Open Access Paper
13 September 2024 Monitoring and inversion of three co-seismic events on the Balkan peninsula
Author Affiliations +
Proceedings Volume 13212, Tenth International Conference on Remote Sensing and Geoinformation of the Environment (RSCy2024); 132120F (2024) https://doi.org/10.1117/12.3035471
Event: Tenth International Conference on Remote Sensing and Geoinformation of the Environment (RSCy2024), 2024, Paphos, Cyprus
Abstract
The Balkan Peninsula is a tectonically active area due to the African-Eurasian collision, compression, and rotation of the Adria microplate in the north-west and rotation of the Anatolian plate in the south-east. The geographic region that comprises the Balkans is located in a very complex geological setting where many tectonic micro-plates meet and hundreds of faults cross the area. These processes generate frequent, though usually small, earthquakes, but occasionally earthquakes with magnitudes above 6.0 occur. In this study, SAR data from Sentinel-1A were utilized to analyze the deformation of co-seismic events. The Okada elastic dislocation model was employed to invert the geometric parameters of the fault and the distribution of co-seismic slip. The results indicate that the maximum uplift and the maximum subsidence deformations obtained from these two methods are comparable. The primary focus of the present study is to create a robust model of the surface displacements that occurred after three earthquakes: the Petrinja earthquake in Croatia on December 29, 2020, the Larissa earthquake on March 3, 2021, and the earthquake on Crete on September 27, 2021. A large earthquake releases sufficient energy to permanently deform the Earth`s crust and causes vibrations, affecting GNSS reference antennas. The examples presented demonstrate the application of space technologies such as GNSS and InSAR for researching and monitoring seismologic zones, highlighting their importance and advantages in establishing patterns in the movements within these zones.

1.

INTRODUCTION

Earthquakes cause deformations of the Earth’s crust due to the release of stress generated by the sliding of tectonic plates along fault lines, plate subduction, underground heat flow, and volcanism. Earthquakes have devastating effects on lives and infrastructure and are among the deadliest and most costly natural disasters on the planet. Satellite observations provide a tool for rapid detection and monitoring of the aftermath of earthquakes, facilitating faster response actions and facilitating the communication during crises. Remote sensing satellites provide data on seismic activity and dynamic change, as well as damage estimates. Small changes in the Earth’s topography can be measured with high precision using synthetic aperture radar (SAR) instruments. Interferometric SAR (InSAR), or radar interferometry, is used to measure changes in the Earth’s topography resulting from various processes, including seismic activity.

The main focus of this research is to present results from interferometric processing of three distinct earthquake events located in different fault structures in Balkan peninsula – two in Greece and one in Croatia (see Table 1 and Figure 1).

Figure 1.

Locations of the epicenters of the studied earthquakes, their magnitudes and the main faults [1].

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Table 1.

Three seismic events with a magnitude greater than 6.0 studied for the region of the Mediterranean Seismic Zone (epicenters are according to EMSC: emsc-csem.org)

NoNameLat, LonMwDate
1Petrinja - Croatia45.42 N, 16.12 E6.4December 29, 2020
2Larissa - Greece39.76 N, 22.21 E6.3March 03, 2021
3Arkalochori - Crete35.11 N, 25.20 E6.0September 27, 2021

Additionally, modeling of the studied events was carried out using the Okada method with different geometries. The results are supplemented with information on well-known faults in the region [1]. The produced InSAR results were compared with GNSS results from permanent networks in order to check them for consistency. The interpretation of space distribution and co-seismic deformation of major earthquakes in the study area may provide insights into the geodynamic and tectonic processes. The Balkan countries are seismically active. In this study, we focus on three locations to determine co-seismic deformations using the differential InSAR (DInSAR) method. Our further studies include comprehensive studies of high-magnitude seismic events that occurred in other Balkan countries, investigating the seismic activity of the following countries: Bulgaria, Bosnia and Herzegovina, Kosovo, Montenegro, North Macedonia, Serbia, Slovenia, Albania, Romania, Greece and Turkey.

2.

METHODS

2.1.

InSAR surface deformation

The basic principle of the DInSAR technique relies on using the phase information of radar waves to obtain the phase of the ground-surface deformation, known as the displacement phase, from the complex interferogram generated from two images acquired in the repeat-pass mode of the satellite.

SAR data that tightly bracketing the earthquake dates were acquired using the Sentinel-1A and 1B satellites from the European Space Agency (ESA). They operate using C-band (radar wavelength of 56 mm) in the Terrain Observation by Progressive Scan (TOPS) mode, which allows for wide swaths and short repeat intervals. Each TOPS image consists of three sub-swaths, with a total swath width of 250 km, providing a complete coverage of the earthquake-struck region from two opposite directions. Image pairs that span the earthquake dates are available from both the ascending and descending satellite orbits. The Sentinel-1 radar remote sensing data was downloaded from the Copernicus Open Access Hub (https://scihub.copernicus.eu/) and processed using the freely available SNAP software. The topography contribution to the radar phase was calculated and removed using digital elevation data from the Shuttle Radar Topography Mission (SRTM) with 30 m resolution [2], [3].

Through the interferometric processing, we always try to eliminate other sources of errors to isolate the contributor of interest, which is typically either elevation phase or displacement phase, with their values wrapped between –π and +π. The unwrapping process, which converts the phase to metric values, is done using snaphu module, a plugin installed in SNAP software. Interferometric fringes, which represent a full 2π cycle, appear on an interferogram as cycles of arbitrary colours. Relative ground movement between two points caused by the seismic event can be calculated by counting the fringes and multiplying them by half of the wavelength. The closer the fringes are, the greater is the strain on the ground. The good candidate images for producing good interferometric results must have strong similarities and therefore the coherence between the master and the slave images must be taken into consideration. Loss of coherence can be caused by temporal factors due to different acquisition times, geometric factors due to orbit errors, or volumetric factors due to vegetation [4]

The satellite SAR imaging geometry along the ascending and descending orbits are connected with projection relation between the Line-of-Sight (LoS) displacement and the 3D motion components. For mapping the deformations in the region of interest interferometric images at intervals 6 or 12 days were produced. The data were selected to primarily reflect the co-seismic effects of the main event, excluding consideration of foreshocks and aftershocks as much as possible. To enhance the quality of the results, SAR data from both ascending and descending orbits of the satellite were processed using SNAP, freely provided software by ESA specifically developed for SAR data analysis. Further processing of the LoS results was conducted to decompose them into real movements in vertical (up-down) and horizontal (E-W) components

2.2.

Okada’s model

Okada’s model, introduced in 1985 [5] and enhanced in 1992 [6], offers an analytical solution for surface deformation and internal displacements caused by seismic events for both point and finite rectangular sources in a homogeneous elastic half-space. This model is crucial for understanding interactions between seismic sources and the Earth’s crust, widely used in seismology, earthquake engineering, InSAR techniques, volcano and tsunami monitoring, and early warning systems [7-10].

Key parameters in Okada’s equations include the focal mechanism of the earthquake (strike, dip, rake), the source location (latitude, longitude, depth), and source geometry (length, width for rectangular sources). These parameters determine the characteristics of surface deformation and enable precise modeling of seismic events [11]. Additionally, the material properties of the Earth’s crust, such as elastic moduli and Poisson’s ratio, influence the propagation of seismic waves and deformation response and needs to be considered as well. By incorporating all these factors, researchers can better understand earthquake mechanics, assess seismic hazards, and improve early warning systems.

2.3.

GNSS

Monitoring and inversion of the co-seismic events help further understand the geometry and properties of Earth’s crust movement and tectonics in the region. We demonstrate the scientific utility of ground motion observations and the benefits of using GNSS for these measurements, while discussing the current role of GNSS in seismic monitoring. Permanent GNSS stations in the study areas are utilized to identify seismic events in the GNSS time series. Jumps in the horizontal and vertical components of the station’s position are recorded and used to accurately verify ground displacements at that point.

3.

RESULTS AND DISCUSSION

3.1.

Croatia earthquake, the Petrinja area, on December 29, 2020

The tectonics of Croatia, in particular are governed by the thrusting of the Adriatic (Adria) microplate under the European lithosphere. The region is complex as there are multiple tectonic units and regional-scale structures, originating numerous crustal faults. The interaction between the main tectonic units causes earthquakes in the upper crust, distributed along the active faults in the region. Most earthquakes occur in the western (coastal) area due to the collision between the Adriatic Platform and the Dinarides. The seismogenic faults in this region are primarily reverse faults, with tectonic movements that have predominantly tangential components [12].

The main shock of the Petrinja earthquake occurred on 29 December 2020 with a magnitude Mw6.4. The earthquake occurred along the central part of Popusko-Petrinja strike slip fault within the Eurasia plate, at a depth of 10 km with an epicentre at 45.422°N 16.255°E, three kilometers west-southwest of the Petrinja town [13]. The earthquake and its aftershocks caused most of their damage in the alluvial plains of the rivers Kupa and Sava, where liquefaction occurred in the loose layers of sands and silty sands. The earthquake was preceded a day before by an earthquake Mw5.2 and a series of smaller ones, and followed by many aftershocks, for a 2-year period, as reported by the Croatian Seismological Survey.

This is the largest earthquake to occur in Croatia since the advent of modern seismic instrumentation. Figure 2 illustrates the contours of the shake map according USGS [14]. It was preceded by two sizeable foreshocks, a magnitude 4.7 and a magnitude 5.2 on December 28. About one in twenty earthquakes have foreshocks. An earthquake of similar size to the recent main shock occurred in 1880 near Zagreb and since 1900 happened only three earthquakes with magnitude around 6 within 200 km distance from the Petrinja town.

Figure 2.

Contours of the shake from the event [14].

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The main shock and most of the aftershocks of this event are located on the Petrinja fault, which is well described in the European Seismogenic Fault Database (EDSF) [1]. We investigate this event using InSAR data (see Figure 3), supplemented with seismological and geological studies.

Figure 3.

Interferometric image of the Petrinja - Croatia earthquake area on December 29, 2020, acquired from ascending 146 (24 December 2020 – 30 December 2020) and deformation map along the LoS direction.

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The interferometric results presented on Figure 3 show similar values of displacements as in [15, 16]. The earthquake is modelled using the Okada method with various geometries, as depicted in Figure 4. Several tests were carried out for different length, width and slip over the fault. The computed vertical co-seismic deformations illustrate the uplift and the subsidence around the earthquake epicentre. Comparing the results from Okada model and InSAR images it is seen that the best coincidence is achieved for L = 14.5 km, W = 8.6 km and Tangential slip = 1.22 m.

Figure 4.

Co-seismic displacements calculated by means of Okada model according three types of geometry for Croatia earthquake.

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As referenced in [14], focal mechanism solutions for the event suggest that rupture occurred on a nearly vertical fault striking either to the southeast or southwest. The event’s location and depth indicate that it was an intraplate earthquake within the Eurasia plate, rather than along a major tectonic boundary.

Tectonics of the Mediterranean Sea, in the convergent boundary region between Africa and Eurasia, are complex, and involve the motions of numerous microplates and regional-scale structures. The Adriatic block, situated immediately west of today’s earthquake, is believed to move somewhat independently from Eurasia and Africa. This movement drives surrounding faulting in Italy and along the eastern Adriatic coast, from Croatia to Albania.

3.2.

Crete earthquake September 27, 2021

Crete Island, situated at the southernmost part of the Hellenic Arc, experiences high seismic activity attributed to the subduction of the African plate beneath the Eurasian plate. Both extensional and compressional stresses that exist in this area result in the creation of an extremely complex tectonic environment characterized by significant horizontal and vertical movements. The Heraklion tectonic graben to the north and the Messara tectonic graben in the southwest dominate the central Crete region on shore [3]. The Heraklion graben is bordered on the west by the Ida Mountains and on the east by the Dicti Mountains along the eastern Psiloritis and Kastelli fault zones, respectively. Both fault zones follow an almost NE–SW direction, with the eastern Psiloritis fault zone being more prominent due to the topography of the Ida Mountains [3].

The differential interferograms and displacement maps are shown in Figure 5. They show clear LoS co-seismic displacements caused by the Arkalochori earthquake. The main deformation zone depicting an elliptical shape with a major axis of 10 km in the SW-NE direction and a minor axis of 7 km [15]. The interferograms show a similar basic shape of surface subsidence, although their maximum values are different due to their opposite satellite views. Two interferograms with phase values were then used to estimate the joint seismic deformation of the ground. To calculate terrain displacement, an unwrapping process was performed and the phase unit was transformed into satellite LoS distance units for each interferometric pair.

Figure 5.

Interferometric images (left) and deformation maps in LoS direction (right) of the Crete earthquake.

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The two interferograms are of good quality and contain the phase difference between the main and undersampled images obtained from the primary seismic event and its aftershocks up to September 29. Due to the short temporal and geometric baselines (12 days), there are no regions of low coherence in the interferograms. Six fringes forming a lobe are evident in both the ascending and descending enveloped phase interferograms (see Figure 5). This asymmetric displacement pattern is characteristic of normal-fault earthquakes, indicating that subsidence is greater than uplift. Each interference fringe is a phase shift that corresponds to a movement of 2.8 cm in a satellite LoS [15].

The LoS displacement map in the ascending geometry shows negative LoS displacement values up to 18 cm after the earthquake with Mw6.0, including the ground deformation from all seismic events between September 23 and October 5 (Figure 5, top panel). In contrast, the LoS displacement map in the descending geometry shows negative LoS values up to 20 cm after the same earthquake, including all aftershocks up to October 1 (Figure 5, bottom). The dominance of the negative vertical displacement component in both LoS image geometries suggests a normal slip event. The rupture may be related to the west-dipping normal fault on the eastern side of the Castelli graben [16]. The subsidence is evidenced by our InSAR results and is clearly visualized on the longitudinal profile in Figure 6.

Figure 6.

A profile line reveling displacement of the Crete descending orbit 19Sep2021_01Oct2021, obtained from deformation map in the LoS direction.

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In addition to the maps produced by SAR, modelling of the event was carried out using the Okada method with different geometries (see Figure 7). In the modelling of Crete earthquake, we have tested several types of geometries, varying the length, the width and the tangential slip. We tested also several focal mechanisms, since some of them are quite different. We obtain the best fit for strike=218, dip=57, rake=-85 (GFZ data center: https://geofon.gfz-potsdam.de/) and for tangential slip=0.35 m. The results are supplemented with information on the well-known faults in the region [1]. The maps generated from SAR data were compared with the results obtained using the Okada method, revealing a good compatibility between the two. The negative values of subsidence are well seen in both InSAR images and Okada’s modelling results (see Figures 5 - 7).

Figure 7.

Co-seismic displacements calculated by means of Okada model for Crete earthquake September 27, 2021.

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According to the results from GNSS data processing for this event it was found that the vertical motion is more clearly demonstrated with recorded value of 14 cm [3], which was evidenced by our InSAR results shown in Figure 5. However, most of the GNSS stations are located more than 20 km away from the epicentre and therefore captured only small displacements on the order of a few milimeters. The GNSS station in Chania from the EUREF network, shown in Figure 8, is a prime example of this.

Figure.8.

GNSS station in Chania from the EUREF network

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3.3.

Larissa earthquake March 3, 2021

The investigated seismic event took place in Thessaly region, located in central Greece, where crustal extension is dominant. However it is not considered a seismically active area [17]. Due to the overall tectonic evolution, major normal faults control the present-day geomorphology and orography of Thessaly. In this region some of the faults are in NW-SE direction, while others, formed more recently, are developing roughly E-W. It is accepted that two of the these faults, namely the Tyrnavos and Larissa faults, are still in their development stage. This ongoing research is important because they are considered possible sorces of surface seismically caused dislocations, such as ruptures or liquefaction features. It is worth noting that other known faults in the area of the event did not exhibit activation cuased by the studied event [18].

In our previous study the surface deformations were determined after the earthquake Mw6.3, occurred on March 3, 2021, 20 km northwest of the Greek town of Larissa. [19] This was followed by a strong aftershock the next day with a magnitude Mw6.1. When studying these type of events, it is very rare to distinguish the deformations of the Earth’s crust resulting solely from the main earthquake as opposed to subsequent aftershocks. Tipically, the applied method registers the total changes in the Earth’s crust over the period between two passes of the satellites, which for the Sentinel-1 mission (for 2021) is a minimum of 6 days. The differences in the values of the displacements of the Earth’s crust, observed from different orbits, underscore the necessity, when studying deformations, to generate interferograms from both types of orbits. This approach provides a more comprehensive understanding and enables comparison with field measurements or results from other sources [20, 21]. Interferometric images and deformation maps along LoS directions for the Larissa earthquake are presented in Figure 9. We generate pairs of images for four different orbits

Figure 9.

Interferometric images of the area of the Larissa earthquake and deformation maps along the LoS direction determined by the two types of orbits.

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To calculate terrain displacement, an unwrapping process was performed and the phase unit was transformed into satellite LoS distance units for each interferometric pair. Decomposition of the upward and downward LoS displacement vectors were performed to extract the vertical (up–down) and horizontal (E-W) components of the ground deformation by merging the ascending and descending interferometric images. Transforming the unfolded phases and geocoding are the final steps before merging the data from the two orbits. The unfolded phases are transformed into radar LoS displacements. Geocoding of the SAR data to a chosen map projection is conducted, ensuring accurate determination of the survey line’s intersection angle with the ellipsoid. To proceed with further processing, marking and derivation of the ellipsoid angle are necessary. Initially, the images are processed separately for each orbit, necessitating the merging the data from different orbits. The subsequent step involves combining them to produce a unified file. This merging process is facilitated by the Collocate operator within the SNAP software, which merges two images from distinct orbits. The horizontal (east-west) and vertical shuffle calculation operator is employed. The resulting product provides information on horizontal and vertical movements, measured in millimeters (see Figure 10).

Figure10.

Deformation maps for Larissa earthquake decomposed to vertical (up-down) and horizontal (E-W) components.

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Information from two data centers for the focal mechanisms for the Larissa earthquake was collected. We tested both of the mechanisms to calculate the co-seismic vertical displacements using the Okada model. Figure 11 illustrates the computations as the left panels show the data from MedNet Regional Centroid - Moment Tensors (INGV) and the right panels present the data from the National Earthquake Information Center (NEIC) of US. In addition, three different configurations for the geometry of the source were selected to be tested, varying the length, the width and the tangential slip over the fault. Maximum and minimum vertical displacements are reproduced by the following geometry: L = 12.7 km, W = 7.7 km and Slip = 1.1 m. The values for the vertical displacements are comparable with those calculated by the DInSAR procedure.

Figure11.

Co-seismic displacements calculated by means of Okada model according three types of geometry for Larissa earthquake.

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Combining both methods, the DInSAR and Okada modelling, we can estimate better the vertical deformations generated after an earthquake. By comparing the results, we can more realistically reproduce the focal mechanisms of an earthquake. GNSS data from EUREF network were utilized for Julian day 062 of 2021, which corresponds to March 3. The time series of coordinates from the GNSS station LARM were analyzed. The LARM point is located southeast of the event’s epicenter. Figure 12 illustrates the recorded displacement observed in the northern and vertical components.

Figure 12.

Time series of LARM GNSS station coordinates from the European Permanent EUREF network.

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The examples provided illustrate the practical application of space technologies such as GNSS and InSAR, in the research and monitoring of seismogenic zones. These technologies underscore their crucial role and advantages in establishing movement patterns within these zones.

CONCLUSION

Three cases were considered, from which the following outcomes were obtained:

  • Only data from one orbit were used, due to heavy forestation and low coherence, as exemplified by the area of Croatia.

  • Deformation maps along the survey line were obtained, as demonstrated by the earthquake on the island of Crete.

  • For the earthquake in the region of Larissa, deformations along the LoS were determined, and both subsidence and uplift were calculated.

In this study, we focused our research on determining the surface deformations following several earthquakes with magnitude greater than 6.0 over the past four years in the region of Balkan peninsula. The results obtained from SAR data processing illustrates the co-seismic deformations in the region, which align well with GNSS data from previous studies and modelling by Okada method. Our primary objective is to demonstrate operational readiness in assessing surface deformation and producing displacement maps that can benefit a wide range of experts. The methodology employed by the authors has successfully generated interferometric images, visually depicting displacements around the epicenters, followed by modeling of these displacements in order to specify the focal mechanisms parameters (using Okada method). Through monitoring and inversion of the co-seismic events we aim to enhance understanding of the geometry and the properties of crustal movement and tectonics in the region.

ACKNOWLEDGEMENTS

The scientific results are part of the work on the project: “Study of co-seismic deformations of the Earth’s crust for the territory of the Balkan Peninsula based on satellite data”, financed by “Competition for financial support of basic research projects – 2023” of Bulgaria. Contract No. KP-06-N74/2 from 14.12.2023.

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(2024) Published by SPIE. Downloading of the abstract is permitted for personal use only.
Mila Atanasova, Hristo Nikolov, Lyuba Dimova, and Reneta Raykova "Monitoring and inversion of three co-seismic events on the Balkan peninsula", Proc. SPIE 13212, Tenth International Conference on Remote Sensing and Geoinformation of the Environment (RSCy2024), 132120F (13 September 2024); https://doi.org/10.1117/12.3035471
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KEYWORDS
Earthquakes

Deformation

Satellite navigation systems

Interferometry

Interferometric synthetic aperture radar

Satellites

Synthetic aperture radar

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