Slope Failure Effects in Rocks at Earthquake - Posočje April, 12. 1998 and European Macroseismic Scale (EMS-98)


  • Renato Vidrih
  • Mihael Ribičič



On 12 April, 1998, the strongest earthquake of the 20th century with an epicentre in Slovenia shook the upper Posočje. Its magnitude was 5.8 and its maximum intensity was between the VII and VIII levels according to EMS. The epicentre was in the lithosphere in the area between the Lepena valley and the Krn Mountain Range, about 9 km deep. Apart from substantial material damage, the earthquake caused considerable changes in the environment, as there were many rockfalls and landslides which in some areas completely destroyed mountain paths. To assess the full effects of an earthquake, one must, besides the reaction of people and their surroundings, the impact on property and damage to buildings, also identify the effects felt in nature. This is why we try to compare earthquake effects on buildings and on nature wherever possible. On this basis, we try to more precisely assess the damage caused to nature which the European Macroseismic Scale (EMS) treats very generally. It is the changes in nature that provide a more reliable definition of macroseismic epicentre and earthquake intensity. The effects between the VII and VIII EMS were felt in Mala vas in Bovec and the villages Spodnje Drežniške Ravne and Magozd. The same level was ascribed to damage to Krn, at the height of 1776 metres, Javoršček, the Krnčica ridge, in the area along the Tolminka river between its source and the Polog mountain and in the mountains above the Lepena valley - Lemež, Sija. For higher levels (VI and above), the European Macroseismic Scale almost exclusively takes the damage caused to buildings into account. In sparsely inhabited areas, as is the area stricken by the earthquake on 12 April, 1998 where only the valleys are inhabited, whilst in the alpine parts there are only alpine dairy farms, hunting lodges, mountain huts and individual mountain farms, use of this scale is difficult and unreliable. The question arises whether assessment of an earthquakeis intensity would not be more precise if it took into account the loss of the natural equilibrium in the form of rockfalls and landslides. Such additional information, if interpreted correctly, can increase the reliability of an earthquakeis intensity assessment. To enable consideration of loss of the natural equilibrium as part of seismological phenomena, a comparison with damage to buildings must be made where possible and the findings extended to the entire area in question. Several approaches are possible to include natural seismological phenomena in the formation of macroseismic scales. According to the first approach, a description of seismological phenomena (like the loss of natural equilibrium) is gradually included in the description of intensity levels, i.e. in the original scale. According to the second approach, Annex C of the EMS treating seismological phenomena is defined more precisely, taking into account the recently gained expert knowledge regarding rock mechanics and spatial analyses made using GIS technology concerning an areais vulnerability which is reflected in the loss of natural equilibrium. The third and most recent approach is to create a separate scale for seismological phenomena linked only to the European Macroseismic Scale through comparative tables. Analyses of the damage caused to nature by the Posočje earthquake show that it would be useful to direct expert efforts towards including the earthquakeis impact on nature in the systematic assessment of earthquake intensity.


How to Cite

Vidrih, R., & Ribičič, M. (1998). Slope Failure Effects in Rocks at Earthquake - Posočje April, 12. 1998 and European Macroseismic Scale (EMS-98) . Geologija, 41(1), 365–410.




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