Holocene Faulting on the Zuogong Segment of the Nujiang Fault Zone, Southeastern Tibetan Plateau

The Nujiang Fault Zone is located in the Three Parallel Rivers region on the southeastern margin of the Qinghai–Tibet Plateau, serving as a discordogenic fault that must be traversed by the Sichuan–Tibet transportation corridor project. Because of the scarcity of direct geological evidence, the late Quaternary activity of the Nujiang Fault Zone remains highly controversial. Focusing on the late Quaternary activity and active intensity of this fault, we conducted detailed field investigations along the Zuogong segment of the Nujiang Fault Zone based on high-resolution remote sensing images. Two trenches were excavated within a linear trough near Ranmi Village. Fourteen 14C samples were collected for AMS dating, and two fault gouge samples were obtained for microstructural analysis, aiming to reconstruct the paleoseismic sequence and estimate the corresponding earthquake magnitude. Clear fault planes were exposed in the trenches, which distinctly offset the middle-late Holocene strata. Three paleoseismic events were recorded by trench exposures: Event E1 prior to 5824 ± 61 cal BP (approximately 7031 ± 688 cal BP), Event E2 at 4560 ± 321 cal BP, and Event E3 at 1464 ± 142 cal BP. Analogous to the northern-central segment of the Longmenshan Fault Zone, where the Ms 8.0 2008 Wenchuan earthquake occurred, and its southern segment was associated with the Ms 7.0 2013 Lushan earthquake, microstructural observations of the fault gouge from the Zuogong segment also revealed typical brittle deformation characteristics. In terms of trenching-offset features, as well as the composition and microscopic properties of clay minerals in the fault gouge, the Zuogong segment exhibited geological signatures that were intermediate between the southern and northern-central segments of the Longmenshan Fault Zone, with greater similarity to the southern segment. Accordingly, the maximum potential earthquake magnitude of this segment was estimated to be Ms 7.0–7.5. The paleoseismic sequence of the Zuogong segment indicated an earthquake recurrence interval of approximately 2500–3000 years, with approximately 1500 elapsed years since the most recent seismic event. Although the full recurrence cycle has not yet been approached, implying low probability of great earthquakes with magnitudes of Ms 7.0–7.5 in the near future, the occurrence of non-characteristic seismic events cannot be ruled out. This study reveals direct evidence for Holocene activity along the Zuogong segment of the Nujiang Fault Zone, providing a critical scientific basis for quantitative seismic hazard assessment and major engineering planning and construction in the research area.