Frequently Asked Questions

Who are we?

We are a team of 10 employees in the Seismological Survey and four seismologists in research and teaching positions in the Geophysical Bureau of the Department of Geophysics at the Faculty of Science, University of Zagreb.

 

What have we done since the Zagreb main shock/earthquake?

In the first months, we have been working nonstop with the management of the Civil Protection from the Ministry of the Interior and the Emergency Management Office of the City of Zagreb to analyze data even for very weak earthquakes which would otherwise go unnoticed. We have also put in place additional measuring instruments in order to better follow seismic activity in the epicentral area of the earthquake, we have gathered macroseismic data (earthquake effects noticed by people or the effects on buildings) and have kept the public informed through the media and social networks. We have done all of this during the quarantine due to the COVID-19 virus (with permission from the appropriate government bodies).

 

What’s what in seismology?

A seismograph is an instrument which detects movement on the surface of the Earth (earthquakes, but also noise).

A seismogram is a record made by a seismograph (e.g. a graph of an earthquake).

A seismologist is an expert who studies earthquakes (the formation and propagation of elastic waves due to the displacement of two blocks of rock across a fault plane).

 

See other basic terms in our glossary (in Croatian language).

 

What to do in the event of an earthquake and how to live with earthquakes

1. How strong of an earthquake would necessitate evacuating the building and how to assess this?

Not all houses, buildings or other structures are built the same, which means that an earthquake of a certain magnitude will not cause the same material damage to all of these structures. Assess the safety of the building you are in: how and when it was built, etc. If you notice visible damage to your building after the earthquake, you need to leave, watching out for objects outside that could fall on you (shingles from the roof, lamposts, ...). Slight tremors and accompanying “rumbles” are certainly not pleasant, but they are not a reason to leave the building.

 

2. What should you do during an earthquake? What if you live on a high floor in a skyscraper? Is it true that you should open the front door as soon as possible because it can get stuck?

During the earthquake, try to stay calm and move minimally, only to one of the safe places in the house/apartment, such as: under a solid table, a door frame, under a solid load-bearing wall, next to a bed ... You should not stand by the windows, under a glass table, next to a wardrobe or shelf from which objects may fall and injure you. Note in advance where the safest places in your home are. If you live in a skyscraper or a high-rise building, never use the elevator during or after an earthquake, and do not run on the stairs because there is a risk of falling due to loss of balance or damage to the stairs themselves. Only when the earthquake is over, and damage to the building is visible, should you carefully exit the building using the stairs. There is a danger that the front door will jam during an earthquake, but opening it during the earthquake itself should not be a priority. Only once the earthquake is over should you check that the door can be opened.

 

We would like to remind you once again of the informational brochure from the Emergency Management Office of the City of Zagreb with advice on how to act during an earthquake which can be found here.

 

3. In the event of an earthquake, how can you care for your helpless and immobile parents? Is it better to take them out onto the street or take them to some part of the house that is considered the safest?

It is best to situate your family and yourself where there is no danger of objects falling on you from a height. If the house/building is disturbed statically, you must evacuate.

 

4. Should we move out of the house in, for example, Čučerje because we don’t know what to expect, i.e. will the next earthquake be stronger and destroy everything?

We are unable to predict when exactly to expect an earthquake, although we are aware that it could happen. If your house is badly damaged, it may be better to call in construction/static experts to inspect it and give you advice.

 

5. Subsequent tremors are normal, but many people still "jump" every time they feel even a small amount of shaking. How to stay calm in the current situation?

It is difficult to make an exhaustive statement that will calm people down because they are understandably afraid. For now, we will definitely "shake" for a period that may last for awhile. People are more likely to be injured due to panic than to be injured by a building collapsing, although both dangers exist.

 

6. How accurate is it now to feel earthquakes that we would otherwise not notice due to the lack of normal traffic (canceled trams, trains and air traffic)? And how common are these tiny earthquakes, say magnitudes 2-2.5, in our area?

It is to be expected that when we listen for something and when there is no city ‘bustle’, we register what we would otherwise miss. Earthquakes are very frequent, with a magnitude of 2-2.5 on the Richter scale in the Republic of Croatia, there are more than 200 of them per year (and if you take into account series of earthquakes, there are even more). Likewise, many people will experience so-called “phantom earthquakes” i.e. in the midst of a traumatic experience after an earthquake, they will feel movement even when it was not there. Otherwise, very sensitive people will feel an earthquake of a minimum magnitude of around 2.0, mostly on the higher floors of a building. Series of earthquakes occur in densely populated areas and are accompanied by sound (such as rumbling or a thunder sound) so individuals may also notice these earthquakes of smaller magnitude.

 

7. Are there "foreshocks" or "pre-earthquakes" that announce the arrival of a big earthquake? Citizens reported rumbling several times in the days before the quake. If so, how do they work?

Foreshocks are defined only after a stronger earthquake. Almost every strong earthquake is accompanied by a series of smaller aftershocks of varying magnitude. Minor earthquakes also occur, not followed by a stronger earthquake. Therefore, minor earthquakes unfortunately cannot be used to predict a stronger earthquake.

 

8. Why are there almost no earthquakes during the day but there are at night? Why do we hear “rumbling” or “humming” every now and then, but don’t feel trembling? Does this mean anything? Many people fear that this means a big earthquake is coming.

After a strong earthquake, a series of subsequent earthquakes occur. Earthquakes do not ask what time it is and do not depend on whether it is day or night. Faults where earthquakes occur are located at depths which are not affected by the difference in daytime and nighttime temperatures, because the difference is too small.

 

Rumbling is a consequence of the so-called seismic P-waves, and tremors are mainly due to the so-called S-waves. S-waves have larger amplitudes, so we feel them more for that reason. You can read more about the different types of waves here.

 

Whether you hear a rumble, feel a tremor, or both depends on the mechanism of the earthquake itself, as well as your location relative to the epicenter. When it comes to "rumbling" or "humming", it is sometimes a good sign, because it means that the surfaces are realigning by sliding slightly, without getting stuck.

 

Questions on the series of earthquakes since March 22, 2020.

1. What is the exact location of the epicenter of the first earthquake on March 22, 2020? Why was the epicenter in Markuševac and not on the already existing fault? Has a new fault appeared?

The epicenter is at 45.85 ° N and 16.03 ° E. From the distribution of weaker, subsequent earthquakes, over time it will be possible to determine the extent of the fault zone that was activated, whether it is a new fault that we did not know about or a known fault, etc. The epicenter of the main and subsequent earthquakes can be seen on our website at this link.

 

2. How big is the displacement that occurred on the currently active fault (cm, m)?

We are unable to provide an answer to this question at this time as it requires additional analysis and calculations. In a back-of-the-envelope estimate, assuming that the fault dimensions are 5 km × 5 km (the real surface has yet to be determined), the expected displacement of the fault blocks could be about 20 cm.

 

3. Is the current situation improving as there are fewer and fewer major earthquakes or is there reason to worry? Has the fault stabilized?

There are a lot fewer aftershocks than in the days right after the earthquake and they are weaker, but we still can't be completely sure that there won't be an earthquake of similar or stronger magnitude. It is possible that the situation at the fault has begun to stabilize, as shown by a series of subsequent medium-strong and weaker earthquakes. We must remain aware of the fact that we live in an area where earthquakes occur, knowing that one day (tomorrow, in 50 years or in 100 years) they will surprise us again, and think about how to react when an earthquake occurs again. Most importantly, we need to construct buildings that will be earthquake resistant and strengthen existing buildings.

 

4. Are the earthquakes of 1880, 1905 and 1906 comparable to today's and if so in what sense? Can their ‘foreshocks’ and ‘aftershocks’ be compared?

Since there are no seismograms for any of the earthquakes before 1906, it is not possible to determine their magnitude, but we empirically estimate it from the intensity of the earthquake. There are empirical equations that we can use to estimate magnitude if we know the intensity or vice versa, but they are only estimates and should be used with great caution. We also learn about previous and/or subsequent historical earthquakes from the literature and testimonies from that time.

 

A historical overview of earthquakes in the vicinity of Zagreb is described in the article by Assoc. prof. Snježana Markušić, here is a quote from the text: “… Earthquakes and seismic activity in the wider vicinity of Zagreb are not unusual. Josip Mokrović, a well-known Croatian geophysicist, calculated that Zagreb shook more or less 661 times from 1502 to 1883. The strongest was the earthquake that happened on a cold Tuesday, November 9, 1880, at 7 hours and 3 minutes. The earthquake had a magnitude of 6.3, with a focus in Medvednica, around Kašina and Planina. The newspapers of that time wrote that Zagreb was "shook strongly", "scared people" and claimed two victims. Their names are recorded in historical almanacs. … According to history books, Zagreb at that time had slightly fewer than 30,000 inhabitants and about 2,500 residential buildings. In total, about 1,400 buildings were damaged or destroyed by the earthquake .... In an almost identical location, as in the case of the earthquake of 1880, two more strong earthquakes occurred: on December 17, 1905 and January 2, 1906. Concerning more recent seismic activity in the wider area of ​​the city of Zagreb, the strongest earthquake that occurred (prior to the most recent one) was on September 3, 1990, with the epicenter in the vicinity of Kraljev Vrh, with a magnitude of 4.7 ”.

 

As for the inquiry about the two earthquakes from 1905 and 1906, it is evident that these were earthquakes that were significantly weaker than the earthquake that most recently struck Zagreb (e.g. both earthquakes caused some damage - it was not comparable to the damage from the 1880 earthquake - but there were no casualties and the earthquakes were much weaker. There is a description of how on the third floor of a house in Zagreb shaking was felt, the chandelier swayed, but nothing fell or was damaged). It is likely that the second event was somewhat stronger than the first, but detailed seismological research, a review of all available historical records, and a re-determination of strength of those earthquakes is necessary to be able to make such a conclusion. Further information is available at the following link.

 

5. After the earthquake, was water pollution noticed at the Zagreb springs from which the city water system is supplied? Have any other changes been recorded in nature?

To the best of our knowledge, no water pollution has been observed or the relevant services have not provided us with such information. Soil subsidence and/or activation of some landslides have been observed - this is being investigated by fellow geologists from the Croatian Geological Survey.

 

Questions about earthquakes in the Zagreb area

1. Which areas of the city of Zagreb are most endangered by earthquakes, how many more "hot spots" do we have besides the already known Kašina fault?

The entire city of Zagreb is endangered by earthquakes because the distance from the possible epicenter is relatively small. Of course, the areas close to the epicenter suffer the most. There are a number of larger or smaller faults around Medvednica, and some even intersect it. The locations of the foci of this series of earthquakes will help us to "draw" the activated part of the fault system.

 

2. How does the structure of the soil and the distance from the epicenter affect how we feel the earthquake? Which areas in Zagreb are the most endangered in that sense, and which are the safest?

In most cases, the farther we are from the hypocenter of the earthquake or its epicenter, the less we will feel its effects. However, damage caused at a location will, in addition to the strength and distance from the epicenter, be affected by the effects of local soil (soil structure, soil properties, etc.). Soil properties can cause an increase in the amplitude of soil movement in an area. The most famous example of the impact of local soil properties is the 1985 magnitude 8.3 earthquake that occurred in Mexico. The area with the greatest damage was as far as 400 km from the epicenter of the earthquake, and it was built on the soft sediments of a prehistoric lake. The soft surface sediments caused an increase in the amplitude of the shaking. Interestingly, a few kilometers away from the epicenter, in an area built of volcanic rocks, the quake was barely felt.

 

So the intensity of the shaking you will feel strongly depends on the properties of the soil beneath the surface. In this context, the most endangered areas in Zagreb are the embankments where buildings were constructed before the 1960s, when no earthquake construction was considered.

 

3. Could Zagreb be hit by an earthquake even stronger than the one in 1880? Is it correct to estimate that the strongest possible earthquake in the area of ​​Zagreb has a maximum magnitude of 6.6?

A magnitude 6.3 earthquake occurred in Zagreb in 1880, and an earthquake of the same strength can of course occur again, and perhaps even a slightly stronger one. Seismotectonic geologists estimate that the seismogenic potential of the seismic zone of the North Medvednica Fault, given its length (approximately 20 km) and depth below the surface (approximately 12 km), can cause an earthquake of maximum expected magnitude estimated at about 6.5 (Richter scale). You can read more about it here.

 

4. Did you predict that a strong earthquake would happen in the area of ​​Zagreb? If not, are you working on a model that could predict something like that?

Earthquakes and seismic activity in the wider Zagreb area are not uncommon (more at this link).

 

However, no one can know for sure if and when the next earthquake will occur. It is possible to estimate the potential maximum magnitude of an earthquake in an area, to monitor relative movements on a part of the surface using GPS and InSAR technology and to calculate the potential for an earthquake of a certain magnitude based on such observations. Also, it is possible to calculate the impact of earthquakes on buildings by analyzing local soil conditions and to construct buildings accordingly. A seismic hazard map prepared by seismologists in 2011 is publicly available, and is part of Eurocode 8, building regulations that are in effect in the Republic of Croatia. This kind of map provides information to construction engineers on the acceleration of the soil that they must take into account in their static calculations. The map can be found here.

 

5. Is there any connection between earthquakes in the Zagreb area and earthquakes in nearby places in Southeastern Europe (Albania, Turkey, etc.)?

There is no direct connection between these areas. The only thing they have in common is that the earthquakes in these areas occur on faults as a direct consequence of the subduction of the African tectonic plate underneath the Eurasian tectonic plate.

 

6. Is it possible for the earth to open up and cause a big rift in the area of the city of Zagreb? Are there already places where such phenomena have been observed and if so, what does it mean?

Such a scenario is not possible in Zagreb. Faults in the Zagreb area occur due to the compression and/or transpression stress regime, and not the extension regime, which is a key factor for such a phenomenon to occur at all. Furthermore, even if the faults in the Zagreb area were exactly such that a rupture in the earth occurs as they activate, much more energy would be needed than what is possible in Zagreb.

 

Simply put, we do not expect a fracture on the faults in the vicinity of Zagreb that would reach the surface itself. Some side effects such as landslides and potholes due to land subsidence and the impact of groundwater have been observed.

 

All other queries

1. Are there people exploring the epicentral area and what equipment is being used? Is it possible to estimate on the spot how much energy has been released and what is the current situation at the hypocenter?

A team of geologists is investigating the situation on the ground at the fault and a team of seismologists is investigating earthquake records. The biggest problem is that earthquakes occur at depths (from less than 1 km to several hundred kilometers), and only their effects on the surface are visible. Damage to buildings and changes in the landscape in the epicentral area, and beyond, as well as human experience, are important in determining the intensity or effects of earthquakes. Based on seismological data, it is possible to estimate the location, depth and magnitude of the earthquake, as well as the type, displacement and position of the fault at which the earthquake occurred. However, there is no way to assess the magnitude or current situation at the hypocenter itself because it is located at various depths.

 

2. Why can't you estimate the exact time when an earthquake will occur, as well as its strength and location?

The location of an earthquake is related to active faults that are mostly known, so it is possible to know in advance approximately where an earthquake may occur. The strength of an earthquake depends on the strength of the rocks in the fault zone - stronger earthquakes are possible only in places where the rocks are stronger, i.e. where a large amount of energy can accumulate without being released prematurely. Based on historical and statistical data and knowledge of fault locations, it is possible to assume the potential strength of a future earthquake. But due to the non-homogeneity of the medium and the non-periodically long “charging” and short-term “discharging” of energy at faults in the form of an earthquake, it is impossible to predict exactly when the next earthquake will occur.

 

3. What is the difference between Omori's Law and Gutenberg-Richter's Relation? Which of these two laws is currently valid in the Zagreb area and what does it mean?

Omori's Law gives the relation on the frequency of occurrence of subsequent earthquakes in time, and it is exclusively valid for aftershocks. The Gutenberg-Richter Relation gives the relationship between the magnitude and the total number of earthquakes in an area over a period of time, and it is valid for individual areas on Earth, but also when the Earth is viewed as a whole. Aftershocks follow both of these laws: over time there are fewer earthquakes (Omori's law) and the "trend" is such that there are always more earthquakes of smaller magnitude (Gutenberg-Richter relation).

 

On Seismological Surveys and Centers

1. It is common for false "reports" about earthquakes and misinformation to circulate on social networks. Where is the best place to find official earthquake data?

Official earthquake data is best sought from official bodies. You can view all earthquake reports on our official website:

http://www.pmf.unizg.hr/geof/seizmoloska_sluzba/izvjesca_o_potresu

 

Furthermore, follow the instructions of the Civil Protection Headquarters and the government. The 112 center has the capacity to receive a larger number of calls.

 

2. Why do you inform after EMSC and why do your magnitudes differ from the ones they publish?

EMSC is a European-Mediterranean Seismological Center that receives real-time information from a number of seismological surveys in Europe, including ours, and publishes automated locations. We do not currently have direct announcements of automatic analysis. All of our information is controlled and analyzed by a seismologist. A few of us do all the work that hundreds of people would do in larger organizations. The automatic location is great for quick information, but we give Civil Protection controlled results to know how to react in the right manner. A difference in magnitude can occur due to the difference in the data used (number of seismological stations, model used to calculate the location, etc.).

 

3. Can we obtain information on earthquakes recorded by your instruments?

We appreciate the desire of individuals to analyze data themselves, but we are cautious when it comes to data sharing - mostly due to the prevalence of pseudoscience, the great possibility of conducting meaningless and/or erroneous analyses and making unsubstantiated conclusions that may have unintended consequences. Please leave this part of the job to the experts.

 

4. Why didn't you tell us all of this earlier?

Seismologists try, in addition to their regular work, to inform the public in various ways. Whenever earthquakes are a current topic in the Croatian media, seismologists have warned of seismic activity in the area of ​​Zagreb and the whole of Croatia. They have participated in numerous events such as the Festival of Science, gave lectures in schools, libraries, etc. Many schools came to visit the Department of Geophysics at the Faculty of Science where they were introduced to the seismology and seismicity of Croatia, as well as other geophysical phenomena and processes. We always respond to calls from interested parties. There is also Geofizika Uživo (Geophysics Live), a one-day open door event of our Department and part of the Day and Night at the Faculty of Science, which has been held regularly in April since 2014. Through short popular lectures, demonstrations of experiments, posters and acquaintance with the legacy of Andrija Mohorovičić, we familiarize people with earthquakes and their effects on society. Unfortunately, this year Day and Night at the Faculty of Science has been postponed until further notice due to the situation with the coronavirus epidemic. For more information see this link. We definitely invite you to visit us when the first opportunity arises! We should also mention the Facebook page Geofizika Uživo (Geophysics Live), which is active throughout the year with the aim of introducing geophysical topics to everyone who is interested.

 

5. Where can I find out more?

We recommend the following educational websites:

https://www.usgs.gov/faq/natural-hazards

https://www.iris.edu/hq/educational_resources

or come study with us

https://www.pmf.unizg.hr/geof/en/study_programmes

 

This text, as well as a number of other texts and materials on our website, were prepared primarily by the volunteer work of us seismologists and students at the Geophysical Department of the Faculty of Science, mostly after the increased public interest in our work.

The text was translated by Chelsea Sanders.