Rhodope Geodynamic Hazards, Late Alpine tectonics and Neotectonics International Conference - May 2001

Fig. 1 Active fault in Pirin Mountain

  The strongest earthquake in SE Europe occurred on April 4, 1904. Its magnitude is estimated at 7.8 by Christoskov and Grigorova (1968) and this value is accepted in the world earthquake catalogues (Shebalin et al., 1974). It is well known as Kroupnik Earthquake. Other strong earthquake with M=7.2 occurred just 23 minutes before the one mentioned above.
The location of the epicentral zone of both earthquakes has not been clarified yet. Macroseismic data show that the most significant damages and losses of lives were located in the area Kroupnik-Pehchevo-Kochani (the last ones are now in the FYR of Macedonia now). This line coincides with the Kroupnik normal fault. The macroseismic map of the 04.04.1904 earthquake (Fig. 2) shows the impact of this earthquake (or earthquakes) on the area was of intensity between X and VIII.

Fig. 2. Isoseist map of Kroupnik Earthquake (04.04.1904) according to Boykova and Nikolov (1999)

  The Kroupnik fault is a system of smaller sections with dominant strike NE-SW and angle of dip 50-62¤ (Vrablyanski, 1974). The fault is left-lateral oblique slip. There are enough evidences to be proposed that the main faulting was realized along the Kroupnik fault during the seismic events in 1904. Traces of these seismic events have been clear yet till now east of the Kroupnik village (Fig. 3). On the fault surface, traces of tectonic movements (slickensides) are still preserved.
The careful analysis of these slickensides has shown that they are connected with the evolution of the faulting process during the last seismic events in the epicentral zone of the Kroupnik Earthquake. Practically, they give a good possibility for a reconstruction of the tectonic stress field, acting when the main seismic events occurred.

Fig. 3. Remains of the surface faulting from the 1904 Kroupnik Earthquakes

  Clear traces of movements along the Kroupnik fault, which can be surely defined as the youngest ones, and probably, connected with the faulting process during the last earthquakes in 1904, were found in four outcrops along the fault surface. The computer program FAULT of Dr. R. Caputo (created in 1989) was used. This program has 3 procedures for reconstruction of the main stress axes using data of spatial orientation of the slickensides and the type of the movement.
All data from 4 points were summarized to determine the regional orientation of main axes of the tectonic field (Shanov, Dobrev, 1999). The hypothesis that the movements were simultaneous during the earthquakes was accepted for this purpose. So, the obtained summarized solution is c1=>83¤/32¤, c2=>184¤/9¤ and c3=>281¤/37¤ (Fig. 4).

Fig. 4. Reconstruction of the principal axes of the tectonic stress field in the investigated section of the Kroupnik fault (summarized solution).

  One of the most discussed problems was the place of the reported 2 m high barraging of Struma river after surface rupturing of the flood-plane terrace (Hornes, 1904). Following the remaining traces of the surface rupturing along Kroupnik fault (see the photo - Fig.2) the most probable terrain was located near the contemporary river bed, where the barraging took place. Vertical electrical sounding was performed along 3 profile lines using 4-electrodes "Schlumberger" array of maximum length AB/2 = 70 m.
The aim was to detect traces of the rupturing under the alluvium cover. This cover is composed by alternation of sandy clays, sands and gravel with electrical resistivity from 20-30(om), up to more than 1500(om). The VES curves interpretations were made by using computer programs for geoelectrical interactive modeling with automatic fitting, approximating with a minimum difference between the measured and the calculated VES curves. The program permits presentation of the measured apparent resistivity pseudosections and inverse model resistivity sections along the profiles (Shanov et al, 1999).

Fig. 5. Electrical resistivity pseudosection along VES profile III.

  The electrical resistivity cross section (Fig. 5) shows the result of the interpretations of the vertical electrical soundings along Profile III. These interpretations permit to postulate the existence of two steps of normal faulting. The SE part of the geo-electrical profiles is presented by alternation of layers with different electrical resistivity due to the different alluvial materials - sand, sandy clay and gravel. The same layer alternation is deeper from 3 m (profile III) to about 5 m (profile I) at the NW side of the fault trace.
A layer of gravel with electrical resistivity more than 400(om), covering the probable paleo-soil, has been deposed over the subsided block. This type of sedimentation is normal for the inner side of barraged river bed. The high electrical resistivity of this gravel layer is well presented on the apparent resistivity pseudosections. A seismological station equipped by S-13 vertical seismograph has been established in the region (near Kroupnk village) in mid 80-is. Since this time the reliable epi- and hypocentral determination of the local events have been established. For example more than 2 500 small earthquakes have been registered during the last 20 years (Toteva et al.,1999). This means that this zone is the most active one on the Bulgarian territory (Fig. 6).

Fig. 6. Epicentral map for all earthquakes registered with magnitudes M>2.0
for the period 1983 - 1989 for the region and surroundings (after Toteva et al., 1999)

  New station - equipped with a "Lenartz" 3 - component seismograph and analog record on the magnetic tape has been established together the with radio timing receiver by the European Geodynamic Center - Luxembourg in cooperation with the Geophysical Institute at the Bulgarian Academy of Sciences.

Boykova A., Nikolov G., 1999. Reconstruction of isoseist map of Kroupnik Earthquakes (04.04.1904, M=7.5) using geostatistical methods. Second Balkan Geoph. Congress, Istanbul, Turkey, July 5-9, 1999. Book of Abstracts, p.230-231.
Caputo R., 1989. FAULT - A Programme for structural analysis. University of Florence, Department of Earth Science, 55 p. Christoskov L, Grigorova E., 1968 Energetic and time-space characteristics of the large earthquakes in Bulgaria after 1900. Bull. Geoph. Inst., BAS, 12, 79-107 (in Bulgarian).
Hornes, R. 1904. Berichte uber das Makedonische Erdbeben von 4.IV.1904, Mit-telungen der Erd-beben. Kommission der Kaiserlichen Akademie der Wis-sen-schaften, Wien, XXIV.
Shanov S., Boykova A., Stoev D. 1999. Geophysical model of the co-seismic rupturing from the April 4-th 1904 Kroupnik Earthquake (M=7,8, SW Bulgaria). Second Balkan Geophysical Congress and Exhibition, Istanbul, Turkey, July 5-9, 1999, Book of Abstracts, p.232-233, (extended abstract).
Shanov S., N. Dobrev., 1999. Reconstruction of the tectonic stress field for the Krupnik 1904 strong earthquake. Proc. Symp. Geodynamic investigations connected with the earthquakes of 1904 in Kresna-Kroupnik., Blagoevgrad, 27-28 April,. 117-124. (in Bulgarian).
Shebalin N. V., (editor), 1974 Catalogue of Earthquakes. UNESCO, Skopje, Parts I and II. Proceedings of the Seminar on the Seismotectonic Map of the Balkan region.
Toteva T., S. Rizhikova, B. Ranguelov., 1999 Recent seismicity of the Kresna source zone (1989-98). Proc. Symp. Geodynamic investigations connected with the earthquakes of 1904 in Kresna-Kroupnik., Blagoevgrad, 27-28 April; 65-77. (in Bulgarian)
Vrablianski, B. 1974. Main lines of tectonic activition of the Earth's crust in Bulgaria during the anthropogean. - C. R. Acad. Bulg. Sci., 27, 7; 953-956.