Interviews

Croatia Earthquakes: Why, Where and When They Happen

By 19 January 2021
Why, where and when do Croatia earthquakes happen?
Why, where and when do Croatia earthquakes happen?

January 19, 2021 – The relief effort is nationwide, international. Media pages are awash with the aftermath and repercussions. The devastating earthquake in Petrinja has created unforgettable images and changed lives forever. With the ground still shaking from sizeable aftershocks, we caught up with one of the country's leading geologists, working in the field near Petrinja, to as him why, where and when Croatia earthquakes happen?

“Once in 100 years”, they said, after the large earthquake hit Zagreb in March 2020. But, in late December, another. This time near Petrinja. Then, unbelievably, an even greater tremor - the biggest yet - on the following day. The aftershocks are considerable. They arrive after those from March's earthquake had only just begun to subside. It's a little wonder people can't sleep at night.

Stood on this shaky surface, our nerves on edge and with too many questions to ask, TCN tried to find some solid ground by turning to science. We spoke with one of the best-placed people in the country to tell us all about Croatia Earthquakes - why they happen, where they happen and when they will happen. We interviewed Josip Stipčević of the Geophysics Department, University of Zagreb, while he was on-site in Petrinja.

JosipStipevi.jpgJosip Stipčević and the Geology Department of the Faculty of Science, University of Zagreb © University of Zagreb

My name is Josip Stipčević and I'm an assistant professor in the Department of Geophysics at the Faculty of Science, University of Zagreb. At our faculty, part of what we do is explore underground and undertake research using seismic waves. We record earthquakes. The geology department explores rocks.

Seismologists and geologists have instruments all around Croatia that record earthquakes. Immediately following the large earthquake near Petrinja, I joined geology colleagues in the near vicinity of the earthquake to look for surface ruptures, visible cracks on the surface. Because of the weather conditions and type of ground, it was important we go immediately because some of these expressions of the earthquake may be quickly lost.

The work we did will form the basis of a report that takes in geological and seismological findings. It's important to integrate these different sets of data with satellite and GPS measurements to learn as much as possible about the earthquake - why it happened, where it happened, what actually happened - how it progressed. You want to build up the most detailed and accurate account of the event. By doing this, it may give us a better understanding of what might take place in the future.

What we already knew was that this is an area where Croatia earthquakes happen. What we don't know is how often they happen or exactly why they happen - what are the forces that drive this build-up of strain in the earth's crust?

Croatian_Geological_Institute.jpgOutdoor educational board constructed by Professor Stipčević's colleagues from the Croatian Geological Institute © Croatian Geological Institute

There were quite a lot of us working in the area, over at least two different sites. It will take months more to analyze all of our findings. I was with a group from the University of Zagreb but there were colleagues from the Geology Institute also. A lot of us.

Croatia Earthquakes: Why do they happen?

Our Earth is a geologically alive planet. The Earth's core is hot. It is gradually cooling, over billions of years since it was formed. The heat must be released. There is convection taking place within the earth - a heat transfer. This is what drives the movement of the solid, outer layer of the earth which, using technical terms, we call the lithosphere. On the top of the lithosphere, there is a crust - like the outer layer of an onion. Here, the convection of the earth drives the movement of different tectonic plates that sit on the surface.

The_Lithosphere.pngThe Lithosphere, or 'Earth's crust' © KDS44

The surface of the Earth is broken up into several major parts. These are what we call the tectonic plates. It's like a jigsaw puzzle, except not all of the pieces fit so comfortably. These tectonic plates are moving because of the convection. They interact with each other. In some places, you have a divergence - where the plates are moving away from each other and new plates are forming. Then, you have plates where there is convergence - the plates are coming together. In those places, one plate is often going underneath the other or, like here - where we have two continental plates coming together, neither of which can sink beneath the other (because continental plates are more buoyant), we have an interaction where the plates collide. It is this collision that creates all of the mountain ranges in Europe - the Pyrenees, the Alps, the Dinaric Alps and so on. When the tectonic plates collide, there is an expression in the build-up of the energy. That is what an earthquake is.

tectonic_plates.pngThe major tectonic plates of the Earth in the present day © Public domain

The major plates in our region are the African plate and the Eurasian plate. They interact through the Mediterranean. But, it's not so simple. You also have small, fragmentary parts of the plate that are ‘stuck’ between these larger plates. One of these fragments, which is still attached to the African plate, is the Adriatic plate. It exists in the area of the Adriatic sea. Because of the movement of the Adriatic plate, you have material on both sides which is strained. This strain, or stress-energy, builds up in the crust and is released in Croatia earthquakes.

The_Adriatic_Plate.jpgThe Adriatic Plate © Public domain

It is this collision that has formed all of the mountains that exist all around the Adriatic - the Apennines, the Alps and the Dinaric Alps. It is also responsible for the range of volcanoes we find running down the west of Italy and the ones more towards the south of Italy, some of which are still active.

There are two kinds of tectonic plates - continental plates and oceanic plates. They are both different. The oceanic plates are thin and dense, very heavy. The continental plates are thick and more buoyant, less dense. It is so buoyant that it cannot sink back down into the mantle - the deeper parts of the Earth. But, the oceanic plates are dense enough to sink back into the mantle. When that happens, one expression is the formation of volcanoes.

The_volcanoes_of_Italy.jpgThe volcanoes of Italy, caused by the Adriatic Plate © Public domain

The Adriatic plate is partly oceanic and partly continental. Broadly speaking, the oceanic part of this plate is sinking beneath Italy, producing volcanoes. In Croatia, we mostly have the continental part of this plate. It cannot sink, so it instead collides and we have Croatia earthquakes. We had volcanoes here maybe 20 or 30 million years ago, but the part of the oceanic plate responsible for those was consumed. I'm speaking in very broads terms here - some of what you're asking me is really quite heavy stuff, ha! It's much more complex when you delve into it.

Croatia Earthquakes: What are the fault lines?

If you take a pencil between your hands and try to break it, the stress you create will find a point at which the pencil will break. The break in the pencil is like a fault line. It's a different kind of strain within the earth's crust, but the same principle applies. The force is absolutely ginormous and this action has been happening for billions of years, in our region alone it has been happening for many millions of years.

Because this has been taking place over such a long period of time and because the movement is still happening, some of the fault lines become inactive. Others are still active and new ones may even be created. In other parts of the world, these fault lines can run hundreds of kilometres long.

San_Andreas_Fault.jpgThe San Andreas Fault in California © John Wiley User: Jw4nvc - Santa Barbara, California

We don't really speak of 'active faults' because it's so hard to measure them. Some of them exist very deep in the earth. Some of them have surface expressions, but not all. So, it's not easy to say 'we have this many active faults here in Croatia'. You can say that in other parts of the world - everyone has heard of the San Andreas fault in California, it is a huge surface expression. Here in Croatia, the fault lines are smaller. The interaction is not so vigorous as in California, which is where the Pacific Plate and the North American Plate meet.

Croatia Earthquakes: Where do they happen?

Rather than active faults or fault lines, it is more accurate in Croatia to speak of active fault areas. We know which areas are tectonically active - where you may experience Croatia earthquakes. Those areas are southern Dalmatia, the Rijeka region, the Zagreb region and the Petrinja / Sisak region.

RTL_Television_depicted.jpgHow Croatia's RTL Television depicted Croatia's fault lines - or fault areas - in their graphic © screenshot

In Dubrovnik, you had one of the most major Croatia earthquakes of the last millennium during the 17th century. It was considerably larger than the one we just had in Petrinja. The whole city was devastated. Extensive damage. Dubrovnik and southern Dalmatia is the area that is most prone to larger Croatia earthquakes. I have just received a grant from the Croatian Science Foundation to explore just this. We are due to start in just a couple of months.

Dubrovnik_from_before_the_major_earthquake.jpgA painting of Dubrovnik from before the major earthquake of the 17th century © Public domain

From what we know, the areas of the country which experience the least seismic activity are Istria and some parts of Slavonia and Baranja. Other parts of Slavonia do have some seismic activity - there were famously earthquakes in the Dakovo area in 1884 and in a wider area of Slavonia in 1964. But these were only moderately strong. From what we know, Istria, Slavonia and Baranja are definitely the safest places where you will not experience a large earthquake. In Istria, you do not see any seismic activity at all. This is because Istria is the only part of Croatia which is on the Adriatic plate. All of the rest of Croatia is on the European plate.

Croatia_location_map.pngCroatia: the areas within red circles are presumed - for now - to be at extremely low risk of a major earthquake © NordNordWest, adapted

Each mountain you see in Croatia is essentially a fault area. That's where the ground somehow had to rise. It is only a question of when that fault line was active. It might have been millions of years ago and the mountain is merely evidence that this once happened, like with older mountain ranges such as the Appalachians in America or the Scandinavian mountains in Sweden and Norway. Or, it might still be happening, in younger mountains like the Alps, the Dinaric Alps and the Himalayas - the earth is active there, there is a collision, the mountains might still be growing. This is where geologists come into the picture. We look at the rocks and we can say when that interaction happened and if it is still happening.

The_Dinaric_Alps.jpgThe Dinaric Alps - a relatively young mountain range. They run down the entire length of the Croatian coast © Pavle Cikovac

It has been said that the fault lines on which the Zagreb earthquake of March occurred and the fault lines on which the Petrinja earthquake occurred are separate. Is it, therefore, correct to say that the Zagreb earthquake of March is unconnected to the Petrinja earthquake?

Basically, we would say yes. They are unconnected. The forces on the tectonic plate are acting on a large scale. The expression of these tectonic forces is different in different regions. From our measurements, we know that these fault systems - Zagreb and Petrinja - are not directly connected. They may be connected in some way, which is not straightforward to explain and not so immediate, but it is not like they are the same crack in the earth. They do not interact directly. The movement on one fault line cannot produce earthquakes on the other.

Petrinja__Croatian_Geological.jpgTop: the fault areas of Croatia. Bottom: The fault area around Petrinja © Croatian Geological Institute

There was an earthquake in Banja Luka. Is that earthquake connected to the one near Petrinja?

They had an earthquake there, yes and an even more devastating one in 1969. That activity does take place in the same fault area as the Petrinja earthquake, yes. But, the connection between the two is still not established. We can only speculate that the stresses and strains on one part of the area can produce earthquakes in another. It is possible that we may have a better answer to this once we have completed all the research we are currently doing. We may be able to say, yes, what happens in Banja Luka directly affects what happens here, or vice versa. You can already do this in other fault areas, such as the one which runs from Istanbul all the way to the east of Turkey.

1969_earthquake_in_Banja_Luka.jpgAftermath of the 1969 earthquake in Banja Luka © Public domain

Croatia Earthquakes: When do they happen?

We can only say what we know from the past and use some measurements that are available to us to guess the probability of Croatia earthquakes happening within a certain period of time. If you hear someone say “Yeah, I know when the earthquake is going to happen”, that’s the time you need to stop listening to that person. They obviously don't know what they are saying. No scientist would say that. What we know for sure is that we don’t know that. A broad estimate, using the data we have from history, is that the probability of a stronger earthquake happening here, something the magnitude of 6.5, is roughly 10% every 50 years. This means that such an earthquake does happen here, but only around once in every 500 years when using a scale of thousands of years.

movement_direction_of_the_earth_in_the_Petrinja.jpgThe fault area and movement direction of the earth in the Petrinja area © Croatian Geological Institute

The second earthquake in Petrinja was a large earthquake. The one the day before, and the Zagreb earthquake in March, were moderately large. Yes, it is unusual that we have experienced these three incidents in just one year, but it is certainly not unheard of. It is possible, like I say, that there is some connection that we don't yet know about between these fault lines. It's an area where research is ongoing and that requires more.

We have experienced three earthquakes in one year. Taking into account that the broad statistics say large earthquakes are predicted to happen within a certain frequency, are we now at a greater risk of another large earthquake happening or can we say that we are at a lesser risk because we have these three already behind us?

It is a difficult, difficult question. The stress was locked in a fault. Once that stress is released, you are much safer. But, if the stress is released in one fault, it may be that it increases the stress on another fault. So, it's hard to say. But, from what we currently know, we should now be safe. But (laughs), nobody can say with absolute certainty that there won't be another earthquake in this area for, say, another 10 years. The reassurance people needed by people who live in a seismically active region comes not from being told “don't be afraid of earthquakes, one will not come” but from constructing buildings that can cope with the earthquakes. But, I am a geologist, not a builder, so I cannot talk about that aspect.

This article was originally published on 8 January 2021

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