Post-20 ka Earthquake Scarps Along NE-Tibet's Qilian Shan Frontal Thrust: Multi-Millennial Return, ∼Characteristic Co-Seismic Slip, and Geological Rupture Control

Abstract

Earthquake slip distributions help constrain rupture behavior and return times. Here, by combining 0.5-m-resolution image and topographic data interpretation with fieldwork, we quantify the co-seismic throw distribution and rupture geometry of the 1609 Hongyazi earthquake along the Qilian Shan Fodongmiao fontal thrust (NE Tibet). We measured 396 vertical offsets across mapped geomorphic units along the thrust trace. The 1609 rupture was ∼118-km long, with average co-seismic throw and slip amounts of ∼1.8 ± 1.0 and ∼2.9 ± 1.4 m, respectively, consistent with a magnitude Mw of 7.4 ± 0.3. Along the ∼88-km-long western stretch of the thrust, our data also reveal five distinct clusters with peak cumulative throws (∼4.9, 9.3, 13.8, 18.0, and 22.2 m), implying older ruptures with nearly characteristic slip during five Mw ∼ 7.6–7.8 events, three of which were also identified in trenches. Two of these earthquakes ruptured the entire thrust, while three mainly broke the western and middle segments. The thrust geometry and segmentation are influenced by geological inheritance. The middle segment emplaces Silurian granites atop Quaternary alluvium, which may account for its irregularly cusped trace, and favor rupturing during large, Mw ∼7.8 events with multi-millennium (∼3.7 +0.5/−2.2 ka) return times. Since ∼20 ka, the Fodongmiao Thrust has kept shortening and uplifting Tibet's northeastern boundary at steady rates of ∼1.4 and 1.1 mm/a, respectively. The remaining oblique component of convergence across the ∼800-km-long pushup between the Altyn Tagh and Haiyuan faults is absorbed by slip partitioning along the left-lateral Changma and Lenglongling faults.