2021 | 74/2 | 189–208 | 14 Figs. | 2 Tabs. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society 1. INTRODUCTION Ground shaking during an earthquake usually causes major dama ge to structures. However, earthquakes also induce ground failures that can significantly contribute to total earthquake loss. “The term ground failure is a general reference to landslides, liquefaction, lateral spreads, and any other consequence of shak- ing that affects the stability of the ground” (USGS Earthquake Glossary, 2021). Unfortunately, numerous cases worldwide witness that not enough attention is directed toward the identification of various types of ground failure which usually follow very strong or se- vere earthquakes (BIRD & BOMMER, 2004). The absence of ground failure data may lead to unrealistic projections of losses caused by a particular earthquake. Furthermore, evidence of such processes/phenomena represent an important input into the production of reliable susceptibility maps and microzoning, which are important in the proper evaluation of seismic hazard in a particular region. Most of such databases in the literature are related to landslides (KEEFER, 1984; RODRÍGUEZ et al., The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series Davor Pollak1,*, Vlatko Gulam1, Tomislav Novosel1, Radovan Avanić2, Bruno Tomljenović3, Nina Hećej1, Josip Terzić1, Josip Stipčević4, Mario Bačić6, Tomislav Kurečić2, Mario Dolić1, Iris Bostjančić1, Lara Wacha2, Ivan Kosović1, Marko Budić2, Matija Vukovski2, Nikola Belić2, Marko Špelić2, Vlatko Brčić2, Josip Barbača2, Branko Kordić2, Damir Palenik2, Radovan Filjak2, Tihomir Frangen1, Mirja Pavić1, Kosta Urumović1, Marin Sečanj7, Bojan Matoš3, Marin Govorčin5, Meho Saša Kovačević6 and Lovorka Librić6 1 Croatian Geological Survey, Department of Hydrogeology and Engineering Geology, 10 000 Zagreb, Croatia; (*corresponding author: dpollak@hgi-cgs.hr) 2 Croatian Geological Survey, Department of Geology, 10 000 Zagreb, Croatia 3 University of Zagreb, Faculty of Mining, Geology and Petroleum Engineering, Department of Geology and Geological Engineering, 10 000 Zagreb, Croatia 4 University of Zagreb, Faculty of Science, Department of Geophysics, 10 000 Zagreb, Croatia 5 University of Zagreb, Faculty of Geodesy, 10 000 Zagreb, Croatia 6 University of Zagreb, Faculty of Civil Engineering, Department of Geotechnical Engineering, 10 000 Zagreb, Croatia 7 Independent researcher, Katarine Zrinski 6, 10370 Dugo Selo, Croatia doi: 10.4154/gc.2021.08 Abstract The most recent major earthquake series struck near Petrinja (December 29th 2020 M 6.2), and triggered extensive ground failures in the wider area of Petrinja, Sisak and Glina. Coseismic ground failures including subsidence dolines, liquefaction and landslides have been document- ed over a large area by various experts and teams. These data are stored in the newly created inventory, which is openly presented in this paper. This inventory is administered and updated by the Croatian Geological Survey, and will be available online via a Web Map Service (WMS) (www.hgi-cgs.hr). The aim of the inventory is to not only provide data for the development of susceptibility maps and more detailed exploration for possible remediation measures, but also to define the priorities for immediate action. The earthquake triggered the rapid development of dropout dolines which endanger the local populations of the villages of Mečenčani and Borojevići. This is still an ongoing process in the vicinity of the houses and therefore in-situ exploration started immediately. Liquefaction related to alluvial sediments of the Sava, Kupa and Glina ri- vers occurred almost exclusively in loose and pure sands, and was accompanied by sand boils, subsidence and lateral spreading. Liquefaction also presents a greater hazard because settle- ment of houses and river embankments occurred. Lateral spreading caused failures of river flood embankments and natural river banks. According to the data known to date, the majority of the coseismic landslides were reactivated with minor displacements. Despite that, it has been re- cognised that houses at the edge, or in landslide colluvium suffered greater damage than other houses located outside the landslide impact zone. 1999, BOMMER & RODRÍGUEZ, 2002; GORUM et al., 2011) or liquefaction (YOUD & HOOSE, 1978), but there are also more recent examples of inventories comprising all the typical forms of ground failure caused by earthquakes (PRESTININZI & ROMEO, 2000). It is well known that Croatia lies in a seismically active re- gion of the central Mediterranean, characterized by seismicity that is unevenly distributed across the country, both in the Adri- atic (HERAK et al., 2005; HERAK et al., 2017b; GOVORČIN et al., 2020) and in the Pannonian basin area (HERAK et al., 2009; HERAK et al., 2020; MARKUŠIĆ et al., 2020). Historical and instrumentally recorded data indicate the three most seismically active areas are: NW Croatia (greater Zagreb area), the Croatian Primorje (greater Rijeka area) and Dalmatia. They are clearly distinguished on the seismic hazard map for the Republic of Croatia (HERAK et al., 2011) (Figure 1). Since the largest city in Croatia is also situated in a seismically active area, the group of authors have created a database of geotechnical boreholes which contains relevant data for soil behaviour and site response analy- Article history: Manuscript received January 26, 2021 Revised manuscript accepted March 01, 2021 Available online May 06, 2021 Keywords: Petrinja, earthquake, ground failures, inventory, dropout dolines, liquefaction, landslides G eo lo gi a C ro at ic a Geologia Croatica 74/2190 ses for the City of Zagreb (KVASNIČKA et al., 1998; KVASNIČKA & MATEŠIĆ, 2001; MATEŠIĆ & KVASNIČKA, 2007; MATEŠIĆ et al., 2011). Although there have been a number of ML ≥ 6.0 earthquakes registered in the last millennium, historical data about earth- quake-triggered ground failures in Croatia are scarce and incom- plete. As a result, such an inventory does not yet exist. However, the most recent December 29th 2020 M 6.2 earthquake near Petrinja, which together with the foreshock on December 28 ( M 5.0) and the following aftershock series that is still ongoing, triggered extensive coseismic ground failures and initiated the creation of such inventory. Therefore, the focus of this work is to present the structure of the initial inventory and data on coseis- mic ground failures collected during first two weeks of the Petrinja earthquake series after the main shock of M 6.2 that oc- curred on 29th December 2020. The work also indicates the most affected zones, and provide information to a wider audience, the general public, local and regional administrations and Civil Pro- tection for further reference. We also hope that open access to the collected data will motivate other earth-scientists, experts, and the public to provide their contribution to the database, in order to establish the most complete insight into all coseismic ground failures and processes triggered by the Petrinja earthquake series. The database would also enable more realistic estimations of the total earthquake damage and loss. For this reason, the complete table of inputs collected so far is presented here in order to transparently provide information that can be further used to develop susceptibility maps, identify the most critical surface damage zones and ensure data for seis- mic microzoning and hazard assessment. Detected coseismic ground failures will be additionally explored in much more detail to provide better insights into local conditions, triggering forces and failure mechanisms, in order to provide parameters for pos- sible remediation measures. The database can also serve as a valu able factor in the development of the design and construction policies for the rehabilitation of the buildings and infrastructure in the area, thus ensuring the long-term safety of residents. The data presented in this preliminary inventory was mainly collected by geologists from the Croatian Geological Survey (HGI), supplemented by inputs from other earth-scientists and geotechnical engineers from the University of Zagreb (The Fa- culty of Mining, Geology & Petroleum Engineering and the Fa- culty of Science; Faculty of Civil Engineering), other civil engi- neering experts, local citizens and the media. Therefore, it is important to point out that not all of the entries presented here are verified yet by geologists. In addition, the inventory for this particular earthquake series should be considered as preliminary, since we expect that additional data will be collected over time. The up to date database is presented online via Web Map Service (WMS), and will be continuously maintained and updated by the HGI (www.hgi-cgs.hr). 2. OVERVIEW OF GEOLOGY AND SEISMICITY OF THE RESEARCH AREA 2.1. Geology The Petrinja earthquake series (December 2020 – January 2021) took place along the southwestern margin of the Sava Basin of the Pannonian Basin System (PAVELIĆ & KOVAČIĆ, 2018, SAFTIĆ et al. 2003 with references). A simplified geological map of the earthquake series affected area (the research area) is shown Figure 1. The location of Croatia marked blue on the European map (right). Geographical location of the research area (red rectangle) at the seismic hazard map for the Republic of Croatia (HERAK et al., 2011). Red symbol indicates the location of the earthquake epicentre (29.12.2020). Map shows peak ground acceleration (PGA) for 10 % probability of exceedance in 50 years in units of standard gravity (g) for reference rock soil (vs30 = 800 m/s). This corresponds to PGA-values that return on average every 475 years. G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 191 Figure 2. The geological map of the research area (originally in the scale 1:100.000) with the locations of ground failures. G eo lo gi a C ro at ic a Geologia Croatica 74/2192 on Figure 2, modified after the Basic Geological Map of SFRY 1:100.000, sheet Sisak (PIKIJA, 1987) and the Basic Geological Map of the Republic of Croatia 1:100.000, sheet Bosanski Novi (ŠIKIĆ, 2014a). The research area is mainly covered by Neogene and Qua- ternary deposits, while older pre-Neogene basement rocks (mostly Palaeogene) are only locally exposed in the SE part of the area (PIKIJA, 1987). In the following text we shall briefly de- scribe the major characteristics of the stratigraphic units pre- sented on the map (Figure 2). Some more details are presented for units that are subjected to ground failure. The oldest exposed rocks in the research area are of Middle to Late Jurassic age (J2,3), represented by metasandstones, slates, phyllites, marbles and greenschists, as well as by metamorphosed magmatic rocks (ŠIKIĆ, 2014b, RAFFAELLI & MAGDALENIĆ, 1970; MAJER, 1984; MAJER & LUGOVIĆ, 1985). The thick- ness of this unit is up to 400 m (ŠIKIĆ, 2014a). Small outcrops of the Cretaceous sedimentary sequence (Late Senonian – K2 3) con- sist of limestones with pyroclastics and radiolarian chert interca- lations. Palaeogene deposits (Palaeocene-Eocene – Pc, E) are mainly conglomerates (Figure 2). The conglomerates are carbonate-free, polymictic with various amounts of sandy matrix. Along with conglomerates, intercalations of sandstones, shales and coal seams, marls and limestones are present. An estimated thickness of these deposits is around 200 m (ŠIKIĆ, 2014b). Ottnangian deposits (M2) are mostly represented by coarse- and fine-grained clastics (ŠIKIĆ, 2014b). Conglomerates, gravels and sands are mostly seen in the lower part of the sequence, while the upper part is composed of an alternation of marls, clays and limestones. The thickness of the Ottnangian clastics is assumed to be 250-300 m. Between the river Kupa in the northwest and Mečenčani vil- lage in the southeast (Figure 2), Badenian deposits (M4) are ex- posed. They unconformably rest over Palaeogene and Ottnangian sediments, above distinctly recognized erosional boundary. In the basal part they are represented by conglomerates, gravels, sandstones and biocalcarenites that grade upward into finer- grained clays and marls, and into bioclastic and biogenic lime- stones in the uppermost part of the sequence (PIKIJA, 1986; ŠIKIĆ, 2014b). The thickness of Badenian sediments is variable, reaching up to 300 m. Sarmatian deposits (M5) continuously follow on top of Bad- enian deposits. They began with coarse-grained clastics com- posed of polymictic conglomerates, gravels, sandstones, sandy clays and marls deposited in coastal environments (ŠIKIĆ, 2014b). In deep-water environments of the basin, laminated sands, marls, and limestones prevail. The thickness of the Sar- matian deposits is a maximum of 100 m. Pannonian clastics (M6) are divided into the Lower and Mid- dle Pannonian. The most frequent sediments of the Lower Pan- nonian are laminated and platy limestones (micrites and clayey micrites), of the Croatica formation, locally interlayered with sands and sandy-marls, and with local occurrences of cm-thick gravel interlayers (ŠIKIĆ, 2014b). In the Glina and Zrin-Dvor val- leys, middle Pannonian, laminated to thin-layered sands and sandy marls alternate with clayey limestones, and poorly lithified sandstones. The Croatica fm. is followed by the Medvedski breg fm., which is represented by massive to weakly layered marls and sporadic clayey limestones and clays. Sediments of the Medved- ski breg fm. are deposited in the more distal part of the lacustrine basin (AVANIĆ et al., 2018b). The thickness of the Pannonian deposits is between 120 to 200 m. Pontian deposits (M7) are mostly characterized by marls. In the lower part, clayey and sandy marls predominate, while in the upper part, clayey siltstones and sands prevail (ŠIKIĆ, 2014b). In the area of the Glina River, layers of coal-clay seams and inter- layers of lignite are found. According to the latest lithostrati- graphic nomenclature, Pontian deposits are subdivided into the Andraševec and Hum Zabočki formations, based on a subdivision known in the area of northern Croatia (KOVAČIĆ, 2004; GRI- ZELJ, 2004). The Andraševec formation is composed of the al- ternation of sands, calcite-rich silts and marls, deposited since the Late Pannonian to the Late Pontian (AVANIĆ et al., 2018a). It is continuously overlain by the Hum Zabočki formation of the Late Pontian that mostly consists of fine- to medium-grained sands, silty marls, clays and coal seams (cm to dm thick). The thickness of the Pontian deposits varies between 150 to 450 m. Pliocene deposits (Pl) are composed of coarse- to fine-grained clastics , mostly sands, gravels, silts and clays with a general coars- ening upwards trend from the Lower to Upper Viviparus beds (MANDIC et al., 2015; KUREČIĆ, 2017; KUREČIĆ et al., sub- mitted) (Figure 2). Lignite layers are also common. Pliocene se- diments are equivalent to the Vrbova fm. of the Sava basin (AVANIĆ et al., 2018a; HALAMIĆ et al., 2019), subdivided into five lithofa- cies units: massive and normally graduated gravels, cross strati- fied sands and gravelly sands, massive and laminated sands, clay silts and heterolithic facies (KUREČIĆ, 2017). Sandstones (litho- arenites) are limonitized and form thin interlayers within the sand The thickness of the Pliocene deposits is 200-400 m (PIKIJA, 1986). Plio-Quaternary clastics (Pl, Q) unconformably cover vari- ous older deposits, from Jurassic to Pontian ages (Figure 2). They consist of sands, gravels and clays, rarely siltites, conglomerates, sandstones, and coal-clays and coal interlayers (ŠIKIĆ, 2014b; PIKIJA, 1986). Sands and gravels alternate irregularly forming interlayers and lens-shaped bodies. Clays are deposited as cm to dm thick interlayers and lens-shaped bodies within the sands. The thickness of these deposits ranges between 50 to 100 m. Loess (Pleistocene – l) is a clayey-sandy silt of aeolian origin and covers older, Neogene deposits (PIKIJA, 1986). The main mineral constituent is quartz (70%), followed by feldspar (16%), rock fragments (10%) and muscovite (4%), with a carbonate con- tent of up to 13%. Carbonate and limonite concretions are often observed. Around Petrinja and Cerje, lenticular interlayers of fine-grained gravels are also seen. Loess has characteristic verti- cal fractures forming loess bluffs. The thickness of loess deposits is up to 30 m. Holocene alluvial sediments (a, ap) are the most widespread Quaternary sediments in the area. Alluvial plain sediments are located in the Sava River valley on the Lekenička Poljana - Sisak - Hrastelnica line (PIKIJA, 1986). Silts predominate over sands and gravels. Oxbow lake sediments form characteristic arched and elongated depressions, filled with mud, silt and sand depos- ited during floods. These deposits are usually up to 5 m thick. Holocene swamp deposits (b) are observed in the morpho- logically lowest parts of the Sava and Kupa River valleys. They are mostly composed of dark clays and clayey silts rich in organic matter (PIKIJA, 1986) 1 to 4 m thick. Diluvial and proluvial sediments (dpr) are predominantly represented by sands and silts, although gravels and larger boul- ders can also be observed. They formed by surface water runoff G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 193 and torrential flows from elevated areas, i.e. as a result of the combined seasonal deposition from mountain streams and creeks and ponds formed along margins of larger river valleys (e.g. the Sava, Una and Glina Rivers) (PIKIJA, 1986; ŠIKIĆ, 2014b). The thickness of these deposits is variable, locally it exceeds 10 m. 2.2. Tectogenesis of the research area The oldest exposed Jurassic and Cretaceous rock complexes of the Banija region are at least partly of ophiolitic origin, emplaced here as a result of obduction that began during the Middle Juras- sic and terminated during Middle Cretaceous times (SCHMID et al., 2020) for a regional tectonic aspect. The Upper Cretaceous volcano-sedimentary complex is locally characterised by pelagic “Scaglia-type” limestones and turbidites, interlayered with pil- low basalts and associated with a bimodal igneous succession comprising gabbros, doleritic dykes and rhyolites, all well ex- posed in the neighbouring Kozara Mts. in northern Bosnia and Herzegovina. There, this complex is interpreted as originating from a deep basin floored by oceanic lithosphere, that probably represented a remnant of the Vardar Ocean (USTASZEWSKI et al., 2009), which finally closed during the Palaeogene along the Sava suture zone (PAMIĆ, 2002; USTASZEWSKI et al., 2010; SCHMID et al., 2020). During the Early and Middle Miocene, the research area was affected by regional-scale extension linked to rift-related subsidence and formation of the Pannonian basin system, that in this area resulted in formation of the Sava basin and the neighbouring Karlovac basin (TOMLJENOVIĆ & CSONTOS, 2001; SAFTIĆ et al., 2003; USTASZEWSKI et al., 2010). After the Pontian and during the Plio-Quaternary the ex- tensional tectonic regime in the Pannonian basin was replaced by reginal-scale compression (BADA et al., 2007), which led to lo- cal inversion of previously active normal faults into faults with reverse sense of slip, often in association with newly formed re- verse and strike slip faults (TOMLJENOVIĆ and CSONTOS, 2001; USTASZEWSKI et al., 2014), such as those responsible for the Zagreb (March 22, 2020) and the Petrinja (December 29, 2020) earthquakes, respectively. 2.3. Seismicity Most of the recorded events of the Petrinja earthquake series (De- cember 2020 – January 2021) so far are attributed to the Petrinja composite seismogenic source that is known in the European Da- tabase of Seismogenic Faults (BASILI et al., 2013) under the code HRCS027. Based on fault characteristics and seismological con- siderations, the maximum expected magnitude that is presumed for this source is 6.5 in the moment magnitude scale. The strongest destructive MW 6.4 earthquake occurred on the 29th December 2020, 12:19 local time with an epicentre in the village of Strašnik, 6 km SW of Petrinja at a preliminary esti- mated hypocentre depth of 10 km. It caused the loss of seven lives and severe damage to buildings and infrastructure in the cities of Sisak, Petrinja, Glina, Dvor, Kravarsko and the surrounding vil- lages, but also in the remote town of Zaprešić, some 60 km NW of the epicentre. The intensity at the epicentre is estimated to be VIII-IX° of European Macroseismic Scale (EMS), described as heavily damaging to destructive. Before this event there were three foreshocks, the strongest of which with a magnitude 5.2 Mw the day before the main shock. Currently, the seismic series is still ongoing with already more than 1000 earthquakes above magnitude ML 1.5 recorded in the first couple of days after the mainshock. Aftershock seismicity is mostly located on the main seismogenic fault (HRCS027) with some earthquakes located on adjacent faults parallel to the main one. So far, recorded seismici ty indicates that a 20 km long section of the fault ruptured with aftershock depths ranging from 5 to 15 km. Results from this up to January 13th 2021, suggest that in Si- sak-Moslavina county, 33 590 buildings were reported as dama- ged and 15 626 were inspected by civil engineers and deemed as follows: – 2292 (15%) are classified as unusable; – 1655 (11%) are classified as temporarily unusable – detailed inspection needed; – 1986 (13%) are classified as temporarily unusable – emer- gency interventions needed; – 5830 (36%) are classified as usable with recommendations for action – 410 (3%) are classified as usable with no damage; – 3453 (22%) are classified as usable with no limitations. Among these, more than 3 000 building inspection reports encompass the problems related to the soil surrounding the build- ing, and the necessity for detailed geotechnical and geological inspection. According to the last census (2011) the population of Sisak- Moslavina county was 172 439, but in the most damaged areas (Petrinja, Sisak, Glina, Dvor and several smaller villages) the population was less than 90 000. Even if we ignore the emigra- tion of the population in recent years, the number of damaged buildings in comparison with the population is substantial. The direct damage of intensive ground-shaking on the build- ings and infrastructure are evaluated by several services and nu- merous experts, mainly civil engineers. Besides, the same earth- quake series also caused a variety of ground failures over a wider area, which represent additional, direct and indirect threats to the local community and their property, which have so far not been considered by those experts and Civil Protection services. 3. THE PRELIMINARY INVENTORY The ground failure inventories related to particular seismic events or series are prepared to improve the ability to assess earthquake- triggered ground failure hazards. Such inventories enable the use of analysis of effects of a single triggering event on particular ground failures or effects. The inventories are most commonly related to particular countries or regions. Therefore, United States Geological Survey (USGS) created an openly accessible, centrali- zed earthquake-triggered ground failure repository in the form of a ScienceBase Community to provide open access to data with the goal of accelerating research progress (SCHMITT, 2017). The digital inventories in that Community are created by both USGS and non-USGS authors. Italy has a very extensive database on earthquake-induced ground effects, which has been upgraded over time. The database contains data regarding landslides, liquefaction, ground cracks, surface faulting and ground changes triggered by earthquakes of Mercalli epicentral intensity 8 or greater, that occurred in the last millennium (MARTINO et al., 2014). The same database was up- graded with new architecture in order to increase its usefulness for both Public Institutions and individual users (CAPRARI et al., 2018). As already mentioned above, historical records of ground failures in Croatia are very scarce. Two catastrophic earthquakes with an estimated X° MCS scale are chronicled. For example, there is a legend about a possible earthquake that struck the island of Pag in the year 361 AD when, according to oral tradition, the G eo lo gi a C ro at ic a Geologia Croatica 74/2194 of the phenomena, e.g. a subsidence doline or landslide. In other cases, single records in the database rather present locations or zones where multiple appearances of some process of phenomena are documented (i.e. sand boils, lateral spreading and different type of ruptures). The inventory contains basic information about ground fail- ures: No. (number of record), ID (label for the observation point), ground failure/effect type, coordinates (coordinate reference sys- tem- HTRS96 Croatia TM), data provider, processing date (field identification date, date of registration) and reliability (Table 2). The reliability of the data in the inventory is defined on the level of expertise by the reporter and the precision of spatial position- ing. Therefore, the processes reported or verified by certified ex- perts with exact coordinates are considered as highly confident. Unverified reports by non-professionals with approximate loca- tions are considered to have low confidence. All cases in between are classified as medium confident. city of Cissa collapsed into the sea. In 1667, the catastrophic Du- brovnik earthquake caused thousands of deaths (the estimations on number of victims differ), totally demolished almost the whole city, triggered rockfalls of huge blocks from Srđ mountain that caused great devastation and initiated a tsunami that damaged ships in the harbour (HERAK et al., 2017a). More exact historical records about seismogenic ground ef- fects are mainly related to the earthquake near Zagreb in 1880. That earthquake with a magnitude of 6.3 initiated: liquefaction at several locations in the zone of Sava alluvium (Stupnik, Jarun, Resnik, Ivanja Reka, Drenje and Trstenik); raised the groundwa- ter table (Krapinske Toplice, Novo Čiče, Lukač); created suffosion processes forming dolines (Stubičke Toplice); ground ruptures (Kašina) and triggered rockfalls (Severin na Kupi) (TORBAR, 1882). In recent history, the earthquake near Zagreb ML 5.5 2020 March 22nd 2020 caused the loss of one life and considerable ma- terial damage (ŠAVOR NOVAK et al., 2020), but ground failures were not reported. However, as reported by BAČIĆ et al. (2020), geological and geotechnical phenomena within the underlying soil yielded significant damage to some of the buildings con- structed in the vicinity of identified faults and landslides. The much stronger Petrinja Mw 6.4 2020 earthquake initiated various processes/effects, which can generally be divided into two main groups: ground failures and other ground effects (Ta- ble 1). Therefore, the database presented here is adjusted for up to date knowledge about historical and recent earthquake-trig- gered ground failures in Croatia. In this paper it presents data collected from 30th of December 2020 until 15th of January 2021 related to the Petrinja 2020-2021 earthquake series only, but as has already been stated, the database will be continuously up- dated online, at the HGI Web Map Service at www.hgi-cgs.hr. It is important to point out that a single record in the database is for some processes/phenomena related to a single occurrence Table 1. The earthquake-triggered ground effects and subdivisions included in the inventory. Ground effects Processes Phenomena/Feature No. of occurrences / locations Ground failures Subsidence dolines Dropout doline 35 Suffosion doline 4 Liquefaction Sand boil 85 Lateral spreading 8 Subsidence 6 Landslides Slide 36 Rockfall 4 Other ground effects Geological Soil rupture 20 Clouding of springwater 1 Infrastructural Road rupture 54 Table 2. The preliminary inventory of ground failures and other ground effects after the Petrinja 2020 earthquake. No ID Ground failure type phenomena/process x y Data provider Processing date Data confidence level SUBSIDENCE 1 IK1 Dropout doline 494638 5015706 HGI 13.01.2021. high 2 IK10 Dropout doline 494230 5016289 HGI 13.01.2021. high 3 IK11 Dropout doline 494219 5016193 HGI 13.01.2021. high 4 IK12 Dropout doline 494236 5016257 HGI 13.01.2021. high 5 IK13 Dropout doline 494010 5016789 HGI 13.01.2021. high 6 IK14 Dropout doline 494002 5016794 HGI 13.01.2021. high 7 IK17 Dropout doline 493781 5017077 HGI 13.01.2021. high 8 IK18 Dropout doline 493677 5017093 HGI 13.01.2021. high 9 IK19 Dropout doline 493597 5017146 HGI 13.01.2021. high 10 IK2 Dropout doline 494663 5015641 HGI 13.01.2021. high 11 IK20 Dropout doline 493589 5017180 HGI 13.01.2021. high 12 IK21 Dropout doline 493598 5017171 HGI 13.01.2021. high 13 IK22 Dropout doline 493425 5017321 HGI 13.01.2021. high 14 IK23 Dropout doline 493431 5017341 HGI 13.01.2021. high 15 IK25 Dropout doline 493449 5017139 HGI 13.01.2021. high 16 IK26 Dropout doline 493459 5017136 HGI 13.01.2021. high 17 IK27 Dropout doline 493499 5017123 HGI 13.01.2021. high 18 IK28 Dropout doline 493556 5017112 HGI 13.01.2021. high 19 IK29 Dropout doline 493564 5017102 HGI 13.01.2021. high 20 IK30 Dropout doline 494514 5015761 HGI 13.01.2021. high 21 IK31 Dropout doline 494496 5015874 HGI 13.01.2021. high G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 195 No ID Ground failure type phenomena/process x y Data provider Processing date Data confidence level 22 IK32 Dropout doline 494462 5015878 HGI 13.01.2021. high 23 IK33 Dropout doline 494386 5015780 HGI 13.01.2021. high 24 IK34 Dropout doline 494321 5015781 HGI 13.01.2021. high 25 IK35 Dropout doline 493266 5016975 HGI 13.01.2021. high 26 IK4 Dropout doline 494186 5015950 HGI 13.01.2021. high 27 IK5 Dropout doline 494138 5015846 HGI 13.01.2021. high 28 IK6 Dropout doline 494100 5015895 HGI 13.01.2021. high 29 IK7 Dropout doline 494123 5016224 HGI 13.01.2021. high 30 IK8 Dropout doline 494120 5016287 HGI 13.01.2021. high 31 IK9 Dropout doline 494219 5016329 HGI 13.01.2021. high 32 IK-37 Dropout doline 493938 5016123 HGI 20.01.2021. high 33 IK-39 Dropout doline 493885 5016101 HGI 20.01.2021. high 34 IK-40 Dropout doline 494125 5016227 HGI 20.01.2021. high 35 IK-41 Dropout doline 492843 5017598 HGI 20.01.2021. high 36 IK15 Suffosion doline 493911 5016862 HGI 13.01.2021. high 37 IK16 Suffosion doline 493861 5017003 HGI 13.01.2021. high 38 IK24 Suffosion doline 493613 5016984 HGI 13.01.2021. high 39 IK3 Suffosion doline 494656 5015991 HGI 13.01.2021. high LIQUEFACTION 40 P-NDTV-12b Sand boil 481448 5032639 HGI 05.01.2021. high 41 P-NDTV-13a Sand boil 481504 5032582 HGI 05.01.2021. high 42 PE-NDTV-02 Sand boil 481883 5032780 HGI 05.01.2021. high 43 PE-NDTV-03 Sand boil 481838 5032783 HGI 05.01.2021. high 44 PE-NDTV-04 Sand boil 481834 5032784 HGI 05.01.2021. high 45 PE-NDTV-05 Sand boil 481814 5032793 HGI 05.01.2021. high 46 PE-NDTV-06 Sand boil 481827 5032750 HGI 05.01.2021. high 47 PE-NDTV-07 Sand boil 481800 5032746 HGI 05.01.2021. high 48 PE-NDTV-08 Sand boil 481736 5032716 HGI 05.01.2021. high 49 PE-NDTV-09 Sand boil 481653 5032627 HGI 05.01.2021. high 50 PE-NDTV-10 Sand boil 481629 5032610 HGI 05.01.2021. high 51 PE-NDTV-11 Sand boil 481436 5032602 HGI 05.01.2021. high 52 PE-NDTV-15 Sand boil 481837 5032832 HGI 05.01.2021. high 53 PE-NDTV-16 Sand boil 481783 5032842 HGI 05.01.2021. high 54 Si-01 Sand boil 491454 5037182 HGI 05.01.2021. high 55 Si-04 Sand boil 491773 5036963 HGI 05.01.2021. high 56 Si-05 Sand boil 491670 5037051 HGI 05.01.2021. high 57 V-01 Sand boil 481401 5033976 HGI 09.01.2021. high 58 RMT-11 Sand boil 476846 5040083 HGI 04.01.2021. high 59 RMT-12 Sand boil 476693 5040607 HGI 04.01.2021. high 60 RMT-17 Sand boil 470568 5030270 HGI 04.01.2021. high 61 RMT-19 Sand boil 472346 5038019 HGI 05.01.2021. high 62 RMT-20 Sand boil 468309 5036590 HGI 05.01.2021. high 63 RMT-21 Sand boil 468320 5036356 HGI 05.01.2021. high 64 RMT-22 Sand boil 468391 5036401 HGI 05.01.2021. high 65 RMT-23 Sand boil 468423 5036435 HGI 05.01.2021. high 66 RMT-28a Sand boil 469995 5025866 HGI 05.01.2021. high 67 RMT-34b Sand boil 477383 5041120 HGI 07.01.2021. high 68 RMT-35a Sand boil 477582 5040954 HGI 07.01.2021. high 69 RMT-36 Sand boil 477608 5040971 HGI 07.01.2021. high 70 RMT-37 Sand boil 477681 5040990 HGI 07.01.2021. high 71 RMT-37a Sand boil 477696 5041003 HGI 07.01.2021. high 72 RMT-37c Sand boil 477722 5041036 HGI 07.01.2021. high 73 RMT-49 Sand boil 470030 5033099 HGI 07.01.2021. medium 74 RMT-50 Sand boil 465677 5046800 HGI 07.01.2021. high 75 MV-1 Sand boil 481233 5034723 HGI 30.12.2020. high 76 MV-26 Sand boil 481241 5034479 HGI 04.01.2021. high 77 MV-28 Sand boil 485589 5036755 HGI 04.01.2021. high 78 MV-29 Sand boil 487149 5035782 HGI 04.01.2021. high 79 MV-31 Sand boil 481356 5033058 HGI 05.01.2021. high Table 2. Continued G eo lo gi a C ro at ic a Geologia Croatica 74/2196 No ID Ground failure type phenomena/process x y Data provider Processing date Data confidence level 80 MV-32 Sand boil 481392 5032955 HGI 05.01.2021. high 81 MV-34 Sand boil 493935 5042867 HGI 05.01.2021. high 82 MV-35 Sand boil 493257 5042439 HGI 05.01.2021. high 83 MV-36 Sand boil 492637 5042213 HGI 05.01.2021. high 84 MV-37 Sand boil 492202 5042157 HGI 05.01.2021. high 85 NP-13 Sand boil 487317 5035347 HGI 04.01.2021. high 86 NP-15 Sand boil 479994 5033591 HGI 05.01.2021. high 87 NP-22 Sand boil 492859 5041123 HGI 07.01.2021. high 88 NP-23 Sand boil 492724 5040426 HGI 07.01.2021. high 89 NP-26 Sand boil 491978 5036711 HGI 07.01.2021. high 90 NP-30 Sand boil 471583 5028895 HGI 07.01.2021. high 91 NP-31 Sand boil 480133 5034511 HGI 13.01.2021. high 92 JB-1 Sand boil 468823 5025602 HGI 07.01.2021. high 93 JB-2 Sand boil 468416 5024971 HGI 07.01.2021. high 94 JB-6 Sand boil 468364 5022176 HGI 07.01.2021. high 95 JB-7 Sand boil 468298 5022237 HGI 07.01.2021. high 96 ŠP-7 Sand boil 485645 5035151 HGI 04.01.2021. high 97 ŠP-8 Sand boil 491626 5042295 HGI 05.01.2021. high 98 ŠP-9 Sand boil 491659 5042255 HGI 05.01.2021. high 99 ŠP-17 Sand boil 485075 5035494 HGI 05.01.2021. high 100 TSS-2 Sand boil 481298 5034640 RGNf 05.01.2021. high 101 TSS-3 Sand boil 493197 5042455 RGNf 05.01.2021. high 102 GEOT-4 Sand boil 481485 5032868 GEOT 12.01.2021. medium 103 GEOT-12 Sand boil 481786 5035259 GEOT 12.01.2021. medium 104 GEOT-14 Sand boil 490382 5039444 GEOT 12.01.2021. medium 105 GEOT-28 Sand boil 481755 5033624 GEOT 12.01.2021. medium 106 GEOT-29 Sand boil 490464 5039161 GEOT 12.01.2021. medium 107 GEOT-30 Sand boil 467978 5022537 GEOT 12.01.2021. medium 108 GEOT-31 Sand boil 481632 5032863 GEOT 12.01.2021. medium 109 GEOT-32 Sand boil 481389 5032981 GEOT 12.01.2021. medium 110 NVI-05 Sand boil 481477 5032892 HGI 19.01.2021. high 111 NVI-05a Sand boil 481463 5032854 HGI 19.01.2021. high 112 NVI-06 Sand boil 481505 5032792 HGI 19.01.2021. high 113 NVI-07 Sand boil 481507 5032791 HGI 19.01.2021. high 114 NVI-10 Sand boil 481372 5032989 HGI 19.01.2021. high 115 GTI-01 Sand boil 481114 5034850 GTI 20.01.2021. high 116 GTI-02a Sand boil 481882 5032867 GTI 20.01.2021. high 117 GTI-02b Sand boil 481878 5032825 GTI 20.01.2021. high 118 VI-03 Sand boil 476421 5040754 HGI 20.01.2021. high 119 VI-09 Sand boil 475259 5037960 HGI 20.01.2021. high 120 VI-10 Sand boil 475561 5038219 HGI 20.01.2021. high 121 VI-11 Sand boil 475469 5038256 HGI 20.01.2021. high 122 VI-12 Sand boil 475035 5037857 HGI 20.01.2021. high 123 VI-13 Sand boil 475055 5037862 HGI 20.01.2021. high 124 VI-15 Sand boil 473643 5038416 HGI 20.01.2021. high 131 Si-02 Lateral spreading 491660 5036857 HGI 05.01.2021. high 132 RMT-35 Lateral spreading 477562 5040939 HGI 07.01.2021. high 133 NP-12 Lateral spreading 487151 5035769 HGI 04.01.2021. high 134 NP-21 Lateral spreading 492335 5042079 HGI 05.01.2021. high 135 GTI-06 Lateral spreading 481089 5034247 GTI 20.01.2021. high 136 VI-06 Lateral spreading 475295 5037837 HGI 20.01.2021. high 137 VI-07 Lateral spreading 475218 5037965 HGI 20.01.2021. high 138 VI-08 Lateral spreading 475348 5037831 HGI 20.01.2021. high 125 P-NDTV-05a Subsidence 481812 5032798 HGI 05.01.2021. high 126 P-NDTV-06a Subsidence 481825 5032746 HGI 05.01.2021. high 127 P-NDTV-11a Subsidence 481438 5032609 HGI 05.01.2021. high 128 P-NDTV-12a Subsidence 481471 5032626 HGI 05.01.2021. high 129 Si-03 Subsidence 491818 5036749 HGI 05.01.2021. high 130 MV-37a Subsidence 492202 5042157 HGI 05.01.2021. high Table 2. Continued G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 197 No ID Ground failure type phenomena/process x y Data provider Processing date Data confidence level LANDSLIDES 139 PE-NDTV-01 Slide 482878 5031095 HGI 05.01.2021. high 140 PE-NDTV-12 Slide 481474 5032619 HGI 05.01.2021. high 141 PE-NDTV-13 Slide 481496 5032594 HGI 05.01.2021. high 142 PE-NDTV-14 Slide 481511 5032676 HGI 05.01.2021. high 143 NP-16 Slide 482924 5025926 HGI 05.01.2021. high 144 ŠP-1 Slide 480887 5030303 HGI 04.01.2021. high 145 ŠP-3 Slide 479527 5029032 HGI 04.01.2021. high 146 TSS-9 Slide 464747 5024336 RGNf 05.01.2021. high 147 GEOT-1 Slide 464873 5027706 GEOT 12.01.2021. medium 148 GEOT-2 Slide 464471 5043053 GEOT 12.01.2021. medium 149 GEOT-3 Slide 475922 5031004 GEOT 12.01.2021. medium 150 GEOT-6 Slide 500430 5010142 GEOT 12.01.2021. medium 151 GEOT-7 Slide 468013 5022625 GEOT 12.01.2021. medium 152 GEOT-8 Slide 485421 5022788 GEOT 12.01.2021. medium 153 GEOT-10 Slide 482610 5023631 GEOT 12.01.2021. low 154 GEOT-11 Slide 467962 5022471 GEOT 12.01.2021. low 155 GEOT-13 Slide 480890 5032753 GEOT 12.01.2021. medium 156 GEOT-17 Slide 476987 5032775 GEOT 12.01.2021. low 157 GEOT-18 Slide 483558 5028393 GEOT 12.01.2021. medium 158 GEOT-21 Slide 480961 5034683 GEOT 12.01.2021. medium 159 GEOT-22 Slide 484231 5027591 GEOT 12.01.2021. medium 160 GEOT-25 Slide 482973 5026073 GEOT 12.01.2021. medium 161 GEOT-27 Slide 486499 5017777 GEOT 12.01.2021. medium 162 IK-KLZ1 Slide 490843 5018292 HGI 13.01.2021. medium 163 NVI-01 Slide 506036 5020302 HGI 19.01.2021. high 164 NVI-02 Slide 495761 5028047 HGI 19.01.2021. high 165 NVI-08 Slide 481404 5032745 HGI 19.01.2021. high 166 GTI-03a Slide 470225 5026717 GTI 20.01.2021. high 167 GTI-03b Slide 470238 5026682 GTI 20.01.2021. high 168 GTI-03c Slide 470121 5026487 GTI 20.01.2021. high 169 VI-01 Slide 477421 5040997 HGI 20.01.2021. high 170 VI-02 Slide 477694 5040845 HGI 20.01.2021. high 171 VI-04 Slide 473997 5038388 HGI 20.01.2021. high 172 VI-05 Slide 475133 5037725 HGI 20.01.2021. high 173 VI-17 Slide 472210 5038568 HGI 20.01.2021. high 174 VI-18 Slide 472124 5038608 HGI 20.01.2021. high 175 HR-DN-02 Rockfall 482859 5028347 HGI 05.01.2021. high 176 RMT-24 Rockfall 462393 5031336 HGI 05.01.2021. high 177 ŠP-16 Rockfall 483128 5028418 HGI 04.01.2021. high 178 IK-ODR Rockfall 480287 5027202 HGI 14.01.2021. high GEOLOGICAL 179 RMT-5 Soil rupture 475149 5033810 HGI 04.01.2021. high 180 RMT-7 Soil rupture 474978 5032745 HGI 04.01.2021. high 181 RMT-8 Soil rupture 474935 5032603 HGI 04.01.2021. high 182 RMT-10 Soil rupture 474902 5032794 HGI 04.01.2021. high 183 RMT-14 Soil rupture 474043 5038129 HGI 04.01.2021. high 184 RMT-27 Soil rupture 471742 5027712 HGI 05.01.2021. high 185 RMT-34a Soil rupture 477383 5041120 HGI 07.01.2021. high 186 RMT-37b Soil rupture 477704 5041022 HGI 07.01.2021. high 187 RMT-38 Soil rupture 477764 5041061 HGI 07.01.2021. high 188 MV-11 Soil rupture 478416 5031477 HGI 30.12.2020. high 189 MV-20 Soil rupture 478261 5031617 HGI 04.01.2021. high 190 NP-25 Soil rupture 491818 5036749 HGI 07.01.2021. high 191 TSS-4 Soil rupture 492419 5042102 RGNf 05.01.2021. high 192 TSS-6 Soil rupture 465989 5044352 RGNf 05.01.2021. high 193 TSS-8 Soil rupture 465701 5046760 RGNf 05.01.2021. high 194 GEOT-9 Soil rupture 475062 5032742 GEOT 12.01.2021. medium 195 GEOT-16 Soil rupture 479668 5039767 GEOT 12.01.2021. medium Table 2. Continued G eo lo gi a C ro at ic a Geologia Croatica 74/2198 No ID Ground failure type phenomena/process x y Data provider Processing date Data confidence level 196 GEOT-19 Soil rupture 481759 5035271 GEOT 12.01.2021. medium 197 VI-14 Soil rupture 473739 5038352 HGI 20.01.2021. high 198 VI-16 Soil rupture 472650 5038286 HGI 20.01.2021. high 253 HR-DN-01 Clouding of springwater 482977 5028375 HGI 05.01.2021. high INFRASTRUCTURAL 199 Si-06 Road rupture 490317 5039630 HGI 05.01.2021. high 200 RMT-3 Road rupture 477178 5033155 HGI 04.01.2021. high 201 RMT-13 Road rupture 476121 5036980 HGI 04.01.2021. high 202 RMT-15 Road rupture 479011 5032783 HGI 04.01.2021. high 203 RMT-16 Road rupture 470485 5030089 HGI 04.01.2021. high 204 RMT-18 Road rupture 473752 5036859 HGI 05.01.2021. high 205 RMT-25 Road rupture 471772 5027624 HGI 05.01.2021. high 206 RMT-26 Road rupture 471744 5027688 HGI 05.01.2021. high 207 RMT-28 Road rupture 469995 5025866 HGI 05.01.2021. high 208 RMT-31 Road rupture 479609 5041418 HGI 07.01.2021. high 209 RMT-32 Road rupture 479117 5041335 HGI 07.01.2021. high 210 RMT-33 Road rupture 477440 5041161 HGI 07.01.2021. high 211 RMT-34 Road rupture 477383 5041120 HGI 07.01.2021. high 212 RMT-40 Road rupture 470620 5039412 HGI 07.01.2021. high 213 RMT-41 Road rupture 469622 5038340 HGI 07.01.2021. high 214 RMT-42 Road rupture 469564 5038235 HGI 07.01.2021. high 215 RMT-43 Road rupture 469564 5038202 HGI 07.01.2021. high 216 RMT-44 Road rupture 469530 5038129 HGI 07.01.2021. high 217 RMT-45 Road rupture 469402 5038127 HGI 07.01.2021. high 218 RMT-46 Road rupture 469364 5038134 HGI 07.01.2021. high 219 RMT-47 Road rupture 469213 5038807 HGI 07.01.2021. medium 220 RMT-48 Road rupture 468228 5037625 HGI 07.01.2021. high 221 RMT-51 Road rupture 464773 5042397 HGI 07.01.2021. high 222 RMT-52 Road rupture 465548 5043364 HGI 07.01.2021. high 223 RMT-53 Road rupture 466068 5044321 HGI 07.01.2021. high 224 MV-2 Road rupture 481193 5034203 HGI 30.12.2020. high 225 MV-3 Road rupture 478036 5032639 HGI 30.12.2020. high 226 MV-8 Road rupture 479017 5032643 HGI 30.12.2020. medium 227 MV-9 Road rupture 478292 5031591 HGI 30.12.2020. high 228 MV-10 Road rupture 478342 5031565 HGI 30.12.2020. high 229 MV-12 Road rupture 478544 5031382 HGI 30.12.2020. high 230 MV-13 Road rupture 478842 5031213 HGI 30.12.2020. high 231 MV-14 Road rupture 479186 5031158 HGI 30.12.2020. high 232 MV-15 Road rupture 479385 5031222 HGI 30.12.2020. high 233 MV-16 Road rupture 479975 5031360 HGI 30.12.2020. high 234 MV-17 Road rupture 480701 5030415 HGI 30.12.2020. high 235 MV-18 Road rupture 480379 5030066 HGI 30.12.2020. high 236 MV-19 Road rupture 480192 5029947 HGI 30.12.2020. medium 237 MV-21 Road rupture 478159 5031553 HGI 04.01.2021. high 238 MV-22 Road rupture 478950 5032157 HGI 04.01.2021. high 239 MV-24 Road rupture 481153 5033888 HGI 04.01.2021. high 240 MV-25 Road rupture 481370 5034062 HGI 04.01.2021. high 241 MV-27 Road rupture 481360 5034587 HGI 04.01.2021. high 242 MV-30 Road rupture 481419 5033078 HGI 04.01.2021. high 243 MV-33 Road rupture 483311 5026330 HGI 05.01.2021. high 244 JB-3 Road rupture 470110 5025591 HGI 07.01.2021. high 245 ŠP-5 Road rupture 483356 5026406 HGI 04.01.2021. high 246 ŠP-10 Road rupture 485695 5035094 HGI 05.01.2021. high 247 ŠP-11 Road rupture 480333 5027108 HGI 04.01.2021. high 248 ŠP-12 Road rupture 479690 5026544 HGI 04.01.2021. high 249 ŠP-14 Road rupture 480866 5030242 HGI 04.01.2021. high 250 ŠP-18 Road rupture 487411 5025085 HGI 05.01.2021. high 251 TSS-7 Road rupture 478145 5031572 RGNf 05.01.2021. high 252 GTI-07 Road rupture 485160 5034297 GTI 20.01.2021. high Table 2. Continued G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 199 3.1. Ground failures Ground failures are largely related to subsidence, liquefaction, and landsliding processes. 3.1.1. Subsidence dolines Subsidence is the sudden sinking or gradual downward settling of the Earth’s surface with little or no horizontal motion (BATES & JACKSON, 1980). Subsidence may be caused by different natu- ral geological processes, but here subsidence caused by suffosion or liquefaction is of primary concern. The ground shaking caused by the Petrinja earthquake series triggered rapid development of subsidence dolines in the wider Mečenčani area, about 20 km SE of the mainshock epicentre. Ac- tually, in this area dolines were registered throughout history and obviously, strong earthquake accelerated the process significantly. Theoretically, two types of subsidence dolines are recognised: suf- fosion and dropout dolines (KRANJC, 2013; Figure 3). Both types are developed by suffosion, a filtration process in which fine par- ticles are washed away from cover sediments overlying carbonate bedrock or other soluble rocks, into their karstified fissures or ca- verns. In the area of Mečenčani, mainly dropout dolines are de- veloped. In this type of doline cover sediments are evacuated downwards through solution pipes in the bedrock creating a void at the bedrock-sediment contact that enlarges by upwards propa- gation through the clastic coverbeds (WILLIAMS, 2004; Figure 3). At the time of writing this manuscript, 35 records were en- tered in the inventory (Figure 4, Table 1), but up to February 15th 2021, more than ninety (90) subsidence dolines were registered in the area between Borojevići and Mečenčani in the Donji Kuku- ruzari municipality. The great majority of these are classified as dropout dolines formed by the sudden (or over a few hours) col- lapse of cover sediments into the underground voids, forming steep-sided cylindrical depressions. The majority of the dolines are between 1 and 10 m in diameter, but the largest one is more Figure 3. The sketches of: a) suffosion doline and b) dropout doline. Reproduced from WALTHAM & FOOKES (2003). Figure 4. Dropout dolines in Mečenčani village: a) endangering adjacent houses (observation point IK31), b) the biggest doline, more than 20m in diameter and about 12 m deep (observation point IK4). a) b) a) b) G eo lo gi a C ro at ic a Geologia Croatica 74/2200 than 20 m in diameter, and is still enlarging. Therefore, their fi- nal dimensions are not yet known. New dolines are still forming, almost twenty days after the main earthquake event. Other than dropout dolines, the suffosion dolines are also registered (at the Mečenčani school backyard and in fields near the village), indicating the gradual subsidence of superficial cov- erbeds which form a funnel-shaped depression at the surface (Ta- ble 1). As opposed to dropout dolines, in this case the process advances from the surface, usually in noncohesive sediment, as the finer fraction tends to move into the cavity underneath, whereas the coarser fraction remains closer to the surface (KRANJC, 2013). 3.1.2. Liquefaction Liquefaction is a phenomenon wherein a mass of soil loses a large percentage of its shear resistance and flows in a manner re- sembling a liquid. The basic cause responsible for the liquefac- tion of saturated sands is the build-up of excess pore water pres- sure due to either cyclic or shock loading of the sand (e.g. during an earthquake). If drainage cannot take place, then the tendency to compress the volume of the grains causes an increase in pore water pressure. In loose sands, pore water pressure can build up to the point where it is the same as the overburden pressure, caus- ing complete loss of strength which initiates liquefaction. How- ever, NORRIS et al. (1998) pointed out that a loose sand does not always lose all strength during liquefaction. Loose sands at low confining pressure, and medium and dense sands undergo only limited deformation due to dilation once initial liquefaction has occurred. Such a response is referred to as ranging from limited liquefaction (in the case of loose and medium dense sands at low confining pressure) to dilative behaviour (in dense sands) (BELL, 2007). Liquefaction is more likely to occur in sandy or non-plas- tic silty soils, but may in rare cases occur in gravels and clays. The increase of the portion of finer particles in the sand somewhat increases the resistance to liquefaction. Also, liquefaction is most commonly reported in the upper 12 to 15 m of the soil profile. The liquefaction of subsurface layers in the research area is recognised by sand boiling phenomena. A sand boil is sand and water that erupts out onto the ground surface during an earth- quake as a result of liquefaction at shallow depths (USGS Earth- quake Glossary, 2021). The research area exhibited many loca- tions with such features, eighty-five are in the database at present (Table 2). Most of the sand boils are aligned in a row along a line ar fracture exposed at the surface (Figure 5). Locations of sand boil phenomena are found in several areas adjacent to the rivers Sava, Kupa and Glina and are clear indications of liquefied zones. Coseismic liquefaction in the research area also initiated late- ral spreading, subsidence and river bank failures. The aforemen- tioned processes are in some cases interrelated. Lateral spreading is a type of movement that refers to movement of rock blocks or very coherent soil masses that rest on soft, deformable material (GONZÁLEZ DE VALLEJO & FERRER, 2011). These movements are due to the loss of strength of the underlying material, which either flows or is deformed un- der the weight of the rigid blocks. Here, the lateral spreading is the result of liquefaction of the saturated and loose sandy layers, and occurred on the flood em- bankments of the river Sava but also on the natural Kupa and Petrinjčica river banks (Figure 6). Horizontal displacements in most of the cases are from a few centimetres to several decime- tres. Subsidence caused by liquefaction is related to larger liqui- fied zones and is registered at six sites. It is recognised or sus- pected in populated areas of Petrinja (Vinogradska and Milana Makanca street zones), Sisak and at the edges of smaller settle- ments Stari Brod, Letovanić, Brest, Drenčina and Palanjek. Ac- cording to our knowledge, the vertical displacements in densely populated areas are up to 10 centimetres. Larger vertical displace- ments of several decimetres are noted in the immediate vicinity of the Kupa and Sava rivers (Figure 7). 3.1.1. Landslides Landslide is a general term covering wide variety of mass-move- ment landforms and processes (including slides and rockfalls) involving the downslope transport of soil and rock material, un- der gravitational influence. Landslides can be triggered by earth- quakes. According to KEEFER (1984) an earthquake with a Richter magnitude of 4 would probably not generate slides, but rockfalls and rock slides could be triggered by the weaker seis- mic tremors. Figure 5. The sand boiling features along linear fractures: a) in Letovanić (aerial photo); b) in Sisak (observation point Si-04). a) b) G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 201 Slide is a landslide characterized by a shearing and rotary movement of a generally independent mass of rock or soil along the curved slip surface (concave upward) and about an axis pa- rallel to the slope from which it descends (BATES & JACKSON, 1980). Rockfall is another type of landslide characterised by pre- cipitous movement, free falling, bouncing or rolling, of a newly detached segment of bedrock of any size from a cliff or other very steep slope (BATES & JACKSON, 1980; DORREN, 2003). Along the detachment surface, little or no shear displacement takes place (VARNES, 1978). Thirty- six slides and four rockfalls have been registered so far, after the Petrinja 2020 earthquake. Most of the registered slides or rockfalls are older landslides reactivated by ground mo- tion. Their nature and horizontal displacements vary greatly (Figu re 8). It has been noted that even minor or not visible dis- placements, together with ground shaking could generate greater material damage on the structures located at the edge or inside the colluvium material. Some of the areas affected by the reacti- vation of landslides (mainly shallow ones), caused by the earth- quake, are Hađer, Stari Farkašić, Petrinja and Sunja. Also, as al- ready mentioned, in some cases slope failures are the consequence of liquefaction and lateral spreading. That is particularly the case in the zones where lateral support of the top layer is absent, domi- nantly along the river banks of the Kupa and Petrinjčica rivers (Figure 6). Rockfalls have also been registered. Most of them are located below known rockfall source areas or steep slopes. Since rock- falls are very rapid to extremely rapid events, ground motions of this earthquake immediately triggered block motions causing rolling and bouncing of the blocks of various sizes into the foot of the slope (Figure 8b). 3.2. Other ground effects Other registered coseismic ground effects will be shortly pre- sented, because these effects still remain to be evaluated and in- terpreted in the near future. So far, the observed ground effects are preliminary differentiated according to the medium in which Figure 6. Lateral spreading and associated processes: a) failure of the flood embankment in Sisak (observation point Si-03); b) road failure in Letovanić (observa- tion point RMT-35). Figure 7. Subsidence and lateral spreading of the embankments near the Sava river: a) in Sisak (observation point Si-03); b) in Palanjek (observation point NP-21). a) b) a) b) G eo lo gi a C ro at ic a Geologia Croatica 74/2202 they are registered. Earthquake-induced ground effects in natural materials are classified as geological, while all other effects on man-made structures are considered as infrastructural (Table 1). A number of soil ruptures are observed in various litholo- gies (Figure 2). In general, two types of soil ruptures can be dif- ferentiated. The first group comprises ruptures closely related to the main seismogenic fault zone that triggered the major earth- quake. The second group comprises soil ruptures that are related to liquified zones and surface subsidence. Fracture displacements are usually up to a few centimetres, rarely several decimetres (Figure 9). At a single location in Hrastovica, the displacement or cloud- ing of springwater is registered. Here, before the main earth- quake, the groundwater catchment collected all waters above the family house, but after intensive ground shaking, the groundwa- ter emergence was scattered in a wider zone causing groundwa- ter eruption inside and around the family house (Figure 10a). Numerous road ruptures are also registered (Figure 10b). The great majority of these fractures are in the wider zone of the activated seismogenic fault (Figure 2). Other road ruptures fol- low minor, local faults, or are related to liquefaction-induced subsidence. Figure 8. Coseismic landslide activation: a) slide in forest road embankment (near Strašnik, observation point ŠP-3); b) rockfall near Pecki that blocked the local road (observation point IK-ODR). Figure 9. Soil rupture: a) near Letovanić (observation point RMT-38); b) near Župić village (observation point MV-11). a) b) a) b) G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 203 4. DISCUSSION AND CONCLUSIONS As seen from the geological map, the locations of noted coseis- mic ground failures are strongly related to the geological setting and composition (Figure 2). Therefore, this chapter provides pre- liminary comments on the relationship between the geological setting and ground failures occurrences in the research area. In addition, details are presented about existing geological and other relevant data, together with recommendations for further explo- rations. The spatial distribution of various types of coseismic ground failures/effects in the Petrinja earthquake seismic series is shown in Figure 2. A similar up to date map view presentation of the collected data, will be available in the form of WMS at the Croa- tian geological survey web portal (www.hgi-cgs.hr). Despite the fact that data collection for this publication was limited to only two weeks after the major December, 29th 2020 Petrinja earth- quake, the spatial distribution of the recorded coseismic ground failures and their generating processes already indicate endan- gered areas. It is possible that further data collection will discover additional locations or areas affected by different post-seismic effects, but here, the comments and conclusions are related to the available information. 4.1. Subsidence dolines area The subsidence dolines are exclusively found in the area around the villages of Mečenčani and Borojevići. This area was surveyed in detail during the hydrogeological investigations for Pašino vrelo (ŠIMUNOVIĆ & HEĆIMOVIĆ, 1998; MRAZ, 2005; MRAZ et al., 2007; LARVA et al., 2010). The terrain is charac- terised by alluvial and proluvial sediments (al-Q2) which overlie Miocene rocks (Figure 11). Freshly exposed steep banks of newly formed dolines facilitate understanding of the nature of alluvial sediments which are soil mixtures of various fractions from clay to gravel. The thickness of these deposits varies from several to ~15 metres in the vicinity of the river Sunja. Older Miocene deposits (M3) are represented by clayey sands and clays. They are overlain by M4 deposits, beginning with conglomerates or brec- cias followed by Lithothamnium limestones. According to geo- physical investigations, those limestones are more than 100 m thick in some parts (KRSNIK, 2005). The Lithothamnium limestones represent the main aquifer for Pašino vrelo. The upper weathering zone in the limestone is from 10 to 25 m thick, with the frequent appearance of wide-open joints, smaller cavities and caverns (MRAZ, 2005). Groundwater level in this area is very shallow, from 1.2 to 2.0 m. All of the fac- tors are favourable for a good yield of the Pašino vrelo which is used for the public water supply. The Pašino vrelo is a natural spring, from an old dropout doline, with a short stream that flow toward the Sunja River. There are several older dolines in the area, and some of them are periodic rather than permanent springs. It is important to emphasize that groundwater extraction at the Pašino vrelo does not significantly influence the total groundwater flow in the area, and the well influence is limited to several tens of metres around it. The closest buildings in villages Figure 10. a) clouding of springwater in Hrastovica (observation point HR-DN-01); b) road ruptures near Pokupsko Vratečko (observation point RMT-43). a) b) G eo lo gi a C ro at ic a Geologia Croatica 74/2204 are some 200m from the spring. Since the aquifer is very karsti- fied, the main task in forthcoming future research will be recog- nizing the main preferential flow paths typical for karst aquifers. Groundwater flow is one of the main causes for the development of dropout dolines and they occur mainly above these zones. This process happens regularly in this area, but over long periods of time. The Petrinja earthquake series significantly accelerated the collapse of several caverns and initiated the rapid development of new dolines, which represents a serious threat to the houses in this area. Since some of the newly formed dolines are developing very closely to family houses and even below them, further detailed research is urgent. Firstly, remote sensing (with UAVs – un- manned aerial vehicles) of the wider Mečenčani and Borojevići area in order to determine the exact positions, sizes, and spatial distribution of the dolines was requested. Analysis of a detailed digital terrain model obtained by UAV data acquisition and the possible detection of newly formed surface depressions can go- vern the positioning of geophysical profiling and core drilling in order to define the depth of soil cover on top of the limestones, detect existing underground cavities and define their size, inter- relationships and provide sampling from deeper horizons. Bore- hole in situ tests, installation of piezometer and laboratory analy- sis of the soil samples may also be needed in order to provide data for the detection of failure mechanisms, and the influence of seis- mic waves and ground water interaction causing rapid develop- ment of dolines in this area. It was suggested that site exploration covers the wider area in order to enable the production of a de- tailed subsidence susceptibility map. In addition, monitoring of the area should be established. Monitoring should include periodic remote sensing, groundwater level monitoring and monitoring of ground shaking intensity. These data, with the results of the other aforementioned investi- Figure 11. The distribution of subsidence dolines in the wider Mečenčani and Borojevići area on the geological map (originally in the scale 1:5 000; MRAZ, 2005). G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 205 gations, should provide insight into the rates of doline develop- ment in time and their relationship to ground conditions. Suffosion processes, enlarging and forming new subsidence dolines in this area are still rapidly progressing, therefore re- search in this area should be given a high priority in dealing with geological hazards induced by earthquakes. Within the forthcom- ing winter and spring, when the hydrological maximum usually occurs in this area (peaks of both the precipitation and the groundwater levels), new dolines can be formed, especially if some stronger aftershocks occur when groundwater levels are high. Locations of the dolines suggest preferential flow paths in the karstic underground, and these zones will be the main target of geophysical profiling (electrical tomography and refraction seismic). These preferential flow paths in karst aquifers are usu- ally caused by fault or fracture zones where the rock mass is highly karstified. Above these karstified rocks, suffosion of Qua- ternary deposits is significantly stronger than in other areas, so the combination of karstification and suffosion weakens these zones, while the earthquake accelerates the whole process. It is important to emphasize that groundwater level (pressure) in the karstic aquifer is usually higher than that in the Quaternary de- posits overlying the aquifer, which also increases the suffosion at the soil/rock contact. 4.2. Liquefied areas Liquefaction is mainly traced by boiling sand phenomena at the surface (Figure 5). According to the preliminary data, most of the phenomena are spotted in the recent alluvial sediments related to the rivers Sava, Kupa and Glina (Figure 2). The alluvium of these recent rivers is dominated by pure sands, only locally by silts. Siltose or gravelly sands, and lenses of pure gravels are also observed (PIKIJA et al., 1986). The ma- terial ejected at the surface is predominantly classified as pure sand, which is confirmed by granulometry laboratory analysis (Figure 12). Locally, liquefaction effects are registered in the flood sedi- ments. These sediments are dominantly fine grained clayey and sandy silts or fine-grained sands (PIKIJA et al., 1986). Due to their depositional nature (sedimentation from an aqueous suspen- sion after flooding), their thickness depends on the terrain con- figuration and is usually up to 5 m. The surface effects of liquefaction are not registered in the minor and thinner alluvium deposits of smaller rivers including the Petrinjčica, Moštanica and other streams (Figure 2). Namely, their alluvial sediments are dominated by gravels (PIKIJA et al., 1986), coarser sediments less susceptible to liquefaction. The distances of the boiling sand occurrences from these ri- vers also confirms the strong relationship between liquefaction and the larger alluvial sedimentary bodies. Namely, distances measu- red from the aforementioned rivers are up to 1000 metres. The exception is the area of the Vinogradska street in Petrinja, where the liquefied area also involves Pliocene sediments in the base (clays, gravels and sands) (according to available geological map in the scale 1: 100 000). Another isolated case of possible lique- faction in Pliocene sediments is registered in Posavci near Kra- varsko, situated far away from any significant river flow (> 8000 m). Therefore, for these reasons, the possibility of liquefaction of saturated sandy layers in Pliocene sediments should be studied in more detail. The preliminary conclusion is that liquefaction almost ex- clusively occurred in widespread and thicker alluvial sediments containing significant beds or lenses of loose and “pure” sand layers. Considering all the collected data in the research area, fa- vourable conditions for liquefaction are related to the Sava, Kupa and Glina alluvium or floodplain sediments which, according to rough and preliminary estimations cover more than 100 km2 (Figure 13). The presented map is based solely on current data- base records and existing geological data at the scale of 1:100.000 and should be considered as a rough estimation. Detailed studies are needed in order to define liquefaction susceptibility zones and mitigation measures. As can be seen, liquefaction-induced lateral spreading and ground subsidence can cause considerable damage to civil infra- structure. In this area, lateral spreads frequently occurred on the fairly flat area alongside streams or river courses causing river bank failures. Ground subsidence usually happened in the near vicinity of the river courses where loose sandy layers are of greater thickness and are completely saturated with water. This resulted in the settling of houses, buckling of roads, and subsid- ence of flood embankments. It is important to note that the set- tlement and tilting of some buildings is also observed in the post- seismic period and even though some of these are marked as ‘usable’ from a structural point of view. Their behaviour is strongly affected by the ground conditions and can lead to state limits being exceeded. It is important to point out that the occurrence of liquefac- tion in some layers is not necessarily associated with the damage of structures. If a thick layer of non-liquefiable soil exists near the ground surface, and if the thickness of the underlying layer of liquefied sand is low, then the damaging effects of liquefaction will not be manifested at the ground surface (ISHIHARA, 1985). Still, considering great potential for damage as a consequence of liquefaction and its related processes, it is suggested that more detailed engineering geological and geotechnical exploration is performed. The exploration should cover the whole Croatian re- gion that meets liquefaction-favourable conditions and are also zones of high seismicity. The aim of such a study should be the mitigation of, and preparedness for liquefaction-induced damage. There are many international researchers who have studied liquefaction potential and defined liquefaction-induced hazards. Maps showing the degree of liquefaction susceptibility are based on the likely response of subsurface materials to seismic loading Figure 12. Granulometric diagram of sand boil material sampled at the surface in Brest Pokupski. The analysis is done in accordance with HRN EN ISO 17892- 4:2016 norm. G eo lo gi a C ro at ic a Geologia Croatica 74/2206 (YOUD & PERKINS, 1987; KAJIHARA et al., 2016). A study about the liquefaction potential of sediments in the Zagreb area was presented by VEINOVIĆ et al. (2007), who also authored a doctoral thesis entitled Evaluation of possibility for liquefaction susceptibility and setting bases for liquefaction zonation in Croatia (VEINOVIĆ, 2007), which could be a good basis for starting research in potentially endangered zones. 4.3. Landslides Landslide locations are scattered across the research area but are mostly related to particular lithologies (Figure 14), being pre- dominantly observed in Pl and Pl-Q sediments. Both units are characterised by clays, sands and gravels, and are well known for frequent landslides in the much wider region. Several landslides in loess sediments are recognised here, which are also observed in the wider Kutina area. Landslides on top of “rocky” M5, M4 and Pc-E units are probably related to soil material in their weathe ring zones. The alluvial (a, ap) and diluvial (dpr) units are presented sepa- rately, because sliding processes on flat areas in these sediments are frequently related to liquefaction processes and lateral spread- ing along river banks, causing river bank failure (Figure 14). Factors to be considered in assessing the potential for earth- quake-induced landslides, according to GONZÁLEZ DE VALLEJO & FERRER (2011) are the presence of unstable slopes or slopes in precarious states of stability prior to the earthquake, steep slopes, soils with low strength or metastable structures (quick clays, collapsible soils) and rock scarps with potential rockfall hazards. Therefore, in estimations of coseismic landslide susceptibility, besides the usual parameters, the aforementioned additional factors should be considered. The authors’ opinion is that due to different geological medium and failure mechanisms, sliding and falling susceptibility should be analysed separately. As for other susceptibility maps, landslide susceptibility analysis should start with inventory. The reliable inventories are based on visual analysis of remote sensing derivates. Besides, ap- propriate to the scale/details in which the map is prepared, a rele- vant geological map is essential. Depending on the specific prope- rties of the area in focus, additional factors and data are combined in order to produce a reliable landslide susceptibility map. Until now in Croatia, various landslide susceptibility maps at different scales were prepared by different engineering-geologists, but for Figure 14. The proportional distribution of landslides in particular geological units. Figure 13. Rough and preliminary estimations of the area with favourable conditions for liquefaction processes (shaded blue). The estimation is based on recent surface processes caused by liquefaction (blue dots) and existing Basic Geological Map in the scale 1: 100.000. G eologia C roatica Pollak et al.: The preliminary inventory of coseismic ground failures related to December 2020 – January 2021 Petrinja earthquake series 207 this region three maps are relevant. For the whole Croatian terri- tory, a small-scale landslide susceptibility map displays general susceptibility for large regions (PODOLSZKI et al., 2014; ERAK, 2018). Regional sliding susceptibility map for the whole of Sisak- Moslavina County at the scale of 1: 100.000 has been prepared for the safEarth project (BOSTJANČIĆ et al. 2021). Rockfall susceptibility maps consider different input factors. For a regional approach, the rockfall inventory is essential. The analysis of large areas subjected to such processes is based on the identification of the rockfall source areas using remote sensing techniques. A detailed bare earth digital terrain model (DTM) obtained by LIDAR can help in defining rockfall source areas. Using a detailed (1:5.000) geological map and the aforementioned remote sensing based inventory, it is possible to produce a reliable rockfall susceptibility map (TOŠEVSKI, 2018). Such a map should indicate zones endangered by falling rocks triggered by intensive precipitation or earthquakes. The procedure for a more detailed rockfall threat assessment, for specific locations, require much more data and field work, and is described in BOSTJANČIĆ & POLLAK (2020). Earthquakes and related natural geohazard processes cannot be prevented, but their effects on construction can be mitigated. Therefore, the focus of geological experts is directed toward haz- ard identification and preventive measures. In order to undertake that successfully, systematic research and extensive studies of various geological hazards are essential. Geological hazards are studied in different projects and their results are applied locally, but central inventories of the geohazard processes are not sup- ported. 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