A cross-border regional earthquake early warning system: PRESTo@CE3RN

Since 2002 the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) in Udine (Italy), the Agencija Republike Slovenije za Okolje (ARSO) in Ljubljana (Slovenia) and the Zentralanstalt für Meteorologie und Geodynamik (ZAMG) in Vienna (Austria), are collecting, analyzing, archiving and exchanging seismic data in real time, initially in the framework of the EU Interreg IIIa Italia-Austria project “Trans-national seismological networks in the South-Eastern Alps”. As outcome of the successful cooperation, in the 2013 OGS, ARSO and ZAMG decided to officially merge their seismic monitoring efforts into the “Central and Eastern European Earthquake Research Network—CE³RN”. This work reports the results of a nine-month real-time test of the earthquake early warning (EEW) algorithm probabilistic and evolutionary early warning system carried out at the CE³RN. The study allowed identifying the actions to be implemented in order to let the CE³RN become in the next future an efficient cross-border EEW system.

     

Impact of mining activities on soils and sediments at the historical mining area in Podljubelj,NW Slovenia

The abandoned Hg mine in Podljubelj was in operation with interceptions from 1557 to 1902. The entire operating period yielded about 110 000 tons of ore, from which 360 tons of Hg was produced. The objective of the research project was to establish the contents and spatial distribution of Hg in soils and stream sediments in the vicinity of the mine. On an area of 88 ha the soil was sampled in a 100 × 100 m grid. Two soil horizons (0–5 cm and 20–30 cm) were sampled in order to distinguish between geogenic and anthropogenic Hg sources. It was established that on an area of about 9 ha Hg content in soil exceeds The New Dutchlist action value for Hg (10 mg/kg). Total Hg concentrations in soil samples vary between 0.17 and 719 mg/kg, with a mean of 3.0 mg/kg. Mercury contents in stream sediments range from 0.065 to 1.4 mg/kg, with a mean of 0.64 mg/kg. The highest determined value in soils was found in the area around the former roasting furnace, where the ore was processed. Increased Hg concentrations were also found on the mine waste dump (108 mg/kg). Mercury contents in soils generally decrease with soil profile depth and with the distance from the mine and from the roasting furnace location. Mercury also appears in higher concentrations along the road that runs through the valley, which results from the use of Hg-bearing ore residues in road construction. The average enrichment factor (EF) of Hg in topsoil with respect to subsoil is 3.3. Calculated enrichment factors show higher values also for Cd (3.2), Pb (2.7), Ca (2.4) and P (1.9). The average enrichment factor of Hg in topsoil with regard to the established Slovenian soil averages (EF_slo) is 19. EF_slo of other determined chemical elements do not exceed 3.0.     

Mapping the thickness of sediments in the Ljubljana Moor basin (Slovenia) using microtremors

The Ljubljana Moor basin is characterized by moderate bedrock topography and thicknesses of Quaternary lacustrine and fluvial sediments ranging from 0 to 200 m. More than 65 boreholes which reached the bedrock were drilled in the area, but their distribution in the basin is very uneven and some data from the boreholes uncertain. There are also no data on S-velocity distribution within the basin, but seismic refraction measurements pointed out a rather uniform increase of P-velocity with depth, great impedance contrast with the bedrock and relatively small lateral velocity variations. The microtremor horizontal-to-vertical spectral ratio (HVSR) method was therefore applied as a complementary tool to seismic refraction survey to map the thickness of sediments. First, microtremors were measured at the locations of boreholes which reached the bedrock and the resonance frequencies determined. The inverse power relationship between the resonance frequency and the thickness of sediments was then determined from 53 data pairs. The quality of the correlation is moderate due to possible heterogeneities in sediments and possible 3D effects in some minor areas, but the obtained parameters correspond well to the values obtained in six other European basins. Secondly, a 16 km-long discontinuous seismic refraction profile was measured across the whole basin, leaving uncovered some larger segments where active seismic measurements were not possible. Microtremors were then measured at 64 locations along the same profile, using 250 m point spacing, without leaving any gaps. The frequency–thickness relationship was used to invert resonance frequencies to depths. These were first validated using the results of the seismic refraction survey, which showed good agreement, and finally used for interpolation in the segments of missing refraction data to obtain a continuous depth profile of the bedrock. The study has shown that the microtremor method can be used as a complementary tool for mapping the thickness of unconsolidated sediments also in areas characterized by moderate bedrock topography. As the input data are always to some extent uncertain, it is important to have a sufficiently large number of borehole data to establish a frequency–thickness relationship, as well as some additional independent geophysical information for its validation.     

Determination of masonry building fundamental frequencies in five Slovenian towns by microtremor excitation and implications for seismic risk assessment

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?C20?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?C11.1?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.     

Special Contribution: ALP 2002 Seismic Experiment

Studia Geophysica et Geodaetica -     

Nationalities of Slovenia — changing ethnic structures in Central Europe

The ethnic structure of Slovenia has in recent decades changed dramatically due to the geo-political rearrangements in Central Europe and due to the socio-economic developments in former Yugoslavia. The mixed, dark-age based ethnic pattern was at first eliminated to be replaced by a similar level of multi-ethnicity of a different origin recently. Ethnic groups of mostly Slavic (Croat, Muslim, Serb) origin have in the past couple of decades migrated to urban centers of Slovenia, thus replacing there former German and Italian minority population. Problems of attitudinal, cultural, social and linguistic nature emerge and impact the poli-structured cultural landscape of the independent nation-state.     

Mercury contents in the vertical profiles through alluvial sediments as a reflection of mining in Idrija (Slovenia)

The influence of geomorphological factors to Hg contamination of the Idrijca River alluvial sediments because of the historical mining and ore roasting activities has been studied. Main source of Hg in alluvial sediments was dumping of ore roasting residues and mining waste into the river channel and its erosion downstream. The position of the material in relation to the geomorphological properties is highly related with its Hg content. Floodplains were found to be the most contaminated geomorphological units (mean Hg content 335 mg/kg), with Hg concentration rapidly dropping in the first terrace (155 mg/kg). The least contaminated material was found in the higher terraces (3.8 mg/kg). Sampling upstream Idrija (average Hg content is 22.1 mg/kg) shows that not only mine and ore roasting plant increased Hg levels in alluvial deposits but also contaminated sites upstream Idrija contribute to Hg contamination. Geochemical background for alluvial sediments for this area is estimated to be 0.75 mg/kg. Downstream Idrija, 9 hotspots were determined where highly contaminated material is actively eroded and carries a high risk of further contamination of the So?a River and northern Adriatic Sea ecosystems.     

Distribution and speciation of mercury in soil in the area of an ancient mercury ore roasting site,Frbejžene trate (Idrija area,Slovenia)

In the initial period of mining activities in the Idrija basin (the16^th and the first half of the17^th centuries), Hg ore processing was performed at various small-scale roasting sites in the woods surrounding Idrija, by roasting ore in earthen vessels. The recovery rate of this method was very low; about half of Hg was lost, causing soil contamination and considerable amounts of waste material that could potentially leach Hg into the surrounding environment. The main aims of present geochemical study were to determine the contents, vertical distribution and speciation of Hg in soils at the roasting site at Frbej?ene trate in order to verify the extreme pollution of ancient Hg ore roasting sites in the Idrija area and to establish their significance in the wider spatial contamination of soils and aquatic systems. Soil sampling was performed at the area of the former roasting site. The organic matter-rich surface soil layer (SOM) and underlying mineral soil were sampled at 63 sampling locations. Mercury speciation was performed using Hg thermo-desorption-AAS to distinguish cinnabar from potentially bioavailable forms. The results indicate extremely high Hg concentrations with a maximum of 37,000 mg/kg in SOM and 19,900 mg/kg in mineral soil. The established Hg median in soil was 370 mg/kg and in SOM 96.3 mg/kg. Spatial distributions of Hg in SOM and soil showed very high Hg contents in the central area and decreased rapidly with distance. The results of Hg thermo-desorption measurements indicated the presence of cinnabar (HgS) and Hg bound to organic or mineral soil matter. A significant portion (35–40%) of Hg in the investigated soil and SOM samples was comprised of non-cinnabar compounds, which are potentially bioavailable. It has been shown that soils contain high amounts of potentially transformable non-cinnabar Hg, which is available for surface leaching and runoff into the surrounding environment. Therefore, contaminated soils and roasted residues at the studied area are important for persistent Hg release into the aquatic ecosystem.