales Tectonicae W(2), 1962 1 I. Pavlides, S. (1996) First palaeoseismological results from Greece. Annali di Geojisica XXXIX(3), 5455555. Ritsema, A.R. (1974) The earthquake mechanisms of the Balkan region, UNDP Project REM/70/i 72, UNESCO report, De Bild, 87 pp. Shebalin, N.V., Karnik, V. and Hadzievski, D. (editors) (1974) Catalogue @‘earthquakes, Part I, 1901-1970 and Part II, prior to 1901. UNDPjUNESCO Survey of the seismicity of the Balkan region, UNESCO, Skopje.
Seismicity of south-western Bulgaria and border regions
325
Sokerova, D., Velichkova, S. and Georgiev, Tz. (1989a) Seismic activity of the Kresna earthquake source (1981-1982). Proceedings of the XIV Congress of the Carpatho-Balkan Geol. Assoc., Sofia, Bulgaria, 1989, 1031-1034.
Sokerova, D., Velichkova, S. and Dineva, S. (1989b) Development of the Kresna earthquake source in SW Bulgaria. Proceedings of the 4-th International Symposium on the analysis of seismicity and seismic risk, Bechyne Castle, Czech., Sept. 4-9, 1989, 93-100.
Sokerova, D. and Velichkova, S. (1989) Velocity characteristics of the Earth’s crust in Bulgaria. Gerlands Beitr. Geojisik, Leipzig 98(5), 398408. Sokerova, D., Velichkova, S. and Dineva, S. (1990) Characteristics of the seismic structures in Bulgaria-Kresna source. Proceedings and activity report of the XXZZgeneral Assembly of the ESC, Barcelona, 17-22 Sept., 1990, 691-696.
Sokerova, D., Dineva, S. and Velichkova, S. (1995) On the Krupnik earthquake source nature. Bulgarian Geophysical Journal Xx1(2), 21-30. (In Bulgarian with English abstract). Solakov, D. and Dobrev, Ch. (1987) Program for calculating of Basic Kinematik Earthquake Parameters on PC. Bulgarian Geophysical Journal Xx111(4), 10&104. (In Bulgarian with English abstract). Vakarelski, I. (1990) Kinematic Parameters of the Earthquakes occurred in the RilaRhodopian region 1988-1989 and compilation of local velocity model. BCs thesis, Sofia State University, (In Bulgarian). Velichkova, S. and Sokerova, D. (1988) Methodical aspects in the determination of velocities of the seismic waves in the upper crust. Recent seismological investigations in Europe, Proceedings of the XIX Gen. Ass. of ESC, Moscow, 1984, 505-5 11. Zagorcev, I. S. (1992) Neotectonic development of the Struma (Kraistid) Lineament, Southwest Bulgaria and Northern Greece. Geol. Msg. 129(2), 197-222. Zagorcev, I. (1992) Neotectonics of the central parts of the Balkan Peninsula: basic features and concepts. Geologische Rundschau M(3), 635-654. Zagorcev, I. (1996) Late Alpine (Palaeogene-Early Miocene) tectonics and neotectonics in the central part of the Balkan Peninsula. Z. geol. Wiss. 24(1/2), 91-112.
Pergamon
SEISMICITY OF SOUTH-WESTERN BORDER REGIONS
S. DINEVA,”
D. SOKEROVA
BULGARIA
AND
and D. MICHAILOV
Acad. G. Bonchev St. block 3, Geophysical Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (Received 27 December 1996; recked 10 August; accepted 14 August 1997)
Abstract-Earthquakes that occurred between 1985-1991 in SW Bulgaria and border regions with Greece and the Former Yugoslav Republic of Macedonia (bounded by 22.O”E22.4”E and 40.8”N-42.4”N) are located to define the characteristics of the present seismicity.
The hypocenters of 2657 earthquakes with magnitudes 0.3 < M, < 5.1 are determined using the program HYPOLOC (modification of the HYP071 program) and a velocity model is obtained for the local area of SW Bulgaria. Earthquakes tend to cluster in seven areas. The seismic activity in the clusters changes with time and correlates mainly with the occurrence of the strongest events in each cluster. The clusters are classified by the density of earthquake epicenters. Following this classification, those with the ‘highest’ seismicity are characterised as the Krupnik cluster, and those with the ‘lowest’ seismicity, the Butkovsko (Kerkini) Lake cluster. The location and shape of the cluster areas are related to boundaries between areas of subsidence (grabens, depressions) and areas of uplift (horsts, swells). A few areas without seismicity, which are related to horst structures are outlined. Historical data exists for each of the obtained cluster areas (from 52) for earthquakes of magnitude larger than 5.0. 0 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION The most significant
event of the recent seismicity
of Bulgaria
and of the central part of the
Balkan peninsula is the strong earthquake of magnitude 7.8 (Krupnik earthquake) which occurred in 1904 in SW Bulgaria near the border with the former Yugoslav Republic of Macedonia. This earthquake is related to very active neotectonic fault structure (Brezani fault zone, Krupnik fault) (Grigorova and Palieva, 1968). In recent years the fault zone of the strongest Krupnik earthquake is characterised by low seismicity. The main purpose of this paper is to precisely define the seismicity in the region of the
*Author acad.bg.
to whom
all correspondence
should
be addressed:
309
Fax: (+ 359-2) 700226; E-mail:
dgm-sid@geophys.
Krupnik earthquake and adjacent area bounded by 22.0 Em-22.4 E and 40.8 N 42.4’N and to outline the source zones during the period I985 199 1. An attempt is also made to relate the present low and moderate seismicity to the strong earthquakes (M > 6) and to the neotectonic structures. The present paper is a summary and development of previous papers concerning the small seismicity in SW Bulgaria and border regions as well as the strong Krupnik earthquake in I904 (Sokerova et ul.. 1989a. l989b, 1990, 1995; Dineva and Sokerova. 1992).
NEOTEC'TONICINFOKMATION The investigated area is a part of the huge Macedono-Rhodope (Morava-Rhodope) Zone, formed as a result of early- to mid-Cretaceous collisions (Dabovski Ct al., 1989. Zagorcev, 1992a). It can be divided into the following neotectonic units: the Struma (Strimon, Kraistid) Zone. the Vardar (Axios) Zone, Serbo-Macedonian Swell (Massif) and the Rila-Rhodope Swell (Massif) ( Fig. I ). In extentional environments (Ritsema. 1974; McKenzie. 1972. 1978; Pavlides et u/.. 1990). as a result of disintegration due to differential vertical motion, a complex mosaic structure of horst and graben structures is formed in the whole area. The graben structures, as neotectonic units. are usually bounded by normal faults crossing the old structures. Both Struma and Vardar Lineaments present first-order lineaments tending NNW-SSE. The Struma Lineament is a fault system 40&60 km wide. It consists of numerous individual faults which form the fault zones. Principal faults strike NNW-SSE (Struma Lineament), SSW-NNE to SW-NE and WNW-ESE to W-E. The grabens are elongated along the strike of the lineament (Blagoevgrad Graben and Seres Graben) but some of them follow the strike of oblique or transverse fault sets (Simitli Graben. Strumeshnica Graben) (Zagorcev. 1992a, 1992b). The Vardar Lineament covers a relatively narrow area along the Vardar valley. This is the most unstable part of eastern Dinarides with neotectonic movements which very often change direction (Arsovski, 1974). Along the Vardar Lineament, with a dominant tendency of subsidence, hors& and grabens are formed. There are also depressions. as the Valandovo one. which are formed during the Quaternary (Arsovski, 1974). The important neotectonic first-order structure. the Middle-Mesta fault zone (lineament). is traced at a distance of about 300 km in west-east direction. The Middle Mesta fault zone and the Belasica Horst present the northern palaeographic barrier between the general subsidence of the Aegean region and the uplift of the Serbo-Macedonian and Rhodope areas (Zagorcev, 1992b). The Serbo-Macedonian Swell is elongated in its central part in NNW-SSE direction and is limited between the Vardar and Struma lineaments. It is characterised by a dominant tendency to uplift since the Pleistocene and Holocene. In Serbo-Macedonian Swell the mountain massifs of Belasica, Ograzden, Osogovo, Vardenik are manifested as neotectonic structures, separated from each other by the graben depressions of Strumeshnica and Kochani, (Arsovski, 1974; Arsovski et cd.. 1975a). The Rila-Rhodope Swell is the largest fragment of the once-united Thracian Massif. It is elongated WNW-ESE and is limited between Marica, Struma and Middle-Mesta lineaments. It shows a stable uplift tendency during almost the entire Phanerozoic, slightly differentiated with marked relative subsidence in the area of Mesta (Nestos) valley
311
Seismicity of south-western Bulgaria and border regions
42.0
41.0
.._. I-‘.--1Miocene-Eady L
^__
23.0
Pliocene-Quatemsry and fViocene continental basins
Neofectonic swells
-
la
Faults and fault zones
Fig. 1. Neotectonic sketch of the investigated region (according to Zagorcev, 1992a, 1996; Pavlides, 1996; imd Bornovas et al., 1974). Neotectonic structures: Faults: EPF - East Pirin; MMF - Middle-Mesta fault zcIne; DG ~ Delcevo; DrG PrF - Predela; WPF - West Pirin; BrF - Brezani; Grabens: BG -Blagoevgrad; Drama; GG - Goce Delcev; KG - Kochani; SdG - Sandanski; SrG - Seres; StG - Strumesnica; VG Valandovo.
(Karagjuleva et al., 1974). The Rila and Pirin horsts in the Rila-Rhodope ondary swells (Zagorcev, 1992b).
Swell are sec-
Transversal fault zones striking SW-NE are most important for the development of the Vardar and Struma lineaments and play an important role in the transverse division of the lineaments and in the bounding horsts and grabens. Usually they are of normal character, sometimes of a left lateral strike-slip character. The intersection with the major faults of Struma and Vardar lineaments are characterised by the most intensive (3-4 km) vertical neotectonic movements and very intensive seismicity (e.g. Brezani fault zone) (Zagorcev, 1992b).
S. Dineva cf ul.
312
STRONG AND MODERATE
SEISMICITY
Few catalogues of historical earthquakes exist for the region under investigation. Earthquake data used in the present study are selected from the following sources: Catalogue of earthquakes in the Balkan region (Shebalin et al., 1974), Catalogue of earthquakes in Bulgaria and border regions (Christoskov er al., 1982; Grigorova et al., 1982), Catalogue of earthquakes in Greece and adjacent areas (Makropoulos and Burton, I98 I ; Papazachos and Papazahou, 1989) Dineva and Sokerova (1979). A map of the earthquakes with magnitudes larger than 4.5 occurring up to 1991 (Fig. 2) is compiled on the basis of all available data. On this map, the dimension of the circles are proportional to the magnitude M, (e.g. Kiratzi et al., 1985) and the year of events for M, 3 5.0 is also shown. (The magnitude values ML for 198551991 are from the present work.)
42.4
,
45
/I,_.
6
/C&L& i-_
j
-j
_$ y--b?:/
___<:i-,
7
kmi
Fig. 2. Map of earthquakes with Ms24.5 during the period 52.-1991. The circle dimensions are proportional the earthquake magnitude following the relation: log r=0.405Ms1.614 derived from the relation S=0.91M,-2.73 (Kiratzi ?I cl/.. 1985). The year of earthquakes with magnitude M,>5.0 is given.
to log
Seismicity of south-western Bulgaria and border regions
313
The strongest event in the region is the earthquake of April 4, 1904 (lo:25 GMT) with a magnitude MS = 7.8 followed by an aftershock sequence that lasted until the beginning of 1907 and was preceded by six-months of foreshock sequences. The earthquake of MS = 7.1 on April 4, 1904 (lo:02 GMT) and the one with MS = 7.8 present a couple of earthquakes (Sokerova et al., 1989b). The foreshock-aftershock sequence outlines an area bounded on average by 22.7”E-23.6”E and 41.6”N42.3”N. After 1907, the seismic activity in the area of the Krupnik earthquake decreased (Sokerova et al., 1989b). Following this sequence, only six earthquakes occurred to date with magnitudes larger than 5.0 (MS = 5.0-5.5), respectively in 1910, 1911, 1917, 1921, 1936 and 1939 and after 1939, only earthquakes with M, = 4.64.8 occurred. There is historical information however about earthquakes that occurred in the same area before 1904: in 896 (MS = 6.9) 1866 (Ms = 7.1), 1893 and 1894 (MS = 5.3) 1895 (MS = 5.1) and 1896 (M, = 5.0). The epicenters of other known earthquakes in the region under investigation can be grouped in a few areas: the Valandovo area, the Radovish area, Gevgelija (Goumenissa) area, the Drama area, the Thessaloniki area, the Mesta river area and the Velingrad area. In 1931 (on March 7th and 8th) two strong earthquakes occurred in the Valandovo area SW of the Krupnik earthquake with magnitudes 6.0 and 6.7. These earthquakes were followed by an aftershock sequence lasting until 1932. There is no information about other strong events in this area. In the Radovish area, located in the north of the Valandovo area an earthquake with MS = 5.0 occurred in 1911. Other strong events occurred in 597 near the Seres town. Further east towards the town of Drama earthquakes were felt in 52 (M, = 6.5), in 1867 (MS = 6.0) and in 1829 (MS = 7.3). The epicenter of the last earthquake is outside the investigated region. An earthquake occurred in the same area in 1985 with M, = 5.1. At the southern border of the investigated region a few strong earthquakes occurred in 700 and in 1902 (MS = 6.6), in 1931 (M, = 5.1) and in 1932 (MS = 6.4). The epicenters of these earthquakes are located NE of the town of Thessaloniki. In the present century, a few earthquakes with MS < 5.1 occurred in the Mesta river area: one in 1944 (Ms = 5.1) and a second one in 1972 with magnitude MS = 4.7, followed by an aftershock sequence. In the Greece-Bulgaria border region, two earthquakes have been recorded, in 1905 (MS = 5.8) and in 1974 (MS = 5.1). The last earthquake occurred near the Butkovsko (Kerkini) Lake. In the south-western part of the investigated area near the town of Goumenissa two earthquakes of magnitudes MS = 5.0 and ML = 5.1 occurred in 1955 and 1990. In the Velingrad area, an earthquake with MS = 5.3 occurred in 1977, followed by an aftershock sequence which lasted until 1978 (Dineva and Sokerova, 1979). In the same area an earthquake with magnitude Ms = 4.2 occurred in 1905. At the NW border of the investigated region an earthquake with M, = 6.9 occurred in 1641 near the town of Kjustendil. SEISMOLOGICAL DATA FOR THE PERIOD 1985-1991 The present study is based on a data set of 2657 earthquakes, occurring between 1985 and 1991 and located in the region between latitude 40.8”-42.3” N and longitude 22.0 -
314 24.2 E. For the determination of the focal parameters of the earthquakes and their magnitude, the data published in the bulletins of Bulgarian, Greek, the former Yugoslav Republic of Macedonian. Albanian. Turkey and Romanian seismic networks are used (monthly bulletins of Bulgarian National Seismological System, seismogram readings at Skopje. Ohrid and Valandovo stations, monthly bulletins of the Geophysical Laboratory of Aristotelian University of Thessaloniki, monthly bulletins of the National Observatory of Athens and the Earthquake Data Reports of the National Earthquake Information Service (NEIS)). The map of stations is shown on Fig. 3. The arrival times used for the location of the earthquakes are from short period seismograph records. The average number of stations used for an event with magnitude 2.0 is
I VRI ) ISR
I 20
Fig. 3. Map showing
22
the location
of seismological
24
6
stations used for determination (196 1991).
26
of hypocenter
parameters
Seismicity of south-western Bulgaria and border regions
315
9, for magnitude 3.0 is 20, and for magnitude 4.0 is 45. The number of used phases (Pg, P, Sg, S) for magnitude 2.0 is 15 on average, for magnitude 3.0 is 35 and for magnitude 4.0 is up to 80. Usually the stations used are at hypocentral distances up to 2”. Only for two strongest events (ML = 5.1) are used data from stations at distances up to 5”. Hypocenters are estimated with the computer program HYPOLOC [Solakov and Dobrev, 19871, which is a modification of HYP071 [Lee and Lahr, 19721. The velocity model obtained for the area of SW Bulgaria is used: d, = 30.0 km, vp = 5.90 km/set and vs = 3.44 km/set; d, = 20 km, vp = 6.70 km/set, vs = 3.90 km/set; half-space: d = CI, vp = 8.00 km/set and d, = 4.52 km/set (Velichkova and Sokerova, 1988, 1989; Vakarelski, 1990). The mean errors of hypocentral parameter estimations are: latitude error, 1.9 km, longitude error, 2.0 km, depth error, 3.4 km, origin time error, 0.38 s, and standard deviation of the hypocenter determination (RMS), 0.43 s. The duration magnitudes are estimated according to the formula: M,(SOF) = I .92 + 2.72 log z -0.026A
(1)
where 5 is the record duration in set and A is the epicentral distance in km (Christoskov and Samargjieva, 1983). Only record durations from Bulgarian short period (S-l 3) seismic stations are used for the determination of M,. The mean error of magnitude estimation of the earthquakes is 0.43 magnitude units. A comparison between (1) - ML (SOF) and duration magnitude values, published in the monthly bulletins of the University of Thessaloniki (M,(THE)) for the period 19901991 for 452 events in the magnitude range 0.70 d M, (SOF) 6 5.10 is obtained: M,(THE)
= 0.81 *M,(SOF) + 0.70
(2)
with a correlation coefficient 0.85. On average the magnitude values M, (SOF) are lower than the M, (THE) (Fig. 4a). The difference is the smallest around M, (SOF) = 3.10, only 0.13 magnitude units, but for ML (SOF) = 5.10 it reaches 1.06 magnitude units. In the range of magnitudes M, > 4.0 there are only 2 events and the correlation between both magnitude values is not reliable. A comparison is made between magnitudes of M, (SOF) and magnitudes from Valandovo station - ML (VAY) (local magnitude act. Hadzievski, 1975) for 479 events in the same period (199G-1991) for the magnitude range 1.OOd ML (SOF) < 4.00. The following relation between the two magnitude scales is obtained: M,(VAY) = 0.82M,(SOF)+0.41
(3)
with a correlation coefficient of 0.74 (Fig. 4b). For magnitude ML (SOF) = 2.50 the magnitude M, (VAY) is 2.42. At the edges of the magnitude interval the difference is bigger: for M, (SOF) = 1.00, ML (VAY) = 1.39 and for MI, (SOF) = 4.00, M, (VAY) = 4.35. The magnitude distribution (Fig. 5) shows that the earthquakes occurring in the investigated region during the period 1985-1991 are predominantly moderate (0.3 I M, < 5.1). There are only two events with magnitudes 5.1,4 events with magnitudes between 4.0 and 5.0 and 86 events with magnitudes between 3.0 and 4.0. There are other 244 events without magnitude values because of the lack of input data on the seismic wave durations. As it is obvious from Fig. 5, the earthquake catalogue is complete for a magnitude level above 1.6. The magnitude-depth distribution (Fig. 6) shows that most events are shallower than 30 km, mainly between depths of 10 and 20 km (37% of all events). A minor seismic activity in the upper part of crust (h < 10 km) is also observed. The strongest events (ML > 4.0) are
316
S. Dineva et al. (a)
6.0 I
/
I
i
50’
m/
I i I
I
0.0 10
0.0
20
40
3.0
5.0
6.0
M,_(SOF)
I /
40
5 e
1
3.0
;
)
E 2.0
I
‘0,
0.0
; 1.0
0.0
20
3.0
5.0
M,WF) Fig. 4. Relations between magnitude
scales: a) M,(THE)
=
60
4o O.XI
M,(SOF) +0.70,
R = 0.85; b) M, (VAY)
=
0.82M, (SOF)+ 0.40. R =0.74.
located at I5 to 30 km depths. There are only a small amount which are of magnitude between 1.O and 3.0.
SEISMICITY The The period cluster
DURING
THE PERIOD
of deep events
(> 30 km)
1985-1991
epicentral map of all 2657 earthquakes is shown on Fig. 7. main outstanding feature of the seismicity in the investigated region during the 19851991 is the grouping of events in a few clusters: a cluster near Krupnik (‘1’) a near Radovish (‘2’). a cluster near Valandovo (‘3’), a cluster near Goumenissa (‘4’).
Seismicity of south-western Bulgaria and border regions
ML(SOF)
Fig. 5. Magnitude
b/M
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8 2
histogram
22
of earthquakes
2.4 2.6 2.8 3
in 1985-1991.
3.2 3.4 3.6 3.8 4
4.2 4.4 4.6 4.8 5
5 2
M/h 0
i
0
2.5
2.5
5
5
1.5
75
10
;..
10
12.5
12.5 15
15
17.S
17.5
20
2a
22.5
X.5
25
2s
p
17 5
27.5 30
39
i..
12.5
32.5 35
35
37.5
17.5
48
48
42.5
12.5 45
45 H/M
0.2 0.4 0.6 0.8
Fig. 6. Magnitude-depth
I
1.2 1.4 1.6 1.8 2
distribution
2.2 2.4 2.6 2.8 3
of hypocenters
3.2 3.4 3.6 3.8 4
in 1985-1991.
4.2 4.4 4.6 4.8 5
(M - magnitude
5.2 MI
II
ML, H - depth in km).
S. Dineva ar d.
42.
23.0
24.0
Fig. 7. Epicentral map of earthquakes in IYXS 199 I. ‘Thecircle dunensions correspond to the magnitude M,. The numbers l-7 correspond to the cluster numbers. The magnitudes of strongest earthquakes in each cluster are given
near Butkovsko (Kerkini) lake (‘S’), a cluster near Drama (‘6’) and a cluster along the Mesta river (‘7’). The rest of the events have a diffused distribution and there are a few areas without any events. The clustering character of the seismicity in the investigated region is also noticed in Sokerova c’t ui. (I 989b) and Dineva and Sokerova (I 992). The cluster ‘1’ (Krupnik area) is well formed and is elongated in SW-NE direction. It includes more than 900 earthquakes (35% of all earthquakes). The earthquakes of this cluster are predominantly of low magnitudes. The strongest event, of magnitude 4.2, occurred on 15.05.1986 is located in the center ofthe cluster. There are 21 earthquakes with a cluster
Seismicity of south-western Bulgaria and border regions
319
magnitudes between 3.0 and 4.0 located mainly at the northern and south-western parts of the cluster. Cluster ‘2’ (Radovish area) is elongated in SSW-NNE direction. This cluster includes about 350 earthquakes. The earthquakes of this cluster are also of low magnitude. The strongest event has magnitude 4.6 (28.09.1985). There are 8 events with magnitudes between 3.0 and 4.0 located at the southern part of the cluster. Cluster ‘3’ (Valandovo area) is elongated in NNW-SSE direction and includes about 150 events. The strongest earthquakes in this cluster of magnitude M, = 4.3 occurred on 31 .01.1990 in the most northern part of the cluster. There are only 4 earthquakes with magnitudes between 3.0 and 4.0. Three of these events are in the most northern part of the cluster area also and the fourth one is in the most southern part. There is information (from the seismogram readings at Skopje) that a number of events in the northern part of the cluster are due to explosions. Cluster ‘4’ (Gevgelja or Goumenissa area) is formed during 1990-1991 after the earthquake of magnitude 5.1 (21.12.1990). The area of this cluster is elongated in NE-SW direction and includes more than 400 earthquakes. The epicenter of the strongest event is located in the south-eastern edge of the area cluster. The strongest aftershocks have magnitudes between 3.0 and 4.0 (12 events). Before 1990 there is only one event (13.05.1989) with magnitude 3.1 located in the northern part of the cluster. Cluster ‘5’ (Butkovsko or Kerkini lake area) is outlined along the Struma river between the Butkovsko (Kerkini) lake and the Greece-Bulgaria border region. This cluster area follows the direction of Struma river in this section (NE-SW). It includes more than 100 events. The earthquakes of this cluster are small. The strongest event is of magnitude 3.9 (25.10.1991). There are 7 events with magnitudes between 3.0 and 3.9. The epicenters of the stronger events are distributed mainly around the Butkovsko (Kerkini) lake. Cluster ‘6’ (Drama area) is formed due to the earthquake of magnitude 5.1 on 09.11.1985. This cluster includes only 60 events. Among the earthquakes there are 6 events of magnitude between 3.0 and 4.0. Cluster ‘7’ (Mesta river area) is outlined along the Mesta river, mainly in NE direction from it. The cluster trends almost in a NW-SE direction. The borders of this cluster are not very well defined and magnitudes of the earthquakes are less than 3.4 (30.09.1987). Around 350 events, or, more than 13% of the total are diffused and they do not form a group. Among these, there are a few with magnitudes between 3 and 4. Well defined areas without earthquakes are outlined to the south and NW of Krupnik cluster, as well as between areas of clusters ‘3’ and ‘5’ (along the Greece-Bulgaria border region). A better idea of the spatial distribution of the epicenters is obtained from the earthquake density map (earthquake numbers per 7 years in grid cells of area 0.05” x 0.05” or 23 km2) (Fig. 8). The isolines of this map outline the clusters and the earthquake density of each one of them. The ‘most active’ cluster area in the investigated region is the Krupnik, cluster ‘1’. For this cluster, the isolines outline an area consisting of two relatively separated parts: the first one with the biggest maximum in the whole region with more than 70 events (at the northern part of the cluster), and the second one with maximum 30 events. The next cluster with the higher density is cluster ‘4’ where one maximum with more than 50 events is observed. Next is cluster ‘7’ with a maximum of 30 events. Two clusters: ‘2’ and ‘3’ have maximum isolines of 20 events and at cluster ‘2’ they appear in two separated places. The cluster with the lower density is cluster ‘6’ with a small maximum of 10 events and last in this classification is the cluster ‘5’ with a maximum of only 5 events.
320
42.1
Fig. 8. Density map of earthquakes in 1985 I YY I. N IS the number of earthquakes mately 23 km’) for 7 years. On the map are shown the neotectonic faults according ( 1996) and Bornovas et al. C1974).
in area 0.05 x 0.05 (approxito: Zagorcev (1996). Pavlides
The seismicity is traced in time with a set of maps for each year between 1985 and 1991 (Fig. 9). Only the area of Krupnik cluster ‘1’ has an almost constant seismic regime during this period. The seismic activity in the other clusters changes with time and correlates mainly with the strongest events in each cluster and their aftershocks. Cluster ‘2’ was formed at the end of 1985 after the earthquake of magnitude 4.6 (28.09.1985). From 1985 to 1989 the seismic activity decreases, after which in 1990 some seismicity increase is observed without any connection to any strong earthquakes (earthquakes in 1990-1991 are of magnitude below 2.9). Cluster ‘3’ is formed from 198.5 to 1990 with some increases of seismicity in 1986. Magnitudes of earthquakes up to 1990 are below 2.9. In 1990, after the earthquake of magnitude 4.3, (31 .Ol. 1990) there is an increasing of seismic activity. More than one third of all events occurred in this year. Cluster ‘4’ formed in 1990 after the earthquake of magnitude 5.1 occurred on 2 1.12.1990 and after it, the seismicity decreases gradually. Only 13 events of this cluster are recorded before 1990. Cluster ‘5’ formed in 1988-1989 when four earthquakes of magnitude 3.1L3.6 occurred. The seismic activity is the strongest in 1991. During this year, one third of the cluster events were recorded, and one event of magnitude 3.9 (25.10.1991).
Seismicity
of south-western
Bulgaria and border regions 321
Fig. 9. A set of epi central maps for 1985-1991 .Vertical
arrows show the strongest
their magnitudes
are given.
earthquakes
in e ach cluster and
322
Cluster ‘6’ was formed there is minor seismicity Cluster ‘7’ originated nitude 3.0-3.4 occurred.
S. Dineva cr d
by the earthquake of magnitude 5. I (09.11.1985). Following that up until 1990. In 1991 some activation is observed. in 1986 and developed during 1987~ 1988 when 5 events of magThis cluster is seen in 1989--1990 as well.
The comparison between the maps of the strong earthquakes during the period 52--l 99 1 (Fig. 2) and all earthquakes in 1985 1991 (Fig. 7) shows, that in the areas of all clusters or close to them there are earthquakes of magnitude larger than 5.0. The comparison between observed seismicity in 1985%1991 (Fig. 7) and the density map with the neotectonic sketch of the region (Fig. 8) shows. that the most active seismic cluster area ‘I’ (Krupnik) follows the direction of Brezani fault zone (Krupnik fault) and crosses the Struma lineament from Serbo-Macedonian swell to Rila and Pirin horsts in a general WSW-ENE direction. The western edge of the cluster area ‘I’ reaches the Bregalnica river near the town of Berovo and the eastern edge reaches the boundary zone between Rila and Pirin horsts. The most active is the NE part of this cluster area. This coincides with the Simitli graben and the conJunction of Brezani and Predela faults. The maximum on the density map corresponds to the intersection of the fault structures at depth (Sokerova cz (I/.. 1995). The second smaller maximum, which is in the south-western part of the cluster area, coincides with the southern edge of the Delcevo graben. The prevailing earthquake fault type in this cluster area is normal with a fault strike ENE--WSW (Dineva and Sokerova, 1992). The orientation of the aftershock area of the earthquake in 1904 is in almost a NESW direction (Sokerova rt II/., 1989b), the same orientation as the cluster. Cluster ‘2’ (Radovish) develops in NED SW direction, crossing the Vardar lineament. at the boundary between a neotectonic depression and an area of uplifting (Arsovski et al.. 1975b). The fault plane solution of the strongest earthquake in this cluster in 1985-1991 (M, = 4.6) is of normal type with a fault plane in NE-~SW direction, e.g. it is the same as the cluster area direction (Dineva and Sokerova. 1992). Cluster ‘3’ Valandovo crosses the Vardar lineament also (direction NNW--SSE). It is developed at the western edge of Belasica horst and Strumeshnica graben (Zagorcev. 1992a). The maximum of this cluster coincides with a neotectonic fault with WNW-ESE direction, separating these two structures. There are only two fault plane solutions for the earthquakes from this cluster (Dineva and Sokerova. 1992). The fault strikes of these earthquakes are NE--SW, e.g. they do not coincide with the elongation of the cluster area. The first one of these solutions is of strikeslip type and the other of dip-slip type. Cluster ‘4’ (Goumenissa) which is developed in NESW direction follows the direction of a neotectonic fault, separating a quaternary basin and an area with uplift (Bornovas rt N/., 1974). The fault plane for the strongest earthquake in this cluster area is in the same direction, NE--SW, as the cluster area elongation. Cluster ‘5’ (Butkovsko (Kerkini) lake) is developed with N&SW direction in the contact area between the south-eastern edge of Belasica horst and the northern part of Seres graben. The maximums of the cluster area coincide with the conjunction of Middle Mesta fault and Struma lineament zone (Zagorcev, 1996). Three of the five fault plane solutions obtained for the earthquakes from this cluster are strike-slip (with strike NE-SW) and two are reverse type (strike N-S) (Dineva and Sokerova, 1992). Cluster ‘6’ which is with N-S direction has a maximum close to the fault bounding from the north the Drama graben (Zagorcev, 1992a). Fault plane solutions obtained for this cluster area (only for three events) have a
Seismicity of south-western Bulgaria and border regions
323
strike almost E-W, the strongest event (M, = 5.1) being of strike-slip type and two others, of dip-slip type. All the fault planes are almost vertical (Dineva and Sokerova, 1992). Cluster ‘7’ with a NW-SE direction coincides with the area between the eastern boundary of Pirin horst (East Pirin fault) and the western boundary of West-Rhodope swell (Zagorcev, 1992a). The elongation of this cluster is in the same direction as the East Pirin fault. The fault-plane solutions obtained for this area do not have a prevailing type. Areas without seismicity coincide with the horsts of West Osogovo, Rila, southern part of West-Rhodope, Ograzden, north Pirin, Belasica, Vertiskos-Krusla, Pangeon.
CONCLUSION The results of the present study are based on the catalogue of 2657 earthquakes in 1985199 1 with magnitudes smaller than 5.1. The depths of earthquakes are to 30 km with a few exceptions to 50 km. The depths of the strongest events range from 15 to 30 km. The seismicity in the region is characterised by clustering. Combining the epicentral map and the density map, the sites and the elongations of seven clusters are defined. Areas without seismicity are also defined. The seismicity time history of the period 1985-1991 is plotted. The seismic activity in the clusters changes with time and correlates mainly with the strongest events in each cluster. Only the Krupnik cluster is characterised by almost constant seismicity. A review of the strong and moderate earthquakes in the region during the period from 52 to 1991 shows that in each cluster, there is at least one earthquake with a magnitude larger than 5.0. An attempt is made to relate the observed clusters with the neotectonic structures. It is established that all clusters are related to boundaries between areas of subsiding (grabens, depressions) and areas of uplift (horsts and swells). Areas of small seismicity coincide with horst structures. Acknowledgements-This research was made possible by Bulgarian Ministry of Education, Science and Technologies contract N0.543/1995. We thank Prof. I. Zagorcev from the Geological Institute of the Bulgarian Academy of Sciences for the articles provided and discussions concerning the tectonics of the region. We also thank Dr. E. Botev from the Geophysical Institute of the Bulgarian Academy of Sciences for the help in collection of bulletin data.
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