During heavy rainfalls between September 17 and 19, 2010 large part of Slovenia has suffered extensive floods that last for nearly two weeks. For the river Iška record discharge of 59.3 m3/s was ...measured on September 19 on the gauging station in Iška vas located at the southern rim of Ljubljansko barje. In the first hour of September 21, 2010 two weak earthquakes (ML=0.6 and ML=0.2) occurred within one minute near Iška vas. They were felt by some inhabitants who reported also a rumbling noise (brontides). During the flood recession period, the water of river Iška started to sink into the gravely stream bed or rocky left banks and the gauging profile completely dried on September 23, day and a half after the first earthquake. Water reappeared again on September 25. In the period September 21 − October 4 additional seven weak earthquakes occurred in the same area. All earthquakes from this series occurred at or near the surface and deviate in hypocentral depth from the seismicity pattern characteristic for the southern rim of Ljubljansko barje, which was analysed for comparison. The epicentres of the first two earthquakes are in good agreement with the location of the dried river bed. It is therefore probable that both phenomena are related. Analyses of seismograms have shown that it is not likely that the observed events are collapse earthquakes, but they are tectonic events. Although earthquakes were relatively weak, it seems that they could be accompanied by small near-surface tectonic movements, because they occurred at the position of a known fault. These movements are probably connected to the opening of pre-existing fissures in the karstified valley bottom, although the primary reason for sinking of the river is that high waters removed the clogged river bed that enables intensive sinking into the river bottom during the flood.
Site effects studies using microtremor free-field measurements were performed recently in five Slovenian towns characterized by increased seismic hazard to determine resonance frequency of soft ...sediments. In this study, microtremor investigations were extended to the measurements inside buildings to determine their fundamental frequencies and to assess the possible occurrence of soil-structure resonance. Measurements were performed in 66 buildings, and by spectral analysis, it was possible to determine reliably fundamental frequencies (longitudinal and transverse) for 58 buildings. Residential masonry houses with two or three floors prevail, but some buildings of up to six-floor height were also surveyed. Microtremor measurements have proved to be an effective tool for assessment of fundamental frequencies of buildings. Such experimental approach is very valuable, as analytical seismic evaluation of an existing building is usually very difficult. Statistical analysis of the fundamental frequency versus number of floors (height) was performed to generalize identification of potential soil-structure resonance. The difference in average fundamental frequency is very clear between buildings with two and three floors. The average value of both horizontal components for two-floor buildings is 9.11 ± 1.94 Hz, and for three-floor buildings, 7.03 ± 1.46 Hz. On the other hand, the difference in frequencies between three- and four-floor buildings (average for later is 6.52 ± 0.80 Hz) is very small. The average fundamental frequencies for five- and six-floor buildings are lower, 4.62 ± 0.64 Hz and 5.30 ± 0.70 Hz, respectively, with no statistically significant difference between both types. Most Slovenian towns are located in shallow sedimentary basins where the free-field soft covers frequencies are in the range 2–20 Hz. On the other hand, houses with two and three floors represent the large majority of the building stock. To assess the possible occurrence of soil-structure resonance in general, an average fundamental frequency ± one standard deviation interval is obtained for these two building heights, which gives the range 5.6–11.1 Hz. The free-field data show that this frequency range occupies from 22 % of the surveyed area in the Kobarid basin to up to 59 % in the Bovec basin and is in between for other three basins. This leads to the conclusion that the possible occurrence of soil-structure resonance is a serious issue for typical geological situations, in which towns are located.
Ljubljana is one of three regions with the highest seismic hazard in Slovenia. In addition soft sediments in the Ljubljana basin have a strong influence on seismic ground motion. We used macroseismic ...data to investigate the influence of local geological structure on earthquake intensities in greater Ljubljana area. We determined intensities for eleven earthquakes according to ground classification based on Eurocode 8 standard (EC8). The results showed a systematic increase in observed seismic intensities, determined according to European Macroseismic Scale (EMS-98), as the seismogeological characteristics of the ground deteriorated. Only one ground type (D) showed slightly lower intensities than expected. This may be due to some unrevealed geological and other factors, or because of very limited macroseismic data available for this particular ground type.
Knowledge about the crustal thickness is one of the key elements in the reconstruction of the regional tectonic history. The Dinaric mountain belt is one of the most enigmatic segments of the ...Alpine‐Mediterranean collision zone, characterized by large variations in crustal thickness and not studied sufficiently. We present a new Moho depth map for the wider Dinarides region which was created using teleseismic earthquake recordings from 87 permanent and temporary seismic stations in the region. Teleseismic data were analyzed using the receiver function method to extract converted P to S waves.
The resulting Moho topography fits well within a structural framework comprising a thicker crust under the Dinarides, which gradually becomes thinner toward the Pannonian and Adriatic domains. The profiles crossing the northwestern Dinarides are marked by a relatively sharp decrease in crustal thickness north of the main thrust front. This transition is followed by significant crustal thinning toward the Pannonian basin. The Mohorovičić discontinuity lies the deepest in the central and southern Dinarides, at depths of over 55 km. Here similarly to the northwestern segment we observe a jump in the crustal thickness when transitioning toward the Internal Dinarides, which hints at possible underthrusting (or subduction) of the Adria plate in this region. Moho depths in the transition zone toward the Pannonian basin and in the Pannonian basin proper vary between 25 and 35 km. In the Adriatic domain, we find crustal thickness ranging from 30 km to more than 45 km around the Central Adriatic islands.
Key Points
New crustal thickness map of the Dinarides and surrounding areas
Thicker crust in the central Adriatic, a deep crustal root in the south Dinarides and a tightly constrained transition from the deep Dinaric to the shallower Pannonian Moho
Jump in the crustal thickness when transitioning toward the Internal Dinarides, which hints at possible underthrusting of the Adria plate in this region
We evaluated the use of ground penetrating radar (GPR) in detecting karst cavities and discontinuities that could form potential landslide surfaces in flyschoid rocks of the Rodež open pit mine in ...Anhovo (W Slovenia). We recorded 21 GPR profiles in three consecutive benches with the unshielded 50MHz rough terrain antenna (RTA) system, and correlated them with the results of detailed structural and lithological mapping of the area. We located several karst cavities and confirmed the presence of discontinuities with the interpretation of GPR profiles alone. However, their correlation with geological and structural data gave a more precise insight into the structural setting of the studied area. The different discontinuity families specified in the Rodež open pit mine are mainly the result of the strike-slip tectonics and gradual anisotropic lithology. The complexity of the intersections of structural discontinuities and the mechanical properties of rocks contribute to the formation of sliding surfaces and the developing of karst features. Cavities and phreatic channels develop in the vadose zone in calcarenites and result from complex structural deformation and karstification factors at the open joints or larger structural fractures.
•Karst features and sliding surfaces pose a hazard for exploitation in open pit mines.•They were successfully investigated with a low-frequency (50MHz) GPR method.•Correlation with geological mapping gave insight into the structural setting.•Cavities in flyschoid rocks develop at open joints and fractures.•A detailed GPR survey could diminish the possibility of work accidents.
The Karstic thrust edge, a pronounced geomorphologic step, which is a result of the tectonostratigraphic evolution of the active Adria–Dinarides thrust zone, represents a major obstacle for the ...planned new railway route Divača–Koper. Thus, the geotechnical and structural properties as well as the geometry of the thrust-fault planes in this area are of great importance. Since geological mapping cannot give insight into the subsurface to reveal a complex 3D structure, and the numerous boreholes needed to investigate the area would be too expensive and time consuming, the application of a geophysical method such as ground penetrating radar (GPR) is needed. To test the method for determining near surface features and detecting low-angle inclined thrusts, a low frequency GPR system with 50 MHz rough terrain antenna was used to record 13 GPR profiles along all three floors of the Črnotiče quarry, where the spatial position of the Socerb thrust fault that separates limestones above and flysch layers below is relatively well documented. The profiles were positioned across selected existing boreholes. The GPR results were correlated with borehole data as well as geological mapping results. The GPR provided not only precise information on the geometry of the Socerb thrust fault, but was also very useful for establishing the position of some known as well as several potential cavities, both air- and sediment-filled. In areas further from the thrust-fault zone, where the limestone is less tectonically damaged, it was also possible to determine apparent dip angles of the strata, which after reconstruction matched the true dips gathered from geological mapping.
The bare karst surface in the southeastern part of Krk Island (Croatia) is characterized by different surface karst features, such as valley-like shallow linear depressions and partially or fully ...sediment-filled depressions of various shapes and sizes. They were noticed due to locally increased thickness of sediment and enhanced vegetation but had not yet been systematically studied and defined. Considering only the geometry of the investigated surface features and the rare traces of cave environments detected by field surveys, it was unclear which processes (surface karstification and/or speleogenesis) contributed most to their formation. The low-frequency ground penetrating radar (GPR) method using a special 50MHz RTA antenna was applied to study and describe these karst features. Three study sites were chosen and 5km of GPR profiles were positioned to include various surface features. The results obtained from the GPR investigation lead to the following conclusions: (1) an increased thickness of sediment was detected in all the investigated depressions indicating their considerable depth; (2) areas between different depressions expressed as attenuated zones in GPR images reveal their interconnection; (3) transitions between surface and underground features are characterized by a collapsed passage visible in the GPR data; and (4) an underground continuation of surface valley-like depressions was detected, proving the speleogenetic origin of such features. Subsurface information obtained using GPR indicates that the valley-like depressions, irregular depressions completely or partially filled with sediment, and some dolines are associated with a nearly 4km-long unroofed cave and developed as a result of karst denudation. In the regional context, these results suggest long-lasting karstification processes in the area, in contrast to the pre-karstic fluvial phase previously assumed to have occurred here. This research is the first application of the GPR method to survey unroofed caves worldwide and the first detailed study of such karst features in Croatia. The low-frequency GPR proved to be an efficient method not only for detecting underground continuations but also for distinguishing and identifying surface features and transition zones between surface and subsurface segments of unroofed caves and can therefore be used for recognizing similar geomorphological features.
•Unroofed caves proved using low-frequency GPR with 50MHz RTA antenna.•Surface and near surface segments of unroofed caves were detected and interpreted using GPR.•Transition zones between surface and underground features were identified and defined on the basis of the GPR image.•A conceptual model of karst denudation and development of unroofed caves in the study area is proposed.
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