Abhisek Basa

• Berstad, Magnus Ketilson Rykke; Torabi, Anita; Silio, Ofelia Soledad; Basa, Abhisek & García-Tortosa, Francisco Juan (2025). Relation between seismites and active faulting. Physics and Chemistry of the Earth. ISSN 1474-7065. 141. doi: 10.1016/j.pce.2025.104167. Fulltekst i vitenarkiv Vis sammendrag

  Seismites occur during an earthquake in soft sediments, when seismic energy increases pore water pressure beyond the overburden pressure, reducing cohesion, and hence deforming the sediments. Therefore, the presence of seismites could reveal the magnitude of an earthquake, although the kinematic and mechanical relation between the active faults and seismites have not been previously understood. In southern Spain, within the Guadix-Baza basin, seismites are found in close proximity to the active Galera fault (GF) and Baza fault (BF) in three localities, Galera, Pilares and Cúllar. This study aims to determine if the location of seismites relative to the active fault, controls seismites spacing, orientation, size, and geometry. A multi-scale approach was used, integrating drone-based photogrammetry, outcrop studies of the geometry and mechanical properties of seismites (Young's moduli). Our results reveal a consistent alignment of the seismites strike with nearby segments of the GF. Furthermore, we observe a variation in the size and frequency of seismites along the fault damage zone. We observed color variations within the concentric layers of seismites (grey and red) that have different Young's moduli values and vary between the studied localities in Pilares and Galera. Our results show that slight variations in the depth of depositional environment of the lacustrine sediments and changes in the mineralogy, together with the positioning of seismites relative to GF and BF likely contribute to the observed differences in seismite characteristics across the investigated localities. Our findings enhance the understanding of fault-seismite relationships and using seismites as past seismicity indicators.

• Basa, Abhisek; Alaei, Behzad & Torabi, Anita (2025). Detailed 3D characterization of fault geometric attributes: Insights from deep learning-based fault imaging in seismic data. Journal of Structural Geology. ISSN 0191-8141. 203. doi: 10.1016/j.jsg.2025.105569. Fulltekst i vitenarkiv Vis sammendrag

  Generation of 3D fault network in conventional seismic interpretation involves creation of structural models by manually tracking the same discontinuity across seismic horizons on several adjacent vertical profiles. This process is often time-consuming, susceptible to human bias and introduces uncertainties in the characterization of fault geometric attributes - such as length, width and throw - that cannot be reliably quantified. In this study, we adopt a data-driven approach and extract the geometric attributes of the individual segments constituting the 3D seismic fault zone architecture in remarkable detail, that provides insights beyond the capabilities of conventional seismic attributes. This is achieved through the application of 3D Deep Learning (DL) Networks trained on 3D synthetic seismic datasets on a seismic survey from the eastern flank of Polhem Subplatform, SW of Norwegian Barents Sea to automatically create probabilistic fault volumes. Detailed extraction of individual fault segments from the fault probability volume allows us to investigate the seismic fault zone architecture at multiple scales. Our results show that the maximum segment length of all the studied faults is present near the upper tips and reduces towards the lower tips. The fault zone widths of individual segments measured on E-W-oriented vertical scanplanes orthogonal to the strike of the dominant fault set mostly vary between 10 and 20m. They exhibit higher values at locations where fault segments link laterally/vertically or interact with antithetic fault sets with similar strike of the dominant westward dipping fault set. Throw estimates and the total accumulated fault zone widths are higher towards the lower tips of the faults. Thus, the application of Deep Learning enables data-driven, high-resolution visualization of the seismic fault zone architecture and facilitates comprehensive extraction of fault geometric attributes, providing a more robust complement to traditional interpretation techniques.

• Basa, Abhisek; Torabi, Anita; Jiménez-Millán, Juan; Alaei, Behzad & García-Tortosa, Francisco Juan (2025). Unraveling the key factors controlling active faulting in Tertiary and Quaternary sequences. Earth and Planetary Science Letters. ISSN 0012-821X. 656. doi: 10.1016/j.epsl.2025.119268. Fulltekst i vitenarkiv Vis sammendrag

  The effects of across-fault variation of mechanical properties and along-fault lithological heterogeneity on the seismicity of active faults is poorly understood. Moreover, the role of chemical processes on the mechanical behavior of active faults has not yet received sufficient attention. Such phenomena have more relevance, especially when faults juxtapose chemically reactive carbonate-dominated host rocks. In this study, we examine the key controls on active faulting by utilizing excellent outcrops of an intrabasinal strike-slip fault cutting across Tertiary and Quaternary sequences. We integrate structural, geomechanical and mineralogical investigations of faults and adjacent host rocks geographically exposed at different structural positions along the length of a 30 km-long, NE-SW-striking Galera Fault (GF) in Guadix-Baza basin, SE Spain. Our results show that fault and host rocks along the GF become carbonate-rich predominantly consisting of calcite towards NE. The thickness of sedimentary cover increases towards SW, where gypsum and phyllosilicate minerals are dominant. Calcite precipitation in fault rocks leads to reduction of porosity and significant enhancement of P-wave velocity (Vp), relative to the adjacent host rocks, and makes the fault zone more prone to seismic slip. On the contrary, phyllosilicates including clay mineral assemblages reduce the strength of the fault rocks and promote aseismic creep. The relationship between Vp and modified estimate of Brittleness Index used in this study indicate that the mineral assemblage of the fault as well as the adjacent host rocks influence seismicity. Our novel approach predicts more seismic activity towards the NE-part of GF due to dominance of calcite-rich rocks and proximity to the basement. This is validated by earthquake records in the studied area, which document shallow earthquakes concentrated towards the NE-part of GF. Thus, our study highlights the importance of adopting interdisciplinary approaches and understanding of the basin architecture controlling the distribution of host rocks to comprehend complex processes associated with active faulting.

• Basa, Abhisek; Torabi, Anita; Vasconcelos, David L.; Bezerra, Francisco H.R. & Nogueira, Francisco C.C. (2024). Architecture of interacting deformation band fault zones in siliciclastic rocks and its influence on geomechanical property distribution. Tectonophysics. ISSN 0040-1951. 875. doi: 10.1016/j.tecto.2024.230226. Fulltekst i vitenarkiv Vis sammendrag

  Spatial organization of strain-hardening zones and the adjacent damage zones associated with faulting of porous siliciclastic rocks influence the across-strike variation of geomechanical properties. The across-fault geomechanical property distribution is expected to change when fault zones of distinct orientations interact. We performed outcrop-scale structural analysis along scanlines trending perpendicular to fault strike to constrain the extent of fault zones oriented dominantly along NE-SW and NW-SE. Young's Modulus (E) and P-wave velocity (Vp) measurements on deformation bands (DBs) and host rocks along such scanlines are further utilized to better characterize the fault zones. The selected outcrop exposes Cretaceous siliciclastic rocks in Rio do Peixe Basin, NE Brazil. In this study, Vp measurements coupled with detailed structural investigation have been integrated with grain size analysis for the first time. We observe that DBs oriented parallel to the major fault sets show grain size reduction along with higher E and Vp relative to the adjacent host rocks, indicating cataclasis and strain-hardening. The expected variation of geomechanical properties across the relatively late-stage NE-SW-trending fault zones gets obscured at sites of intense interaction with the early formed NW-SE-striking fault zones. In such areas, E of DBs consistently increases with proximity to the NW-SE-oriented fault zones revealing that the structural position of pre-existing fault zones mostly controls the geomechanical property distribution at sites of intense fault interaction. Similarly, we observe a direct relationship between Vp and grain size of the host rocks across the late-stage faults where there is relatively less fault interaction. However, this relationship does not exist within zones of intense fault interaction. As DBs are prevalent mostly in coarse-grained rocks, Vp in coarse-grained host rocks increases with proximity to the constrained fault zones. An integrated approach adopted in our study is crucial in delineating the effects of fault interaction on the distribution of geomechanical properties.

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• Munoz, Jhon-Meyer; Torabi, Anita; Basa, Abhisek; Hansen, Torsten Hundebøl & Skurtveit, Elin (2026). Fault-core in overburden section: geomechanical and geo chemical characterisation of the Baza fault core, southern Spain. I Ziegler, Martin; Nicol, Romain; Jaeggi, David & Nussbaum, Christophe (Red.), Caprock Integrity & Gas Storage Symposium 2026. Swiss Federal Office of Topography. s. 41–45. doi: https:/www.swisstopo.admin.ch/dam/en/sd-web/jwHxgZfGm5AB/Conference%20Proceedings%20CIGSS%202026.pdf. Fulltekst i vitenarkiv

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