Largest Earthquakes in Slovenia

Slovenia is a country in Central Europe, with a population of 2.1 million and covering an area of 20,271 square kilometers (7,827 sq mi). A relatively small country has a very diverse topography since it’s made up of portions of four major European geographic landscapes:  
- the European Alps,  
- the karstic Dinaric Alps,  
- the Pannonian and Danubian lowlands and hills,  
- the Mediterranean coast. 

The greatest economic damage in Slovenia is caused by natural hazards affecting agriculture, such as drought, hail, rainstorms, and frosts. Great damage is also caused by floods in settlements and by landslides and avalanches in mountain areas. Heat waves are increasingly frequent, while earthquakes occur periodically¹. 

Even though Slovenia is prone to various natural hazards, big differences exist among Slovenia regions, taking into account its geographic position. For example, frequent rockfalls and landslides mostly affect the Alpine mountains area in the west and north, fires characterize the Mediterranean landscapes in southwestern Slovenia while flooding most often occurs in the Pannonian lowlands in the southeast. Seismic activity is high in the western as well as central and southeastern parts of the country¹. 

Picture 1: Seismicity in Slovenia.

The largest earthquake in Slovenia occurred on March 26, 1511, in Idria, between three and four in the afternoon. The first shock was said to have a strength of 6.8, while the following one had a strength estimated at 7.0 to 7.2. The radius of the earthquake's effects was 750 km. 12,000 people are said to have died as a result of the earthquake². 

It is recorded that the earthquake damaged many towns and castles in Friuli and Venecia, Primorska, Gorenjska, Notranjska, and Dolenjska regions, leaving great damage mainly to churches, castles, forts, and public buildings. Considering the way of construction in those times, it is to be expected that there was a lot of damage even to residential houses that were made of brick. Wooden huts, which were typical in western Slovenia, probably survived the earthquake without serious damage. Estimates have been published that the earthquake claimed 12.000 victims, but these are probably exaggerated³. 

If we look at the 20th century, the largest earthquake occurred on April 12, 1998, in Zgornje Posočje. The magnitude 5.6 earthquake struck at 11:55 local time for Easter lunchtime at a depth of 8 km. Due to its strength, it was felt by the inhabitants of almost entire Slovenia, as well as in Austria, Italy, Germany, and even the Czech Republic. It caused damage to more than 4,000 buildings, and fortunately, it did not claim any human casualties². 

It was reported that the earthquake damaged 4,055 buildings, however, fortunately, the ground shaking did not cause any fatalities, "just" a long-term, 100-million-euro renovation, which was officially completed in 2009. Most of the money, around 80 million, was intended for the renovation of the buildings. In just over ten years, more than 1,800 units were renovated with this money. 

There were more than 400 aftershocks in the first 20 hours after the main earthquake, and more than 9,000 in the following months. The strongest aftershock occurred on May 6 at 4:52 a.m. ET and had a magnitude of 4.2⁴. 

Economic Losses due to Earthquakes

According to the World Bank, the annual average population affected by earthquakes in Slovenia is around 80,000 and the annual average affected GDP is around $2 billion. The annual averages of fatalities and capital losses caused by earthquakes are around 50 and around $200 million. However, the fatalities and capital losses caused by more intense, less frequent earthquake events can be larger than the annual averages. For example, an earthquake with a 0.4 percent annual probability of occurrence (a 250-year return period event) could cause around 1,000 fatalities and $4 billion in capital loss (around 8 percent of GDP)⁵. 

In 2021, the National Seismology Office presented a new seismic hazard map for Slovenia, taking into account the latest seismic and geo-tectonic data. The upgraded danger levels and risk assessment are based on the new findings of the past two decades. It shows peak ground acceleration as well as spectral acceleration to include all possible building frequencies. Active faults and their sources, defined by the Geological Survey of Slovenia, have been included in the map for the first time ever, meaning risk levels are now assessed not just on the basis of previous quakes but also on potential fault hotspots⁶. You can access it here. 

Sources: 
1) Komac, Blaž. 2019. Natural Hazards in Slovenia. Accessed on 29-Aug-2022. Available at: https://link.springer.com/chapter/10.1007/978-3-030-14066-3_17 
2) Ribiske karte. 2020. Najmočnejši potresi v Sloveniji. Accessed on 29-Aug-2022. Available at: https://www.ribiskekarte.si/blog/najmocnejsi-potresi-v-sloveniji/ 
3) Cecić, Ina. Potres 26. marca 1511 in njegove posledice v naših krajih - povzetek. Accessed on 29-Aug-2022. Available at: www.arso.gov.si/novice/datoteke/026427-1511%20povzetek%20za%20novinarje.pdf 
4) 24 ur. 2018. 'Potres v Zgornjem Posočju je bil učni primer za vso državo'. Accessed on 29-Aug-2022. Available at: https://www.24ur.com/novice/slovenija/potres-v-zgornjem-posocju-je-bil-ucni-primer-za-vso-drzavo.html 
5) The World Bank. Europe and Central Asia: Country Risk Profiles for Floods and Earthquakes. Accessed on 29-Aug-2022. Available at: https://www.worldbank.org/en/region/eca/publication/europe-and-central-asia-country-risk-profiles-for-floods-and-earthquakes
6) STA. 2021. New Seismic Hazard Map Shows Earthquake Risks for Slovenia. Accessed on 29-Aug-2022. Available at: https://www.total-slovenia-news.com/lifestyle/7875-new-seismic-hazard-map-shows-earthquake-risks-for-slovenia