The structure of paleoearthquake surface ruptures along the Kubadru Fault (Gorny Altai): Ground-penetrating radar evidence
DOI:
https://doi.org/10.21638/spbu07.2024.205Abstract
The Gorny Altai is part of the system of intracontinental Cenozoic orogens that originated from the Eurasia-India collision. There is crustal contraction, which is followed by numerous earthquakes, and the highest concentration of earthquake sources is observed in the south-eastern part of the region. The system of paleoearthquake surface ruptures in the Kubadru Fault Zone was studied at three representative sites using the GPR method in conditions of weakly consolidated coarse clastic deposits in the upper part of the section and permafrost development. UAV aerial photography and trenching supported the geophysical studies. By using a 250 MHz antenna, we determined morphological parameters of paleoseismic scarps with high accuracy and learned about the fault's deep structures up to 8 m. A 100 MHz antenna provided the same information up to 10 m. Identifying active faults by looking for reflection discontinuities and chaotic reflections is not enough. Additional features from GPR datasets are necessary to detail the internal structure of the seismogenic fault zone. These are subvertical high-amplitude zones that can be traced to depths of up to 6-8 metres. The presence of such zones can be explained by the fracturing of rocks and sediments, their waterlogging, the development of processes of seismogenic fluidization of soils. Deposits of colluvial wedges are distinguished on GPR profiles as depressions filled with layered deposits, with a series of parallel inclined reflectors. A 3D GPR survey helped identify paleofaults over a larger area and eliminated errors on individual profiles. The permafrost table sharply limited the depth of sounding. A low frequency of the spectrum, a smooth decrease in amplitudes to minimum values characterized rocks and sediments in the frozen state. Shallow reflections on the GPR profiles correspond to the lithological complexes observed in the trenches. The acquired findings demonstrate the importance of employing geophysics in investigating the surface ruptures of paleoearthquakes both at the stage of determining the location and kinematics of seismogenic ruptures prior to trenching, and at the stage of substantiating the history of the formation of faults.
Keywords:
surface rupture, paleoearthquake, ground penetrating radar, Kubadru Fault, Gorny Altai
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