A comparative study of bedrock fault scarps by s-UAV and t-LiDAR: Insights into site selection criteria for paleo-seismology studies

Abstract

Normal fault scarps, as classical topographic features and geomorphological markers along mountain range-fronts, form in consolidated bedrock due to faulting in extensional settings. They generally preserve more complete records of paleo-earthquakes than fault scarps in unconsolidated sediments. The reconstruction of paleo-seismic history from a bedrock fault scarp in terms of the times, co-seismic slips and ages by a combination of quantitative morphological analysis, TCNs dating and other physical/chemical index has been proven feasible by several previous studies. However, this success heavily relies on a suitable site selection along the bedrock fault scarp because geomorphic processes are also capable exhuming the fault surface, i.e., an erosion-origin fault surface. To distinguish between tectonic- and fault-origin fault surfaces, four bedrock fault surfaces (MJYC, NMZC, DYC and SYC) in the northern Shanxi Rift have been targeted to carry out a comparative study by combining small unmanned aerial vehicle (s-UAV) surveys and terrestrial laser scanning (TLS). The results can be classified into two groups according to their quantified morphological characteristics and exposed ways. For the NMZC and SYC group, which are in elevated areas away from gullies, the quantitative morphological analysis shows that both bedrock fault surfaces have the characteristics of vertical segmentation. This kind of segmentation indicates that the fault surfaces may be exhumed by repeated seismic events, and the corresponding co-seismic slips are determined by the height of these segments. For the MJYC and DYC group, the quantitative morphology shows a gradually-changing characteristic without segmentation. This gradually-changing characteristic indicates an erosional exposure mode from geomorphic processes, and accordingly the paleo-seismic information cannot be extracted. Thus, results from this study highlights the importance of selecting a suitable study site when conducting paleo-seismic research on bedrock faults. Based on experience obtained in this study combined with previous cases, we propose the characteristics of bedrock fault surfaces suitable for extracting paleo-seismic information and corresponding identification principles. Finally, a general workflow for paleo-earthquake history reconstruction from bedrock fault scarps is proposed to promote paleo-earthquake study in bedrock areas.