Seismic Reflection and Borehole Constraints on the Tectonic Activity of the North Branch of the Lintong–Chang’an Fault: Implications for the Seismic Hazards near the Megacity of Xi’an, Central China

Abstract

Abstract Active faults that develop in urban regions pose significant seismic hazards to cities with densely concentrated populations and wealth, as demonstrated by several destructive earthquake events in the recent decades. Lintong–Chang’an fault is a known active fault, which comprises multiple branches and traverses the urban area of Xi’an in Weihe Graben—a prominent Chinese megacity with a rich 3000-year-old civilization and a population of 13 million. High-resolution seismic reflection profiles and borehole transects, combined with Quaternary strata dating, reveal that: (1) to the south of Shenhe Loess Tableland, two northern branches of the fault zone follow the trend of the middle part and extend to the front of the Qinling mountains in the SW240° direction; (2) the strata since the late Middle Pleistocene on the borehole transect have been offset, with the vertical displacement of the ∼216 ka layer measured at 5.9 ± 0.3 m, the ∼118 ka layer at 3.8 ± 0.3 m, and 41 ka layer at 1.0 ± 0.1 m, indicating an average vertical slip rate of 0.02–0.04 mm/yr for the individual branch at the study site. Notably, the slip rate of the entire fault zone could be two to three times that of a single branch. Despite the relatively low-slip rate, the fault traverses the megacity of Xi’an, is situated in the relay zone of two large, strongly active basin boundary normal faults (Huashan and Qinling Piedmont faults) and is responsible for the formation of Xi’an ground fissures. Hence, it is necessary to pay special attention to this fault.