Application of high spatial resolution optical imageries in active fault investigations: The North Zanjan fault

Articles in Press

Document Type : Research Article

Authors

1 Institute for Advanced Studies in Basic Sciences (IASBS)

2 Department of earth sciences, Institute for Advance Studies in Basic Sceinces

3 Istituto Nazionale di Geofisica e Vulcanologia

Abstract

The slow rate of active deformation and the absence of historical and instrumental moderate to large earthquakes in the Zanjan area have led to an underestimation of the active tectonic and seismotectonic impacts of major tectonic structures such as the Soltanieh and North Zanjan faults. The North Zanjan Fault (NZF), marking the western boundary of the Alborz Mountains, is a dextral reverse fault that directly affects the western districts of Zanjan city, where new urban and industrial sectors are emerging. However, it remains uncertain whether the fault experiences aseismic or seismogenic slip. Obtaining a geologically reliable answer to this question is crucial for our assessment of seismic hazard in the Zanjan region. In the absence of significant seismic activity, and in an effort to characterize the fault's behavior, we conducted an extensive investigation spanning several thousand years of geological activity along the NZF. This study involved a combination of photogrammetric technique, tectonic geomorphology, remote sensing, and field surveys. We utilized a wide array of aerial and specialized datasets, including open-access satellite images (e.g., Google Earth and SASPlanet), Keyhole (KH)9 dataset (1970-1980), 12.50 m ALOSPALSAR Digital Terrain Model, 30 m SRTM data, as well as drone-based Digital Surface Models (DSMs) and ortho-mosaics. Our three-dimensional morphotectonic analysis of both general-scale and newly acquired site-scale DEMs revealed a complex structural pattern for the NZF. This pattern consists of three distinct northern, central, and southern portions, interconnected through arrays of imbricate thrust faults. The primary fault zone has migrated westward from the mountain front into the Quaternary plain. Sharp centimetric lateral and vertical displacements observed in present-day geomorphology suggest abrupt recent movements along the fault, indicating the seismogenic nature of a significant portion of the fault slip. The most recent fault offsets are observed along the southern and central fault zones, where the offsets recorded by active streambeds or gullies do not exceed approximately 0.5 meters vertically and 1.10 meters in total displacements. Assuming a similar order of total displacement for the average slip per event implies a moment magnitude in the range of 6.7 to 7.0 for the last earthquake along the NZF. Our findings provide a solid foundation for future complementary studies on the paleoseismicity and slip rate of the fault. This study demonstrates that multi-scale temporal and spatial datasets can be confidently employed in active tectonic studies on slowly slipping active faults. When accompanied by Quaternary dating, these datasets can unveil the history of faulting in intracontinental regions affected by low rates of active deformation.

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Iranian Journal of Geophysics

Articles in Press, Accepted Manuscript
Available Online from 07 March 2024

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