Department of Seismotectonics
Institute of Rock Structure and Mechanics of the CAS

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Tools for the efficient analysis of surface waves from active and passive seismic data: exploring an NE-Italy perilagoon area with significant lateral variations

A series of reclamation works that took place during the twentieth century, almost completely destroyed the dune system that characterized the eastern part of the Grado-Marano perilagoonal area (NE Italy). Because of the limited data available, so far very little was known about the local subsurface conditions and the present paper presents the main outcomes of the seismic exploration accomplished with a twofold goal: collecting comprehensive data about the subsurface conditions (which geologists need to be able to reconstruct the formation processes of the local geomorphological elements) and testing a series of efficient and unconventional methodologies based on the analysis of surface waves from both active and passive seismic data. The survey was designed and accomplished also considering the local digital terrain model (DTM) and some resistivity and penetrometer data. In the present paper we focus on three main areas and, from the methodological point of view, special emphasis is given to the Holistic analysis of Surface waves (HS) and the Horizontal-to-Vertical Spectral Ratio (HVSR), since both these techniques require simple field procedures and a light equipment. It is also show the wealth of information that the simple spectral analysis of multi-offset passive data can provide in particular for the identification of possible lateral variations. In fact, in spite of the low-energy depositional environment, the area reveals extremely complex with major and abrupt lateral variations that require special care and prevent from using coarse methodologies that cannot properly handle their identification. Collected geophysical data provide a consistent overall scenario: while the area is in general dominated by soft (silty) sediments, the residual dunes are constituted by cemented sandy materials (medium-grained calcarenite) responsible for anomalously high shear-wave velocity (VS) values already at the surface. Parallel to such residual sandy dunes we also identified a series of peat channels characterized by distinctive low VS values due to a significant amount of organic components.

Publication: Giancarlo Dal Moro & Josef Stemberk (2022): Tools for the efficient analysis of surface waves from active and passive seismic data: exploring an NE-Italy perilagoon area with significant lateral variations. Earth, Planets and Space volume 74, Article number: 140 (2022) DOI:10.1186/s40623-022-01698-z

Fig.: Location of the investigated area: a) general overview (South Europe map) and main geological, tectonic and alluvial elements (Fontana et al. 2008). The Grado-Marano lagoon is the area between the Isonzo and Tagliamento rivers; b) study area with highlighted the Beligna and San Marco sites

Ground Fissures within the Ethiopian Main Rift: Tectonic, Lithological and Piping Controls

Ground fissures, especially if they open due to a sudden collapse of the surface, is a serious risk for populated areas. Their common occurrence in unconsolidated sediments of the Main Ethiopian Rift was found to be mostly a result of piping. The fissures start by piping in linear sub-horizontal underground voids, which often propagate upwards resulting in ceiling collapse and formation of deep and long ground fissures with vertical walls. In the southern and central Main Ethiopian Rift the fissures pose a serious risk to infrastructure and settlements. The ground fissures are often linear (up to several kilometres long and often tens of metres deep) and accompanied by sinkholes (along the length). A detailed field mapping of the geological (rock composition, orientation and character of lithological boundaries, primary fabrics and brittle structures) and geomorphological features (especially a length, width and depth of fissures, sinkholes and gullies) followed by in situ seismic anisotropy measurements and a laboratory determination of the geomechanical properties of volcanoclastic deposits was carried out to investigate the ground fissures' origin. The conditions and factors leading to the formation of the ground fissures have been linked to: (a) the presence of regional normal faults and the associated extensional joints and (b) the alternation of lithological units with contrasting hydraulic permeability. The latter corresponds to a sequence of less permeable hard rocks (e.g., rhyolitic ignimbrites) overlain by heterogeneous, soft and permeable, unconsolidated volcaniclastic deposits with a low amount of clay (less than 10%). The ground fissures' occurrence has shown affiliation to areas which have a significantly high seismic anisotropy (more than 20% at the study sites), which can be used as a proxy to map out high risk areas prone to piping and ground fissure formation.

Publication: Valenta, J., K. Verner, K. Martínek, T. Hroch, D. Buriánek, L. A. Megerssa, J. Boháč, M. Kassa, F. Legesse, M. Yakob, B. Kebede and J. Málek (2021). Ground fissures within the Main Ethiopian Rift: Tectonic, lithological and piping controls, Earth Surface Processes and Landforms. 1-17, DOI:10.1002/esp.5227

Fig.: Ground fissures in the MER, and especially their sudden opening, is one of the most significant geoohazard in the area. The fissures open in unconsolidated sediments and are long (even several kilometres), narrow (0.5 to several metres), deep (usually several meters but maximum depth measured was 60 m) and fast opening (even several hundred meters in half a year). The fissures are oriented perpendicular to the main direction of extension (parallel to the Rift).

Fig.: WEBNET seismic stations (triangles) with available data (31 May 2014).
WEBNET location of strongest event (star) is also depicted.

Focal Mechanisms of West Bohemia, Central Europe, Earthquakes - End of May 2014: Evidence of Volume Changes

West Bohemia is a region with a lot of mineral springs and gas outflows. We focused on the strongest earthquake over the past thirty years (May 31, 2014; Mw~3.8) and on two smaller ones (Mw~2.9; 2.5) from the same day. The main goal of this study is to contribute to clarification of the nature of earthquake swarms in the western edge of the Bohemian Massif. Negative value of the isotropic part of full moment tensor could be related to the closing of cracks and fissures during a rupture process.

Publication: Křížová, D. and J. Málek (2021): Focal Mechanisms of West Bohemia, Central Europe, Earthquakes-End of May 2014: Evidence of Volume Changes, Seismological Research Letters 92(6), 3398-3415, DOI: 10.1785/0220200389

Fig.: Focal mechanism plot for the strongest earthquake (31 May 2014, 10:37:21.11) with signs of first motion for available WEBNET data. Black squares mean "up" and white "down"; T and P axes are depicted by circles.

Further important publications and outputs

Dal Moro, G., and G. F. Panza (2022). Multiple-peak HVSR curves: Management and statistical assessment, Engineering Geology 297, February 2022, 106500, DOI:10.1016/j.enggeo.2021.106500

Dal Moro, G., and J. Stemberk (2022). Tools for the efficient analysis of surface waves from active and passive seismic data: exploring an NE-Italy perilagoon area with significant lateral variations, Earth, Planets and Space, 74, 140 (2022), DOI: 10.1186/s40623-022-01698-z

Nováková, L. (2022) Chapter title: Tectonicaly Significant Fault Zones in Central Europe (Germany, Czech Republic and Poland) and Their Surface and Subsurface Outcrops: Franconian Line, Hronov-Porici Fault, Sudetic Marginal Fault and Lusatian Fault. Book: Structural Geology and Tectonics Field Guidebook – Volume 2, Springer, DOI: 10.1007/978-3-031-19576-1

Cabieces, R., A. Olivar-Castaño, T. C. Junqueira, J. Relinque, L. Fernandez-Prieto, J. Vackář, B. Rösler, J. Barco, A. Pazos, and L. García-Martínez (2022). Integrated Seismic Program (ISP): A New Python GUI-Based Software for Earthquake Seismology and Seismic Signal Processing, Seismol. Res. Lett. XX, 1–14, DOI: 10.1785/ 0220210205

Majewski, R. S., Valenta, J., Tábořík, P., Weger, J., Kučera, A., Patočka, Z., and Čermák, J. (2022). Geophysical imaging of tree root absorption and conduction zones under field conditions: a comparison of common geoelectrical methods. Plant and Soil, 1-27, DOI: 10.1007/s11104-022-05648-2

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