LAUSR

Abstract Rupture behavior, one of the quantitative features of active fault, is fundamental for understanding crustal deformation mechanisms and assessing regional seismic risk. The 120-km-long Tianqiaogou-Huangyangchuan Fault (THF) is an important branch fault of the Qilian-Haiyuan Fault System (QHFS), the main boundary fault zone at the emerging northeastern margin of the Tibetan Plateau. Mismatching with the fast strike-slip rate, there is no M ≥ 5 earthquake recorded along the THF. To obtain more precise information of the rupture behaviors along the boundary structure, we conducted a 1:50,000 active fault mapping and paleoearthquake trenching on the THF. Two trenches respectively illustrates at least 3 rupture events combining the radiocarbon dating and the OxCal modeling. According to the overlap of their age ranges and the analysis on completeness of the sedimentary records, it is determined at least 5 paleoearthquakes occurred along the THF in the Holocene with respective age ranges of 485 ± 675 BCE, 2085 ± 295 BCE, 5590 ± 1090 BCE, 7910 ± 970 BCE, and 9975 ± 955 BCE and moment magnitudes estimated to be 7.3–7.4. Meanwhile, the detailed investigations on paleoseismology and tectonic geomorphology show the THF was not involved into the seismogenic structure of the nearby 1927 Gulang M8 earthquake. The Monte Carlo statistical analysis of these paleoearthquakes yields a mean recurrence interval of 2.4 ± 0.3 ka, illustrating a strong activity of the THF that accords with its slip rates and an important role as a tectonic boundary accommodating the eastward extruding of the northeastern Tibet relative to the Gobi Alashan block. Moreover, based on its strong activity and long elapsed time of at least 1.7 ka, we propose the THF is another seismic gap with a potential of M ≥ 7 earthquakes similar to the Tianzhu gap near the triple junction in the QHFS, showing a high seismic risk along the THF and in this region.