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New data on seismotectonic activity in Croatia

Seismic and structural data obtained by two projects financed by the Ministry of Science, Education and Sports of the Republic of Croatia allow us to define and characterize the major seismogenic zones of the Croatia, here briefly described. The seismogenic zone of Žumberak – Samoborsko gorje experienced the strongest known earthquake on February 11th 1699. The earthquake of August 13th 1887 (Io = VII°MCS) caused heavy damage on churches and houses in the town of Jastrebarsko and the surrounding villages. Three fault-plane solutions (FPS) calculated for this zone point to generally N-S to NW-SE trending greatest principal stress direction and the prevalence of strike-slip tectonics accommodated by steeply dipping NE–SW striking sinistral and/or NW-SE striking dextral fault sets (HERAK et al., 2009). Pliocene-Quaternary transpressional tectonics controlled by a generally N-S trending greatest principal stress direction is also evidenced by structural data, which indicate the youngest displacements along the southern margin of the Žumberak Mt. accommodated by sinistral NE-SW striking fault set. The seismogenic zone of Pokuplje lies along the Kupa River where it transects through theNW-SE trending hills of Vukomeri~ke gorice. The most important event here is the one of October 8th 1909 (M=6.0, Io=VIII °MCS). Two FPS available from this zone consistently indicate a moderately plunging SW-trending pressure axis, with potential earthquake generating fault corresponding either to NW-SE striking dextral or NE-SW striking sinistral fault set. The former seems to be more plausible seismogenic source and would be a part of the Sava fault zone, which is in this segment documented as the NE-dipping boundary normal fault along the southwestern margin of the Pannonian basin during the Neogene. At present, however, this fault segment would be reactivated and inverted accommodating dextral and reverse motions due to recently NE-SW directed compressional stress. This interpretation is additionally supported by a cross-sectional view of hypocentres indicating a seismogenic zone that dips to the NE with an angle of 60°, a dip angle typical for normal faults. The seismogenic zone of Medvednica experienced strong seismic activity in the 18th, 19th and in the beginning of the 20th century. The strongest earthquakes occurred on October 13th 1775 with the epicentral intensity of VII–VIII °MCS, on November 9th 1880 with the intensity VIII °MCS (known as the great Zagreb earthquake), on December 17th 1905 (Io = VII–VIII °MCS) and on January 2nd 1906 (Io = VII–VIII °MCS). Calculated and available FPSs indicate seismogenic activity on (1) reverse ENE-WSW striking faults and (2) along dextral or sinistral NW-SE and ENE-WSW striking faults, respectively. The hypocentres in the western part of this zone lie along a steeply SSE-dipping plane that is in agreement with the Quaternary active SE-dipping reverse fault mapped along the northern margin of Mt. Medvednica. Geometry and kinematics of this fault nicely corresponds with the NE-SW striking and SE-dipping nodal plane calculated by FPS, which indicate a reverse fault plane with a top-to-the-NW motion direction. Another two FPSs indicate seismogenic structures corresponding either to the NW-SE striking dextral or the NE-SW striking sinistral faults. The seismogenic zone of Ivanščica – Kalnik recently experienced pronounced seismic activity with only moderate events of macroseismic intensity up to VII °MCS (e.g. June 1st 1993). All three FPS from the central and eastern part of this zone (HERAK et al., 2009) indicate prevalence of compressional tectonics under N–S trending P-axis, accommodated by the E–W striking reverse faults that are in a good agreement with faults mapped along the northern and southern margins of Ivanščica and Kalnik Mts. The seismogenic zone of Bilogora – Drava extends in NW-SE direction for about 75 km between towns of Koprivnica and Virovitica. Three out of four earthquakes known from this area with I – VII °MCS had epicentres near the town of Koprivnica. The March 27th 1938 earthquake (Io = VIII °MCS) destroyed many houses and churches in the town of \ur|evac and its surrounding villages. This is the first event in Croatia for which FPS could have been computed. Potential seismogenic structure most probably corresponds with the NW–SE striking nodal plane dipping at 75° to the NE, which would nicely correlate with the major NW–SE striking Bilogora fault zone characterized by reverse and dextral faults which affected Pliocene-Quaternary strata. The seismogenic zone of Ilirska Bistrica – Senj is the NW-SE trending, 60 km long and 30 km wide zone of epicentres along the coastline of Kvarner extending from Ilirska Bistrica (Slovenia) to Senj at the NW flank of Mt. Velebit. The oldest known strong event here is the Vinodol 1323 earthquake (Io=9), later followed by earthquakes of 1750, 1870, 1878 (Io=7-8) and 1916 (ML = 5.8). Two seismogenic sources have been delineated in this zone. The first, Ćićarija fault system represents the frontal NE-dipping thrust system in the northern part of the external Dinarides and makes the eastern boundary of the gently folded and aseismic block of stable Adria (Istria peninsula). Instrumentally recorded seismicity with ML < 3.0 is partly consistent with two major thrusts that at depth of 3 km merge into the major NE-dipping fault. This fault extends to depth of ca. 18 km where it ramps from the regional NE-dipping décollement within the pre-Mesozoic basement. The second Ilirska Bistrica – Senj fault system is composed of at least six faults in en échelon relationship traced along ca. 50 km long and 10 km wide zone. In its central part, this fault system is represented by the Grobnik-Bakar fault, i.e. some 60° NE-dipping fault that ramps at depth of 20 km from the regional décollement within the pre-Mesozoic basement. Surface structural measurements and fault plane solution data point to combination of strike-slip (dextral) and reverse (top to SW) motions along this fault system. The seismogenic zone of Dalmatia is 350 km long and 100 km wide extending from Zadar up to Dubrovnik, including the coastline range and its hinterland as well as the island belt of the central-Adriatic archipelago. Here, damaging earthquakes are more frequent towards the SE, the strongest recorded at Mt. Dinara in 1986 (ML = 5.5) and 1990 (ML = 5.6), in Sinjsko polje in 1898, 1899 and 1907 (Io=8-9), in offshore Makarska in 1962 (ML= 5.9-6.1) and in Makarska hinterland in 1923 and 1942 (ML = 6.2). Further SE in Ston-Dubrovnik area evidences of pronounced seismicity date back to ancient and medieval times (e.g. in 1481, 1482, 1504, 1520, 1530 and 1563). The most important is certainly the great Dubrovnik earthquake of 1667 (Io=10), which was preceded by intense activity spread between Kotor (Montenegro) and Dubrovnik. In the area of Ston, some 40 km NE from Dubrovnik, strong events occurred in 1850 (Io=8-9) and in 1996 (ML = 6.0). The major seismogenic source in the southern Dalmatian sector of the Adriatic coastline region is the Ston-Dubrovnik fault system. It comprises a set of NW-SE striking and NE-dipping thrusts traced from offshore Ston to offshore Dubrovnik area, formed as out-of-sequence thrusts with respect to the Latest Miocene frontal thrust in this part of the external Dinarides. Surface structural measurements, FPS data and geometry of thrusts point to the prevalence of reverse top-to SW movements. The seismogenic area of Adriatic offshore of Croatia turned to be quite active especially in its central-southern sector, i.e. SE of Zadar – Ancona line. Contemporary seismicity in this sector has been revealed as quite intense and rather localized around islands ofVis, Jabuka and Palagruža.The most recent earthquake series are those of Palagruža in 1988 (main shock ML = 5.4) and Jabuka in 2003 (main shock ML = 5.9 ; HERAK et al., 2005). The most representative seismogenic source in this area is the Jabuka-Andrija thrust system. It splays off from the Paleozoic– basement at depth of ca. 15 km, cut through Neogene and Quaternary horizons up to the sea-floor. Other thrusts are described as blind thrusts with Quaternary strata clearly deformed above their tips. Our data suggest that the ongoing seismotectonic activity along the Jabuka-Andrija thrust system is strongly localized, being associated with still active growth of salt diapirs, which are the most representative structural features of active tectonics in this part of the Adriatic offshore.

seismicity; active tectonics; Croatia

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