Description: 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

Description: Published

Description: Gather Online

Description: 2T. Deformazione crostale attiva

Description: The Apennines-Sicilian-Maghrebian fold-and-thrust belt originated from the subduction of the Alpine Tethys and the later collision of drifted continental blocks against the African and Apulian paleomargins. From North to South, the Sicilian Fold-and-Thrust Belt (SFTB) is divided in four main tectono-stratigraphic domains: (1) the Calabro-Peloritani terrane, drifted from the European margin, (2) the remnants of the Alpine Tethys accretionary Wedge (ATW) related to the subduction of the Tethys, (3) the folded and thrusted platform (Panormide) and deep-water (Imerese-Sicanian) series of the offscrapped African margin, and (4) the African foreland (Hyblean). Unfortunately, scarce quality seismic lines and outcrops of key tectono-stratigraphic units make the structure and dynamic evolution of the central-eastern part of the SFTB controversial. First, this study outlines through a review of the tectono-stratigraphic evolution of the central-eastern sector of the Sicilian orogen, the major remaining issues concerning: (1) the occurrence of inferred Alpine Tethys units far from the region where the remnants of the ATW outcrop (Nebrodi Mountains); both, in a forearc position above the Peloritani block to the North and in an active foreland context along the present day southern front of the belt; and (2) the diverging tectonic styles, from stacked large-scale tectonic nappes to foreland imbricated thrust systems rooted into a main basal décollement. Secondly, new constraints are given using analogue modeling to test mechanically the hypothesized structural and tectono-stratigraphic evolution of the SFTB. The experiment simulates the orogenic evolution of the SFTB at crustal-scale, from the Oligocene Tethys subduction, to the Middle Miocene-Late Pliocene continental collision between the European and African paleomargins. The tectono-stratigraphic synthesis is used to model the first-order mechanical stratigraphy of the sedimentary units involved in the Sicilian belt, as well as the imprint of the African margin structural inheritance. The experiments succeed in reproducing the general structure and tectono-stratigraphic evolution of the SFTB. In particular, the models support field observations hypothesing a gravity-driven origin of the inferred Alpine Tethys units intercalated within the forearc and foreland syntectonic sedimentation. Moreover, the model testifies of the main tectonic steps that led to the SFTB building. First, a low-tapered accretionary wedge was accreted above the Alpine Tethys oceanic crust from the Oligocene to the Early Miocene. The following underthrusting of the stretched African continental margin and its frontal Panormide platform shortened and thickened the accretionary wedge. This phase provided favorable conditions for significant pulses of reworked Alpine Tethys units that intercalated within the forearc and foredeep successions. During the Middle-Miocene, the décollement of the African Meso-Cenozoic cover (Panormide platform and Imerese-Sicanian deep-water basin) enhanced a deep-seated deformation phase, along with duplexing of the Panormide platform beneath the Alpine Tethys wedge leading to its emersion. Since the Late Messinian, activation of basement faults led to a generalized emersion of the orogenic units through large-wavelength fold culminations accompanied by syntectonic deposition at their southern limbs. Concurrently, the prism front was partly indented to the southeast by the thick and locally already emerged Hyblean platform.

Description: Published

Description: 103257

Description: 1T. Struttura della Terra

Description: JCR Journal

Description: This article has been accepted for publication in Geophysical Journal International ©: The Authors 2022. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Uploaded in accordance with the publisher's self-archiving policy.

Description: 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.

Description: Published

Description: 1394–1418

Description: 2T. Deformazione crostale attiva

Description: JCR Journal