Environmental effects and seismogenic source characterization of the December 2020 earthquake sequence near Petrinja, Croatia

Abstract

Summary On 29 December 2020, a shallow earthquake of magnitude Mw 6.4 struck northern Croatia, near the town of Petrinja, more than 24 hours after a strong foreshock (Ml 5). We formed a reconnaissance team of European geologists and engineers, from Croatia, Slovenia, France, Italy and Greece, rapidly deployed in the field to map the evidence of coseismic environmental effects. In the epicentral area, we recognized surface deformation, such as tectonic breaks along the earthquake source at the surface, liquefaction features (scattered in the fluvial plains of Kupa, Glina and Sava rivers), and slope failures, both caused by strong motion. Thanks to this concerted, collective and meticulous work, we were able to document and map a clear and unambiguous coseismic surface rupture associated with the main shock. The surface rupture appears discontinuous, consisting of multi-kilometer en échelon right stepping sections, along a NW-SE striking fault that we call the Petrinja-Pokupsko Fault (PPKF). The observed deformation features, in terms of kinematics and trace alignments, are consistent with slip on a right lateral fault, in agreement with the focal solution of the main shock. We found mole tracks, displacement on faults affecting natural features (e. g. drainage channels), scarplets, and more frequently breaks of anthropogenic markers (roads, fences). The surface rupture is observed over a length of ∼13 km from end-to-end, with a maximum displacement of 38 cm, and an average displacement of ∼10 cm. Moreover, the liquefaction extends over an area of nearly 600 km² around the epicenter. Typology of liquefaction features include sand blows, lateral spreading phenomenon along the road and river embankments, as well as sand ejecta of different grain size and matrix. Development of large and long fissures along the fluvial landforms, current or ancient, with massive ejections of sediments is pervasive. These features are sometimes accompanied by small horizontal displacements. Finally, the environmental effects of the earthquake appear to be reasonably consistent with the usual scaling relationships, in particular the surface faulting. This rupture of the ground occurred on or near traces of a fault that shows clear evidence of Quaternary activity. Further and detailed studies will be carried out to characterize this source and related faults in terms of future large earthquakes potential, for their integration into seismic hazard models.

Rapid Remeasure of Dense Civilian Networks as a Game‐Changer Tool for Surface Deformation Monitoring: The Case Study of the Mw 6.4 2020 Petrinja Earthquake, Croatia

“…The fault is discretized into 2 km wide sub-faults defined by 15 independent nodes along-strike and 8 along-dip. We perform inversions with different dips of the fault ranging from purely vertical to 70°SW, in agreement with the available data (Baize et al, 2022;Xiong et al, 2022). We invert for the rake and slip amplitude at each node using the horizontal components of both continuous GNSS and civilian benchmark displacements.…”

“…Interseismic velocities (blue arrows) are plotted relative to stable Eurasia (Zurutuza et al, 2019). Focal mechanisms of historical earthquakes are plotted together with known active or potentially active faults (see Baize et al (2022) and references therein). Inset: present-day regional kinematics (Nocquet, 2012).…”

“…Finally, the coseismic displacement field seems unaffected by landslides or liquefaction zones (from Baize et al. (2022)), except for one station near Sisak that was therefore discarded (Figure 2 and Data Set S2 in Supporting Information ).…”

“…The code solves the equations from (Okada, 1985) relating surface displacements to slip on dislocations at depth within a homogeneous elastic half‐space. We use the mapped surface rupture and the published hypocenters distribution (Baize et al., 2022) to constrain the geometry and position of the fault which is 28 km‐long with a strike of N130°, and extends up to 14 km at depth. The fault is discretized into 2 km wide sub‐faults defined by 15 independent nodes along‐strike and 8 along‐dip.…”

“…Seismic catalog, surface rupture evidences, traces of liquefaction or coseismic landslides are extracted from Baize et al. (2022). (b) Daily time‐series of continuous station SISA (Sisak city).…”