Surface rupture and slip distribution along the Lenglongling fault in the NE Tibetan Plateau: Implications for faulting behavior

Abstract

The Lenglongling Fault (LLLF) is located in the western section of the “Tianzhu seismic gap” along the Qilian-Haiyuan fault zone in the NE Tibetan Plateau, between the 1920 M8.5 Haiyuan and the 1927 M8.0 Gulang earthquakes. There is little information about the paleoseismic history of the LLLF and understanding the faulting history of the LLLF is important for determining the seismic hazards of both the fault and the seismic gap. Using UAV-based photogrammetry, high-resolution satellite imagery, and field investigations, we mapped the surface trace of the fault in detail and measured 192 left-lateral offsets of geomorphic markers along the fault trace. A binned cumulative offset probability density (COPD) distribution was calculated to analyze single-event and multi-event cumulative offsets. As a result, we find that the LLLF has fresh scarps along the entire ∼120 km length, indicating that the most recent event might have ruptured the full-segment of the fault. The coseismic offset of the most recent event changes along the fault trace and has an asymmetrical bell-shaped distribution. The maximum offset was ∼7 m, located in the middle-eastern section of the fault, and the offset decreases progressively towards both ends. The minimum offset, located at the western end, was ∼3.0 m, and the average coseismic offset was ∼4.8 m. The cumulative offsets of the four most recent paleoearthquakes are multiples of the coseismic offset of the most recent event, and show similar slip distributions, indicating that the fault ruptures with a characteristic slip. The fault slip distribution, estimated recurrence interval, and estimated moment magnitude imply that the LLLF releases accumulated strain energy mainly by repeatedly producing Mw 7.5–7.8 earthquakes every 1000 ± 200 years. We also show that the binned COPD technique performs well to separate single-event and multi-event cumulative offsets.