Abstract
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
The Bovec basin, which is located in the alpine valley of the Soča river (NW Slovenia), was recently struck by two strong earthquakes (1998 and 2004) which caused extensive damage of maximum ...intensity VII-VIII EMS-98. Macroseismic data for both events showed large variations in damage to buildings within short distances and indicated strong effects of sediments on ground motion. A site effects study was therefore performed using H/V spectral ratios from earthquake data and from ambient noise, as well as standard spectral ratio technique using the reference station located on the edge of the basin. Following the July 12, 2004 (M
w
= 5.2) earthquake, six strong motion seismic stations were deployed in a profile across the Bovec basin to record the aftershock sequence. Accelerograms of eight stronger aftershocks (M
L
= 2.5-3.6) and additional ambient noise measurements were used in the study. Spectral ratio analyses showed that ground motion amplification occurs mainly in a frequency range of 5-10 Hz, with corresponding amplitudes in the range of 6-11. The observed range of amplification cannot be related to the total thickness of Quaternary sediments, which is up to 100 m in the Bovec basin. The variability in the main peak frequencies and in their amplitudes is therefore explained by the complex geological structure of the basin, filled with heterogeneous glacial and fluvial sediments. Irregular layers of conglomerate within sand/gravel deposits and layers of tillite result in large impedance contrasts at several interfaces within Quaternary sediments. Spectral ratios from earthquake data are therefore quite complex and show a broad range of ground motion amplification. On the other hand, ambient noise data revealed only the first stronger impedance contrast which is related in the border areas to the flysch bedrock and in the central part of the basin to a shallow layer of conglomerate. Comparison of the two H/V analyses showed that the amplitude obtained from ambient noise data is always lower than the amplitude from earthquake data. The difference can be as much as a factor of two. These quantitative results apply to weak ground motion and because of nonlinearity cannot be directly extrapolated to damaging earthquake situation. Since one and two-story houses prevail in the Bovec basin, with the main building frequency in the range of 6-11 Hz, the danger of soil-structure resonance is considerable in the area. It was presumably one of the main reasons for the relatively high level of damage observed from both earthquakes.
UDC 550.837.7:551.435.84(497.4) Andrej Gosar: Analysis of the capabilities of low frequency ground penetrating radar for cavities detection in rough terrain conditions: The case of Divaca cave, ...Slovenia High frequency ground penetrating radar (GPR) is usually applied for cavities detection in a shallow subsurface of karst areas to prevent geotechnical hazards. For specific projects, such as tunnel construction, it is important to detect also larger voids at medium depth range. However, dimensions of classical rigid low frequency antennas seriously limit their applicability in a rough terrain with dense vegetation commonly encountered in a karst. In this study recently developed 50 MHz antennas designed in a tube form were tested to detect cave gallery at the depth between 12 m and 60 m. The Divaca cave was selected because of a wide range of depths under the surface, possibility of unknown galleries in the vicinity and a rough terrain surface typical for Slovenian karst. Seven GPR profiles were measured across the main gallery of the cave and additional four profiles NE of the cave entrance where no galleries are known. Different acquisition and processing parameters were analysed together with the data resolution issues. The main gallery of the cave was clearly imaged in the part where the roof of the gallery is located at the depth from 10 m to 30 m. The width of the open space is mainly around 10 m. Applied system was not able to detect the gallery in the part where it is located deeper than 40 m, but several shallower cavities were discovered which were unknown before. The most important result is that the profiles acquired NE of the cave entrance revealed very clearly the existence of an unknown gallery which is located at the depth between 15 m and 22 m and represents the continuation of the Divaca cave. Access to this gallery is blocked by the sediment fill in the entrance shaftof the cave. The results of the study are important also for future infrastructure projects which will involve construction of tunnels through karstified limestone and for speleological investigations to direct the research efforts.
Slovenia is an earthquake-prone country with a moderate seismic hazard, characterized by relatively long recurrence intervals for strong earthquakes. With newly compiled data and revised info on ...active faults of the region, we are now able to supplement and enhance the probabilistic seismic hazard assessment, which was previously based mainly on the seismic catalog. The core of the seismic hazard model is the seismogenic source model combining various seismogenic representations i.e. area and gridded sources, and active faults. The ground motion model is characterized by a backbone model with regionalized parameters, which was also used to update the European seismic hazard model. We considered epistemic uncertainty in the most influential input parameters, implemented in a logic tree with 1377 branches. Considering the existing and forthcoming requests of the seismic design standard—Eurocode 8, we have developed the peak ground acceleration map of Slovenia and spectral acceleration maps for ten spectral periods, as well as seismic hazard curves and spectra for selected locations. The highest peak ground acceleration values (0.325 g) are at the western border of Slovenia with Italy. Other high-hazard areas extend across the Dinarides, in southeast Slovenia, and around the capital Ljubljana.
Located at the northeastern corner of the Adria microplate, the Alps‐Dinarides junction represents a key region for understanding how the Adria microplate interacts with stable Europe. However, ...little is known on how the present‐day deformation imposed by the rotation of the Adria microplate is absorbed across the Dinarides. Using morphotectonic analysis based on satellite and aerial images, accurate topographical maps, and digital elevation models combined with field investigations, we mapped in detail the three main active faults of the Northern Dinarides. Geomorphic and geological cumulative displacements ranging from a few meters to several kilometers have been identified on those faults and dated for the most recent ones using 36Cl exposure dating. Those results yielded a total right‐lateral motion of 3.8 ± 0.7 mm/yr oriented N317. Comparing our results with the motion expected from Adria rotation models suggests that the Northern Dinarides absorbs most of the predicted Adria‐Eurasia motion, thus representing the eastern boundary of the microplate. However, a significant E‐W component is lacking, suggesting that part of the stress imposed by the microplate rotation is transferred farther to the east. Finally, bounds placed on the Plio‐Pleistocene kinematics confirm that faulting onset occurred during the Early Pliocene and evidence a significant kinematic change at the Early/Middle Pleistocene boundary.
Key Points
Pleistocene slip rates of the Dinaric faults are derived from displaced markers and 36Cl CRE ages
Chronological bounds are placed on the kinematic evolution of the Dinarides over the Plio‐Pleistocene
Deformation in the Dinarides absorbs most of the Adria microplate rotation (versus stable Eurasia)
The Bovec basin, which is filled with glacial and fluvial sediments, has recently been struck by two strong earthquakes (1998 and 2004) which caused extensive damage (VII–VIII EMS-98). Strong site ...effects resulted in large variations in damage to buildings in the area, which could not be explained by the surface variations in Quaternary sediments. The microtremor horizontal-to-vertical-spectral ratio (HVRS) method was therefore applied to a 200 m dense grid of free-field measurements to assess the fundamental frequency of the sediments. Large variations in the sediment frequency (3–22 Hz) were obtained, with most of the observed values in the range 6–12 Hz. The observed frequencies cannot be related to the total thickness of Quaternary sediments (sand, gravel), but can be explained by the presence of conglomerate or lithified moraine at shallow depths. The results were compared also with the velocity structure derived from seismic refraction data. Microtremor measurements performed in several two and some three- and four-storey houses (masonry with RC floors), which prevail in the Bovec basin, have shown that the main building frequencies in the area are in the range 7–11 Hz. This indicates that damage to houses in both earthquakes in some parts of the basin was enhanced by site amplification and soil-structure resonance. Areas of possible soil-structure resonance were identified in the settlements Bovec–Brdo, Bovec–Mala vas, Čezsoča and Kal-Koritnica. Considerable changes in fundamental frequencies within short distances were established in the town of Bovec. Their values are as high as 22 Hz in the central part of the town, but diminish to 6–11 Hz in the adjacent Brdo and Mala vas districts. This is in agreement with the distribution of damage in both earthquakes, which was considerably higher in Brdo and Mala vas, although the houses in the central part of the town are older.
Microtremor investigations have proved an effective tool for assessment of site effects in cases of complex geological structure commonly encountered in young Alpine basins filled with glaciofluvial sediments which are partly cemented. Lithified layers can considerably change the fundamental frequency and, consequently, the site effects. By taking additional measurements in buildings possible soil-structure resonance can be identified.
The studied area of the northwestern (NW) Dinarides is located in the northeastern (NE)
corner of the Adriatic microplate and is bordered by the Adriatic foreland,
the Southern Alps, and the ...Pannonian basin. Its complex crustal structure is
the result of interactions among different tectonic units, the most
important of which are the Eurasian plate and the Adriatic microplate.
Despite numerous seismic studies in this tectonically complex area, there is
still a need for a detailed, small-scale study focusing mainly on the upper,
brittle part of the crust. In this work, we investigated the velocity
structure of the crust with one-dimensional (1-D) simultaneous hypocenter–velocity inversion
using routinely picked P- and S-wave arrival times. Most of the models
computed in the combined P and S inversion converged to a stable solution in
the depth range between 0 and 26 km. We further evaluated the inversion
results with hypocenter shift tests, high- and low-velocity tests, and
relocations. This helped us to select the best performing velocity model for
the entire study area. Based on these results and the seismicity
distribution, we divided the study area into three subregions, reselected
earthquakes and stations, and performed the combined P and S inversion for
each subregion separately to gain better insight into the crustal structure.
In the eastern subregion, the P velocities in the upper 8 km of the crust
are lower compared to the regional velocities and the velocities of the
other two subregions. The P velocities between 8 and 23 km depth are
otherwise very similar for all three models. Conversely, the S velocities
between 2 and 23 km depth are highest in the eastern subregion. The
NW and southwestern (SW) subregions are very similar in terms of the
crustal structure between 0 and 23 km depth, with slightly higher P
velocities and lower S velocities in the SW subregion. High
vP/vS values were obtained for the layers between 0 and 4 km depth.
Below that, no major deviations of vP/vS in the regional model from
the value of 1.73 are observed, but in each subregion we can clearly
distinguish two zones separated by a decrease in vP/vS at 16 km
depth. Compared to the model currently used by the Slovenian Environment
Agency to locate earthquakes, the obtained velocity models show higher
velocities and agree very well with some of the previous studies. In
addition to the general structural implications and the potential to improve
the results of seismic tomography, the new 1-D P and S velocity models can
also be used for reliable routine earthquake location and for detecting
systematic travel time errors in seismological bulletins.
Located in central Europe at the junction of the Southern Alps and the Dinarides, the ≈100km long Idrija fault, striking N310 and dipping ≈80°NE is often considered as the potential source of the ...historical 1511 earthquake (estimated macroseismic magnitude: Mm 6.8). Using 1/25,000 to 1/5000 topographical maps, satellite images (SPOT5), 12.5m illuminated DEM and airborne LiDAR data-derived 1-m-DEM, we examined in detail the Idrija fault trace along a 20-km-long swath in the central portion of the fault. Combining topographic and remote sensing data allows characterizing the recent activity along the Idrija fault and estimating its cumulative displacement along strike.
Between Tolmin and Godovič, the fault trace is mostly linear but appears divided into three disconnected segments of 11–20-km-long. Morphological evidences such as offset streams and ridges and major drainage abandonments suggest ongoing movement. At three sites, field observations and LiDAR-DEM analysis allow measuring cumulative dextral offsets comprised between 35 and 65m. We found two larger offsets of ≈140m and ≈360m at the southeasternmost site. The vertical component is significant and revealed by a rake ranging from 0 to 33°. The derived N303–N310 striking slip-vector is in agreement with the motion along this fault as suggested by the geodesy. Assuming that the smallest measured offsets have been recorded after the last glacial maximum (LGM, about 20ka ago), the minimum slip-rate for the Idrija fault is about 1.5mm/yr over this period of time.
•We provide a detailed mapping of the central portion of the Idrija fault trace.•We quantified cumulative offsets along the fault ranging between 35 and 360m.•A significant vertical component has been found beside the main horizontal motion.•The calculated slip-vector across the fault is in agreement with geodetic data.•The estimated Idrija fault slip-rate is comprised between 1.8 and 3.9mm/yr.
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