In 2020, three large earthquakes occurred in the area around Zagreb. An earthquake with a magnitude of 5.6 on the Richter scale occurred on 22 March, another with a magnitude of 5.2 on 28 December and an even stronger one on 29 December, with a magnitude of 6.3. At the end of March, the staff of the ELKH CSFK GGI Kövesligethy Radó Seismological Observatory completed maps of the surface change caused by the spring earthquake and published them on the website of the NKP SZET project, the details and significance of which are summarized in this article. The researchers are to publish the findings of the satellite comparison of the December earthquakes.

Although the seismic activity of the Pannonian Basin is moderate, historical data has also shown seismic events that have caused serious destruction.  For example, based on the historical data, seismologists have estimated that an earthquake that occurred close to the city of Komárom in 1763 reached 6.1 on the Richter scale. Seismic events of similar magnitude can have unpredictable, severe consequences in today’s urbanized environment due to the vulnerability of critical infrastructure.  This means that it is essential to better understand the geological processes and structures to which seismic events known from the past and expected in the future are linked. In addition to the large amount of structural research using geophysical measurement and drilling data, it is now possible to use satellites to observe the deformations caused by earthquakes. The goal of the seismotectonics project implemented within the framework of the Hungarian National Excellence Program (NKP SZET) is to create a seismotectonic model of Hungary that will help seismic risk analyses of various scales and thus mitigate the potential consequences of earthquakes. The March series of earthquakes in Croatia served as a kind of pilot project, a natural laboratory for researchers studying the Carpathian-Pannonian region to see how the analysis of seismological station data, the reconstruction of the deep stress field and the satellite observation of permanent surface deformations can provide an insight into the nature of earthquakes.

In addition to many other applications, state-of-the-art satellite Earth observation programs (e.g. the European Space Agency’s (ESA) Copernicus Program) use special data processing techniques called satellite radar interferometry (InSAR) to enable surface impressions of the planet’s internal processes with high accuracy, observed in high spatial and temporal resolution. The essence of this is that between the recordings of the ESA Sentinel-1 satellite pair for remote sensing made at different times, changes in the phase information (interference principle) are determined. With this method, surface deformations of less than half a wavelength can be mapped with an accuracy of a few millimeters based on images from a satellite passing at an altitude of approximately 700 km (Figure 1). Based on the phase changes of the Sentinel-1 recordings before and after the March 22 earthquake, permanent surface movements related to stress release can be observed. By ‘summing up’ the interference bands, the total deformation can be produced, and by analyzing the images with different satellite geometries, nearly three-dimensional information about the surface displacements can be obtained (Figures 2 and 3). Together with the nesting mechanism calculations obtained from the analysis of the seismic wave, these results serve to refine and validate the existing geological and tectonic models, and they can also inspire the creation of new ones.

Figure 1 An interferogram for the Zagreb earthquake series based on Sentinel-1 ascending satellite orbits (17 and 23 March). The large spatial scale signal on the interferogram reflects the changes in the state of the atmosphere between the times of the recordings, as the microwave signal does not propagate in a vacuum but in the constantly changing state of the atmosphere of the Earth. In the middle of the figure, the almost concentric deformation pattern north of Zagreb caused by the main earthquake is still clearly visible. The phase change is approximately six radians compared to the points south of the city that can be considered immobile. The epicenters of the March 22 and subsequent earthquakes are indicated by yellow circles, and the color code on the right shows the depth of the earthquakes (source: USGS)

Figure 2 Vertical deformations caused by the March 22 earthquake near Zagreb based on data from the Sentinel-1 satellite

Figure 3 Horizontal (east-west, positive displacement: east) deformations during the March earthquake near Zagreb