Late-Breaking Session: The December 2020 earthquake sequence in Petrinja, Croatia, and its seismotectonic and geodynamic environments


Late-Breaking Session: The December 2020 earthquake sequence in Petrinja, Croatia, and its seismotectonic and geodynamic environments
Co-organized by G3/GM9/NH4/SM4
Convener: Stéphane Baize | Co-conveners: Sara Amoroso, Lucilla Benedetti, Petra Jamšek Rupnik, Branko Kordić, Snjezana Markušić, Bruno Pace, Stefano Pucci
vPICO presentations
| Thu, 29 Apr, 15:30–17:00 (CEST)

vPICO presentations: Thu, 29 Apr

Chairperson: Stéphane Baize
Mathieu Causse

Here, I use seismological observations (~70 broadband stations at distances between 100 and 400 km from the source) to characterize the rupture properties of the Petrinja mainshock (Mw 6.4). First, I perform a spectral analysis of the P-waves to compute the corner frequency. In order to remove the wave propagation effects and isolate the source properties, I use the largest foreshocks and aftershocks (Mw>4) as empirical Green’s functions (EGFs). Assuming a Brune’s source model, the obtained stress drop is ~20 MPa. This rather large value is in agreement with the short rupture length of ~8 km inferred by InSAR data (Ganas et al. 2021). In addition, the weak azimuthal variations of the corner frequencies indicates a bilateral rupture, that is a rupture nucleating close to the fault center. Second, I compute the apparent source time functions (i.e. the source time functions “seen” from any station) using an EGF deconvolution approach. The results indicate an average rupture duration of 5-6 s with weak azimuthal variation of the apparent rupture duration, in agreement with the spectral analysis. Finally, I perform a Bayesian inversion of the apparent source function, in order to obtain a kinematic model of the rupture propagation (slip distribution, rupture velocity). The preliminary results reveal a slow velocity of the rupture propagation. Such a slow rupture velocity associated with a large stress drop has been observed on other faults with slow slip rates (e.g. Causse et al. 2017). This work provides insight on the rupture process of this major event on a poorly documented fault. I am fully open for collaborations to further develop and enrich this study.

Causse, M., G. Cultrera, L. Moreau, A. Herrero, E. Schiapappietra and F. Courboulex. Bayesian rupture imaging in a complex medium. The 29 May 2012 Emilia, Northern Italy, earthquake (2017), Geophysical Research Letters, DOI : 10.1002/2017GL074698.
Ganas, A., Elias, P., Valkaniotis, S., Tsironi, V., Karasante, I., Briole, P., 2021, Petrinja earthquake moved crust 10 feet, Temblor, http://doi.org/10.32858/temblor.156

How to cite: Causse, M.: Rupture analysis of the 2020 Petrinja earthquake based on seismological observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16577, https://doi.org/10.5194/egusphere-egu21-16577, 2021.

Josip Stipčević, Valerio Poggi, Marijan Herak, Stefano Parolai, Davorka Herak, Iva Dasović, Michele Bertoni, Carla Barnaba, and Damiano Pesaresi

The Department of Geophysics, University of Zagreb and the Italian National Institute of Oceanography and Applied Geophysics (OGS) installed on January 4th 2021, five temporary seismic stations near the town of Petrinja, Croatia, in the aftermath of  the 29 Decembre 2020 Mw 6.4 earthquake. The stations equipped with a seismometer and a strong motion sensor, recorded the aftershock sequence beginning six days after the mainshock allowing to augment the permanent seismic network in the area improving the azimuthal coverage and providing additional near‐field observations.

In this presentation we summarize the motivation and goals of the deployment; details regarding the station installation, instrumentation, and configurations and observations from the network. The collected data set will be useful for carrying out several seismological studies including the analysis of variability of strong ground motions in near field, the determination of the aftershocks source parameters,  the estimation (if any) of rupture directivity of small events, the clustering of events in space and time, the better imaging of the fault zone, the evolution of crustal properties within and outside of the fault zone.

How to cite: Stipčević, J., Poggi, V., Herak, M., Parolai, S., Herak, D., Dasović, I., Bertoni, M., Barnaba, C., and Pesaresi, D.: First results from temporary deployment of small seismic network following the Mw=6.4 Petrinja earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16579, https://doi.org/10.5194/egusphere-egu21-16579, 2021.

Vanja Kastelic, Simone Atzori, Michele M. C. Carafa, Marin Marin Govorčin, Davorka Herak, Marijan Herak, Bojan Matoš, Josip Stipčević, and Bruno Bruno Tomljenović

The ongoing Petrinja earthquake sequence interests a structurally complex area characterized by the transition between the Dinarides and the Pannonian Basin structural units. The sequence mainshock (December 29, 2020; Mw = 6.4) struck in the vicinity of the Petrinja town and caused significant damage in the human and in the natural environments. The preliminary seismological and geodetic analyses indicated a dextral strike-slip NW-SE oriented fault as the event source. Numerous geologic surface deformation patterns have been identified in the aftermath of the main event, including collapsed sinkholes, liquefaction, different forms of landslides, and surface fractures which nature and causative process require further detailed studies.
The aim of our contribution is to apply a multitude of different geophysical, geodetic and geologic methodologies to decipher the Petrinja seismogenic fault geometry in the light of its ongoing earthquake sequence. We will show how the different datasets converge in delineating the fault geometry and discuss their diverging aspects and implications. Our preliminary analyses on the geometric and kinematic characteristics of the mainshock (as well as those of the foreshocks and aftershocks) point to an important structural complexity. This aspect helps us to better understand the seismotectonic framework of the Petrinja seismogenic fault and other regional seismogenic faults of similar geologic and geodynamic setting.

How to cite: Kastelic, V., Atzori, S., Carafa, M. M. C., Marin Govorčin, M., Herak, D., Herak, M., Matoš, B., Stipčević, J., and Bruno Tomljenović, B.: Petrinja Seismogenic Source and its 2020-2021 Earthquake Sequence (central Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16585, https://doi.org/10.5194/egusphere-egu21-16585, 2021.

Olga Bjelotomić Oršulić, Matej Varga, Tomislav Bašić, and Tvrtko Korbar

The precision of geodetic measurements is reliable as much as the reference on which the measurements relies on. From the aspect of todays' most used geodetic method, the GNSS measurements, its reference is defined through a national reference frame established with corresponding reference stations. Hazardous earthquake of M=6.2 occurred in NW Croatia at the very end of year 2020. Earthquake was one of the most hazardous natural phenomena in Croatia in the last century. Due to the tremendous damages left behind, in which also one of the national GNSS reference station temporarily out of the service, we analyzed how much earthquake had impacted the surrounding reference stations and overall the Croatian national reference frame CROPOS. The presentation shows the analysis of GNSS time series in order to determine the scale of displacement of the CROPOS CORS GNSS reference stations due to the earthquake. The results show the greatest shift of 5 cm east on Sisak reference station, with stations in circumstances of 100 km impacted by the earthquake and shifted between 1 and 2.5 cm positional and 2-4 cm in height. Identified displacement of national reference frame and the ground displacement over the affected area will have domino effect on the geodetic field measurements and cadastral survey on that area.

How to cite: Bjelotomić Oršulić, O., Varga, M., Bašić, T., and Korbar, T.: Distorsion of Croatian national positional reference system CROPOS after the earthquake M6.2 in NW Croatia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16581, https://doi.org/10.5194/egusphere-egu21-16581, 2021.

Olga Bjelotomić Oršulić, Tvrtko Korbar, Danko Markovinović, Matej Varga, and Tomislav Bašić

At the very end of the year 2020, at 29th of December, hazard earthquake of M=6.2 hit near Petrinja, at NW of Croatia. Earthquake have been felt in a circumstance of a 400 kilometers, leaving in an epicenter vicinity inconceivable damage, devastated towns and obstructed lives. In order to obtain the first emergency crisis numbers over the impact of the earthquake on a ground motion, we have analyzed open satellite radar images of Copernicus Sentinel-1 along with the seismic faults. Multiple spatio-temporal Copernicus Sentinel-1 C-SAR images were used and processed for the differentiating the before and after earthquake state of the art. This presentation shows the results of the SAR conducted analysis, with the results of ground displacement in vertical up-down and horizontal east-west direction. The results show the vertical ground displacement to extent of -12 cm at southern area to +22cm at north-west part of a wide area covered by the earthquake impact regarding the epicenter. The horizontal displacement is detected in range between 30 cm towards west and 40 cm towards east is detected around the epicenter area, and +/-5cm horizontal displacement over a wider affected area indicate a spatial extent and hazardous impact the mainshock event made. The SAR results were verified by including the analysis over one station from the national positioning reference frame CROPOS. Accordingly, we obtained matching results of 5 cm easting shift and -3 cm subsidence on Sisak GNSS CROPOS station which coressponds to our SAR findings. Furthermore, geological interepretation of new findings is given based on results detecting Pokupsko and Petrinja fault.

How to cite: Bjelotomić Oršulić, O., Korbar, T., Markovinović, D., Varga, M., and Bašić, T.: Ground displacement over Petrinja area caused by earthquake M6.2: interdisciplinary analysis of geodesy and geology based on analysis from SAR Sentinel-1 data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16582, https://doi.org/10.5194/egusphere-egu21-16582, 2021.

Athanassios Ganas, Sotiris Valkaniotis, Panagiotis Elias, Varvara Tsironi, Ilektra Karasante, Pierre Briole, Eugenio Sansosti, and Vincenzo De Novellis

On December 29, 2020, at 11:19 UTC, a strong (M6.4), shallow earthquake occurred in the central region of Croatia. The epicentre was located near the town of Petrinja, about 40 km to the south of the capital, Zagreb. Here we present a preliminary analysis of the geodetic data (differential InSAR & GNSS) and preliminary estimates of the slip that occurred on the fault during the earthquake and subsequent aftershocks. We picked InSAR data to invert for the seismic fault assuming linear rheology and Okada-type dislocation (rectangular) source with non-uniform slip. The interferograms show an asymmetric, four‐lobed pattern, centered on a NW‐SE oriented discontinuity that is in agreement with the right-lateral plane of the moment tensor solutions for the mainshock. We found that the Petrijna earthquake ruptured a segment of a strike-slip fault zone that is shorter (8 km) than average and with larger slip (~ 3 m). All parameters of the seismic fault are well constrained by InSAR modeling due to the full azimuthal coverage with both ascending and descending data of good quality. The fit to the fringes is better with a steep dip angle (76°) than with a purely vertical fault. The upper edge of the modeled fault is at a depth of ~1 km, this means that the slip drop from 3 to 0 m in the uppermost kilometer and our geodetic analysis cannot assess whether the fault reached the surface in some sections of the fault, however should this be the case, we expect ruptures at the surface in the range of 0.1 to 0 m for consistency with our model and the structure of the fringes pattern. In particular, preliminary modelling results with distributed fault-slip show that the slip reached a peak of more than 2.5 m at a depth of about 2 km. We also found that, differently from what reported in the European database of seismogenic sources (EDSF), the seismic fault dips westward instead of eastward. Additionally, the 2020 rupture and the InSAR mapped trace do not match the EDSF composite seismogenic fault surface trace. Kinematic analysis of GNSS waveforms at station BJEL (about 70-km east of the epicentre) revealed that horizontal ground motion reached 7-cm (peak-to-peak). The InSAR data revealed a 7 km of right-lateral afterslip on the NW-edge of the rupture, and 5 km to the south of the main fault rupture. In particular, the afterslip data on the NW edge of the rupture document the curved shape of the post-seismic deformation, that highlights the non-planarity of faults in nature and possibly indicating the existence of a ramp structure connecting to the neighboring segment towards north.

How to cite: Ganas, A., Valkaniotis, S., Elias, P., Tsironi, V., Karasante, I., Briole, P., Sansosti, E., and De Novellis, V.: Ground deformation related to slip and afterslip of the 29 December 2020 Mw 6.4 Petrijna earthquake (Croatia) imaged by InSAR, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16576, https://doi.org/10.5194/egusphere-egu21-16576, 2021.

Branko Kordić, Matija Vukovski, Marko Budić, Marko Špelić, Josip Barbača, Nikola Belić, Vlatko Brčić, Radovan Filjak, Tomislav Kurečić, Damir Palenik, Neven Bočić, Jure Atanackov, Miloš Bavec, Rok Brajkovič, Bogomir Celarc, Ana Novak, Matevž Novak, Petra Jamšek Rupnik, Sara Amoroso, Ricccardo Civico, Stefano Pucci, Tullio Ricci, Paolo Bonico, Francesco Iezzi, Bruno Pace, Alessio Testa, Lucilla Benedetti, Maxime Henriquet, Adrien Moulin, Stéphane Baize, Marianne Métois, and Snjezana Markusic

The earthquake with magnitude ML=6.2 that occurred on 29th December 2020 has caused significant material damage to objects and infrastructure in the towns of Petrinja, Sisak,Glina and the surrounding area. According to the satellite interferometry data, the coseismic and postseismic deformation area covers around 500 square kilometers. The existing geodetic benchmarks have been set in the affected towns, and their coordinates have been determined based on previous GPS campaigns. The GPS network was set up and adjusted at the State Geodetic Administration's request for geodetic monitoring of infrastructure and cadastral projects. These points are not primarily intended for high accuracy measurements at the level of a few millimeters, so their accuracy and the absolute shift concerning geodynamic processes in the region should be taken into account. Nevertheless, the data obtained by their observation after the earthquake can provide valuable information about the horizontal and vertical displacements with a certain level of confidence. The field survey has detected disappearance of a large number of benchmarks and some valuable information has been lost. Still, 58 points were found and observed and it has been concluded that 52 points are reliable and can be used for future research. Because the network of benchmarks is not developed in rural areas, there is a gap in the distribution of benchmarks in affected area. Therefore, the additional data was collected using the benchmarks established for the engineering and cadastral projects and studies. From a total of 67 points that have been found and observed, 42 points will be used. Along with the data collected in urban areas, there will be a total of 94 benchmarks. The accuracy of the geodetic benchmark measurements is at the centimeter level, while the values of deformation are at the level of a few decimeters. Therefore, the obtained data can be used to better assess the displacement recorded during the 29 December 2020 event. In the future, field research will focus on finding additional benchmarks to reach a better spatial distribution.

How to cite: Kordić, B., Vukovski, M., Budić, M., Špelić, M., Barbača, J., Belić, N., Brčić, V., Filjak, R., Kurečić, T., Palenik, D., Bočić, N., Atanackov, J., Bavec, M., Brajkovič, R., Celarc, B., Novak, A., Novak, M., Jamšek Rupnik, P., Amoroso, S., Civico, R., Pucci, S., Ricci, T., Bonico, P., Iezzi, F., Pace, B., Testa, A., Benedetti, L., Henriquet, M., Moulin, A., Baize, S., Métois, M., and Markusic, S.: Geodetic benchmark displacement measurements following the 2020 Petrinja earthquake in Croatia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16588, https://doi.org/10.5194/egusphere-egu21-16588, 2021.

Matija Vukovski, Marko Budić, Marko Špelić, Josip Barbača, Nikola Belić, Vlatko Brčić, Radovan Filjak, Tvrtko Korbar, Branko Kordić, Tomislav Kurečić, Damir Palenik, Neven Bočić, Jure Atanackov, Miloš Bavec, Rok Brajkovič, Bogomir Celarc, Ana Novak, Matevž Novak, Petra Jamšek Rupnik, Sara Amoroso, Francesca Romana Cinti, Riccardo Civico, Daniela Pantosti, Stefano Pucci, Tullio Ricci, Paolo Boncio, Francesco Lezzi, Bruno Pace, Alessio Testa, Anna Maria Blumetti, Pio Di Manna, Lucilla Benedetti, Maxime Hnriquet, Adrien Moulin, and Stéphane Baize

On December 29th, 2020, a strong Mw 6.4 earthquake hit central Croatia. The epicenter was located approximately 3 km southwest of Petrinja, and the intensity was estimated to VIII-IX EMS. The earthquake led to significant environmental effects related to earthquake magnitude, focal depth, and geological and geotechnical properties of the affected area.
The Croatian Geological Survey (HGI-CGS) conducted extensive geological and geodetic surveys starting a few hours following the main shock to measure the earthquake’s effects,
including those on infrastructures. Ten geologists from the Department of Geology carried out surveys from Decmber 31st, 2020 to January 7th, 2021 along the potential seismogenic source (inferred from geological maps and InSAR data) and in the wider epicentral area that suffered significant damage (e.g., Glina and Sisak).
During a second phase, researchers from the University of Zagreb (PMF UniZG), Slovenia (GeoZS), Italy (INGV, ISPRA, U. Chieti) and France (CEREGE, IRSN) were mobilized to complete the observations. The collaboration with these geologists allowed to deepen the investigations and to bring further detail to quantify the effects. The surveys were then compiled based on data formats used by the European Community, namely those of the INGV EMERGEO team (Villani et al., 2017; for environmental effects including surface ruptures and liquefaction) and those of the SURE group (Baize et al., 2019 for surface ruptures).
These observations revealed that the earthquake triggered a discontinuous, few km-long surface rupture with a maximum displacement of about 20 cm, which is consistent with the lower average of observations made on similar events (Wells and Coppersmith, 1994). Liquefaction spread over several tens of square kilometers mostly in river plains, the most distant being about 20 km from the epicenter (to be confirmed!). Other observed effects include lateral spreading, landslides, groundwater regime changes, rockfalls, and various infrastructure damage.
The compilation of the acquired dataset into a unified database, consistent with database of other historical and recent events, is essential for establishing reliable empirical relations between geological effects and physical characteristics of earthquakes (magnitude, depth). This forms the basis for seismic hazard assessments, whether for “surface rupture”, “liquefaction”, or “ground-shaking” potential.

How to cite: Vukovski, M., Budić, M., Špelić, M., Barbača, J., Belić, N., Brčić, V., Filjak, R., Korbar, T., Kordić, B., Kurečić, T., Palenik, D., Bočić, N., Atanackov, J., Bavec, M., Brajkovič, R., Celarc, B., Novak, A., Novak, M., Jamšek Rupnik, P., Amoroso, S., Cinti, F. R., Civico, R., Pantosti, D., Pucci, S., Ricci, T., Boncio, P., Lezzi, F., Pace, B., Testa, A., Blumetti, A. M., Di Manna, P., Benedetti, L., Hnriquet, M., Moulin, A., and Baize, S.: A database of the environmental effects associated to the December 29th, 2020 Mw 6.4 Petrinja earthquake (Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16574, https://doi.org/10.5194/egusphere-egu21-16574, 2021.

Paolo Boncio, Sara Amoroso, Jure Atanackov, Stéphane Baize, Josip Barbača, Miloš Bavec, Nikola Belić, Lucilla Benedetti, Rok Brajkovič, Vlatko Brčić, Marko Budić, Marco Caciagli, Bogomor Celarc, Riccardo Civico, Francesca R. Cinti, Paolo Marco De Martini, Radovan Filjak, Maxime Henriquet, Branko Kordić, Francesco Iezzi, Lua Minarelli, Adrien Moulin, Rosa Nappi, Ana Novak, Matevž Novak, Bruno Pace, Damir Palenik, Daniela Pantosti, Stefano Pucci, Petra Jamšek Rupnik, Marko Špelić, Alessio Testa, Sotiris Valkaniotis, and Martija Vukovski

The 29 December 2020, Mw 6.4 Petrinja earthquake nucleated at a depth of ~10 km in the Sisak-Moslavina County in northern Croatia, ~6 km WSW of the Petrinja town. Focal mechanisms, aftershocks distribution, and preliminary Sentinel-1 InSAR interferogram suggest that the NW-SE right-lateral strike-slip Pokupsko-Petrinja fault was the source of this event.
The Croatian Geological Survey, joined by a European team of earthquake geologists from France, Slovenia and Italy, performed a prompt systematic survey of the area to map the surface effects of the earthquake. The field survey was guided by geological maps, preliminary morphotectonic mapping based on 1:5,000 topographical maps and InSAR interferogram. Locally, field mapping was aided by drone survey.
We mapped unambiguous evidence of surface faulting at several sites between Župić to the NW and Hrastovica to the SE, in the central part of the Pokupsko-Petrinja fault, for a total length of ~6.5 km. This is probably a minimum length since several portions of the fault have not been explored yet, and in part crossing forbidden uncleared minefields. Surface faulting was observed on anthropic features (roads, walls) and on Quaternary sediments (soft colluvium and alluvium) and Miocene bedrock (calcarenites). The observed ruptures strike mostly NW-SE, with evidences of strike-slip right-lateral displacement and zones of extension (opening) or contraction (small pressure ridges, moletracks) at
local bends of the rupture trace. Those ruptures are interpreted as evidences of coseismic surface faulting (primary effects) as they affect the morphology independently from the slope direction. Ground failures due to gravitational sliding and liquefaction occurrences were also observed, mapped and interpreted as secondary effects (see Amoroso et al., and Vukovski et al., this session). SE of Križ, the rupture broke a water pipeline with a right-lateral offset of several centimetres. Measured right-lateral net displacement varies from a few centimetres up to ~35 cm. A portion of the maximum measured displacement could be due to afterlisp, as it was mapped several days after the main shock. Hybrid surface ruptures (shear plus opening and liquefaction), striking SW-NE, with cm-size left-lateral strike-slip offsets were mapped on the northern side of the Petrinja town, ~3 km NE of the main fault.
Overall, the rupture zone appears discontinuous. Several factors might be inferred to explain this pattern such as incomplete mapping of the rupture, inherited structural discontinuities within the Pokupsko-Petrinja fault system, or specific mechanical properties of the Neogene-Quaternary strata

How to cite: Boncio, P., Amoroso, S., Atanackov, J., Baize, S., Barbača, J., Bavec, M., Belić, N., Benedetti, L., Brajkovič, R., Brčić, V., Budić, M., Caciagli, M., Celarc, B., Civico, R., Cinti, F. R., De Martini, P. M., Filjak, R., Henriquet, M., Kordić, B., Iezzi, F., Minarelli, L., Moulin, A., Nappi, R., Novak, A., Novak, M., Pace, B., Palenik, D., Pantosti, D., Pucci, S., Jamšek Rupnik, P., Špelić, M., Testa, A., Valkaniotis, S., and Vukovski, M.: Surface faulting during the 29 December 2020 Mw 6.4 Petrinja earthquake (Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16575, https://doi.org/10.5194/egusphere-egu21-16575, 2021.

Sara Amoroso, Josip Barbača, Nikola Belić, Branko Kordić, Vlatko Brčić, Marko Budić, Riccardo Civico, Paolo Marco De Martini, Nina Hećej, Tomislav Kurečić, Luca Minarelli, Tomislav Novosel, Damir Palenik, Daniela Pantosti, Stefano Pucci, Radovan Filjak, Tullio Ricci, Marko Špelić, and Matija Vukovski

Earthquakes and related coseismic effects at the surface, both primary and secondary, such as liquefaction and lateral spreading, can impact humans due to induced economic or social disruptions (e.g. slope, bridge and building foundation failures, flotation of buried structures). In this respect, it results of primary interest to map liquefaction induced evidences soon after an earthquake. On the 29th December 2020, a major earthquake (Mw 6.4) occurred in Croatia, close to Petrinja, 45 km south of Zagreb, generating widespread liquefaction and lateral spreading phenomena in a radius of approximately 20 km from the epicentre. A European team of researchers (geologists and engineers), in strict collaboration with the Croatian Geological Survey, performed field reconnaissance campaigns with the aim to provide a detailed identification and characterization of the primary and secondary geological and geotechnical coseismic effects induced by the Croatian earthquakes. Specifically with reference to the liquefaction phenomena, the Working Group integrated the data collected directly in the field with those from remote survey by drone aerial photos acquired in the post-event immediate. The adopted process allowed the collection of the liquefaction record with the highest possible completeness both in terms of pattern and distribution of the phenomena. The database includes several detailed case studies typified by the following characteristics: (1) liquefaction occurring on alluvial plain sites (Kupa river, Sava river and Glina river); (2) blows made by sand and/or gravel with local presence of shells and armored mud balls; (3) lateral spreading phenomena along road and river embankments; (4) sand ejecta of different grain size and matrix, even at the same site; (5) sand and/or gravel ejecta along fault traces. The characteristics of these features are discussed with reference to the alluvial setting and tectonic context. All together, the detailed survey of these recent liquefaction features will assist to build new empirical relations, to update the existing ones and to mitigate the effects of future earthquakes recognizing liquefaction prone areas for a correct land use planning, as for seismic microzonation studies.

How to cite: Amoroso, S., Barbača, J., Belić, N., Kordić, B., Brčić, V., Budić, M., Civico, R., De Martini, P. M., Hećej, N., Kurečić, T., Minarelli, L., Novosel, T., Palenik, D., Pantosti, D., Pucci, S., Filjak, R., Ricci, T., Špelić, M., and Vukovski, M.: Liquefaction field reconnaissance following the 29th December 2020 Mw 6.4 Petrinja earthquake (Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16584, https://doi.org/10.5194/egusphere-egu21-16584, 2021.

Tvrtko Korbar and Snježana Markušić

Devastating M6.2 earthquake (1) hit Petrinja epicentral area (2) on 2020-12-29. M5.0 foreshock on 2020-12-28 (1) caused moderate damage on buildings and forced many inhabitants to move out form their homes. Thus, the foreshock was a kind of lucky event that saved many human lives.

Considering the shallow focal depth (1) and QMTS that show clear strike-slip focal mechanisms (3, 4), surface failures were expected after the mainshock. Immediate reports in media allowed quick online research of surface failures indicating that linear infrastructure damages appear along ~30 km long portion of sinistral NE-SW striking Sisak-Petrinja-Glina-Topusko Fault. Quick field inspection revealed that fresh fault planes in the bedrock appear mostly along longitudinal NW-SE striking (Dinaric strike) Pokupsko-Kostajnica-Banja Luka Fault, and show clear dextral co-seismic stike-slip displacements. The map view time-lapse animation of the seismic sequence (5) revealed that ~20 km long portion of the Pokupsko Fault was (re)activated. The two subvertical  mutually perpendicular faults intersect near the epicenters. The historically important Pokupsko earthquake occured in the vicinity (6), and was used by a famous Croatian geophysicist Andrija Mohorovičić to discover the MOHO discontinuity.

The fault system is textbook example of major failure in the upper crust along the pre-existing fault net (7) at the critical moment of centennial release of generally north-south oriented compressional strain that is accumulating in the crust because of continuous northward movement of the Adriatic microplate (Adria). Up to 10 mm/yr Adria GPS velocities measured in the Adriatic foreland are mostly accommodating along major External Dinarides active faults, since the Internal Dinarides GPS velocities are only 1-2 mm/yr, while the velocities in the Pannonian basin are near zero (8). The dextral Pokupsko-Banja Luka Fault could be one of the main inherited active faults between the crustal segments of the Adria, while sinistral Petrinja fault could represent reactivated Mesozoic transform fault bordering the crustal fragments (9) of once greater Adria (10).

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How to cite: Korbar, T. and Markušić, S.: Petrinja M6.2 earthquake (Croatia) on 29/12/2020 occurred on the intersection of the two regional active faults at the transition between Dinarides and Pannonian basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9434, https://doi.org/10.5194/egusphere-egu21-9434, 2021.

Emanuele Tondi, Anna Maria Blumetti, Mišo Čičak, Pio Di Manna, Zoran Đuroković, Paolo Galli, Chiara Invernizzi, Stefano Mazzoli, Luigi Piccardi, Giorgio Valentini, Eutizio Vittori, and Tiziano Volatili

We provide here a first-hand description of the coseismic surface effects caused by the Mw 6.4 Petrinja earthquake that hit central Croatia on 29 December 2020. This was one of the strongest seismic events that occurred in Croatia in the last two centuries. Field surveys in the epicentral area allowed us to observe and map primary coseismic effects, including geometry and kinematics of surface faulting, as well as secondary effects, such as liquefaction, sinkholes and landslides. The resulting dataset consists of homogeneous georeferenced records identifying 222 observation points, each of which contains a minimum of 5 to a maximum of 14 numeric and string fields of relevant information. The earthquake caused surface faulting defining a typical ‘conjugate’ fault pattern characterized by Y and X shears, tension cracks (T fractures), and compression structures (P shears) within a ca. 10 km wide, right-lateral strike-slip fault zone (i.e. the Petrinja Fault Zone, PFZ). We believe that the results of the field survey provide fundamental information to improve the interpretation of seismological, GPS and InSAR data of this earthquake. Moreover, the data related to the surface faulting may impact future studies focused on earthquake processes in active strike-slip settings, integrating the estimates of slip amount and distribution in assessing the hazard associated with capable transcurrent faults.

How to cite: Tondi, E., Blumetti, A. M., Čičak, M., Di Manna, P., Đuroković, Z., Galli, P., Invernizzi, C., Mazzoli, S., Piccardi, L., Valentini, G., Vittori, E., and Volatili, T.: Coseismic surface "conjugate" faulting of the 29 December 2020., MW 6.4, Petrinja earthquake (Sisak-Moslavina, Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16598, https://doi.org/10.5194/egusphere-egu21-16598, 2021.

Miklós Kázmér and Rosana Škrgulja

Archaeological excavations of the Roman city of Siscia (Sisak, Croatia) found walls of the city, up to 2 m thick, toppled in the moat. Brick masonry wall segments were found in various orientations: tilted, rotated, twisted, toppled, overturned. Foundations display features of twisting and shearing. There are additional shearing planes within the fallen walls, which allowed the segments to extend during collapse. Much of construction material was robbed in later centuries, so original dimensions are estimates only. Subsoil is alluvial sandy clay. We suggest that a major earthquake damaged the city wall of Siscia. Excitated by site effects of loose soil, high peak ground acceleration caused the wall to be sheared off from its foundation, landing it ultimately in the adjacent moat. Rebuilding of the city wall in the late antique period suggests that the first wall collapsed between the beginning of the 3rd and the middle of the 4th century. This earthquake between ~200 AD and ~350 AD is missing from historical catalogues. Both the Antique and the modern earthquakes were of intensity IX. The St. Quirinus site at Siscia is 12 km from the fault which caused the destruction in Petrinja on 29 December 2020, mere 3 km from the fault. We suggest that the Antique earthquake was stronger than the M 6.2 modern event.

How to cite: Kázmér, M. and Škrgulja, R.: A stronger predecessor to the M 6.2 Petrinja, Croatia earthquake in Antiquity – archaeoseismology of the 4th century Siscia event, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16572, https://doi.org/10.5194/egusphere-egu21-16572, 2021.

Petra Jamšek Rupnik, Marko Budić, Matija Vukovski, Branko Kordić, Marko Špelić, Nikola Belić, Damir Palenik, Neven Bočić, Jure Atanackov, Bogomir Celarc, Ana Novak, Matevž Novak, Rok Brajkovič, Miloš Bavec, and Stéphane Baize

After the earthquake of 29/12/2020 in Petrinja (ML6.2, ImaxVIII-IX EMS), an attempt was made to characterize the active structure associated with the earthquake. As a first step towards this goal, we performed a geomorphological analysis in order to contribute to the identification and characterization of the surface expression of the active Pokupsko dextral strike-slip fault. We focused on the area between the southernmost parts of Vukomeričke Gorice and the southernmost parts of Hrastovica Mountain, where the NW-SE striking Pokupsko fault has slipped during and after the recent earthquake (Ganas et al., 2021). Using available 1 : 5 000 scale topographic maps and various 10 m resolution digital elevation model visualizations, we mapped lineaments that could represent relatively recently active fault segments. We used a quantitative approach to perform stream sinuosity analysis (e.g., Leopold et al., 1964; Zamolyi et al., 2010) on major streams crossing the structure to identify distinct changes in channel patterns that may be associated with vertical movement along the predominantly strike-slip fault. We observed changes in the shape of the valleys, especially the changes in width, height, and direction. By summarizing various geomorphological indicators of active fault segmentation at the surface with available geological data (Pikija, 1987) and so far limited field observations, we provide insights into the structure of the Pokupsko fault.
Preliminary results show good agreement between lineament mapping, changes in valley shape, changes in the stream sinuosity index, and (to some extent) previously mapped faults. In addition, some of the changes in stream sinuosity correspond to locations where coseismic surface ruptures occurred during the December 29 earthquake (Budić et al., this session; Pollak et al., 2021). Results suggest that the several-kilometer-wide zone of uplifted Neogene deposits results from the dextral-transpressive structure, which at the surface consists of a series of subparallel fault strands branching off the main fault that runs along the SE slopes of the Hrastovica Mountain. The SW-most fault strands are associated with significant changes in the shape of the valleys: the wide valleys of Petrinjčica, Utinja and Šanja change to narrow and deeply incised as they cross the uplifted structure. Paleocene and Eocene rocks, which otherwise underlie the Neogene, outcrop in the NE parts of the fluvial breakthrough valleys, indicating the uplift of the Hrastovica Mountain. Topographic data show a decrease of the mountain range elevation towards the SW. This evidence suggests that the main fault runs on the NE side of the mountain, strikes NW-SE and dips steeply towards the SW. The fault strike deviates between Župić and Farkašić. The fault plane solution for the December 29 earthquake suggests a nearly pure strike-slip fault, while geomorphic evidence strongly indicates areas of active uplift along the fault, further supported by the general antiformal structure. We interpret this as an indication of either a general current transpressional character of the fault or as local kinematic variations due to segmentation and changes in the strike of the fault; further analyses are pending.

How to cite: Jamšek Rupnik, P., Budić, M., Vukovski, M., Kordić, B., Špelić, M., Belić, N., Palenik, D., Bočić, N., Atanackov, J., Celarc, B., Novak, A., Novak, M., Brajkovič, R., Bavec, M., and Baize, S.: Some geomorphological perspectives on the structure associated with the Petrinja M6.2 earthquake in Croatia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16578, https://doi.org/10.5194/egusphere-egu21-16578, 2021.

Maxime Henriquet, Adrien Moulin, Matija Vukovski, Branko Kordić, Marko Budić, James Hollingsworth, Ryan Gold, Stéphane Baize, and Lucilla Benedetti

The Petrinja-Pokupsko fault-system is a NW-SE right-lateral fault system that ruptured during the 29 December 2020 Mw 6.4 earthquake (~40km south-east of Zagreb, Croatia). Field analysis revealed opening of cracks and offsets of several centimeters (3 to 40 cm) along a ~20 km long fault zone extending from the Kupa river (in the northwest) to the Petrinjčica river (in the southeast). Optical image correlation based on WorldView satellite images has been used to document the first-order near-field rupture signal. The pre-event image was acquired on 7th December 2017, and the post-event image on 15th January 2021. The first results indicate a right-lateral displacement of ≈75 cm with a small (<10 cm) extensional dip-slip component localized on the Petrinja fault. Using 1:5,000 topographic maps, a WorldView-derived DEM (1 m), and field observations, we identified and quantified cumulative dextral offsets along the central and southern section of the fault (south of Župić). Right-lateral offsets range from 5 to 200 m near Križ and Cepeliš (central sector). Diverted streams also extend southeast of the Petrinjčica river, where no surface ruptures have currently been reported to date. To the northwest, perched valleys, wind gaps, and karst features all testify to ongoing uplift across NW-SE-trending anticlines. It is unclear if the primary component of faulting changes from strike-slip (in the SE) to reverse (in the NW), or if these folds merely record a transpressive component across the fault. The activity of this fault system is poorly known. The region experienced a magnitude Mw 5.8 in 1909, ~30 km northwest of Petrinja, which may have been associated with the Petrinja-Pokupsko fault system. The recent 29 December 2020 earthquake confirms the seismic potential of this fault system to generate Mw>6 earthquakes. Since the fault extends farther NW and SE, from the Vukomeričke Gorice hills to Mount Kozara (Bosnia), for a total length of about 100 km, it could generate potentially larger events. It is also noteworthy that the 2020 Petrinja event occurred only 9 months after the Zagreb March 2020 (Mw 5.3) earthquake. This event occurred on an ENE-WSW-trending thrust fault, broadly orthogonal to the right-lateral Petrinja-Pokupsko fault system, ~45 km north of Petrinja, and raises the prospect of potential interplay between strike-slip and thrust faults in moderate strain-rate intra-plate settings. To address this problem, future works will aim at constraining the geometry of this fault network and its seismogenic potential.

How to cite: Henriquet, M., Moulin, A., Vukovski, M., Kordić, B., Budić, M., Hollingsworth, J., Gold, R., Baize, S., and Benedetti, L.: Comparison between the coseismic surface displacement during the 29 December 2020 Mw 6.4 Petrinja earthquake (Croatia) from optical image correlation and long-term geomorphological observations of cumulative displacements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16590, https://doi.org/10.5194/egusphere-egu21-16590, 2021.