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Historical and instrumental records of large earthquakes within the southeastern region of the Tibetan Plateau appeared to show an irregular and complex behaviour in the seismic activity over a relatively short-time period. However, whether this kind of earthquake behaviour can be extrapolated over long-time periods is not well addressed. Herein, we present about an 8000 yr earthquake record using palaeoseismological techniques from the Zemuhe fault (ZMHF) to assess the earthquake behaviour and show the complexity of faulting within the region of the Tibetan Plateau. The ZMHF along with the Anninghe (ANHF) and Xiaojiang (XJF) faults, form the N–S-striking active fault zone at the tectonic boundary of the southeastern margin of the Tibetan Plateau. In the historical record, a large earthquake occurred in 1850 AD at M 7.5 along the ZMHF, with a coseismic left-lateral displacement of 3.6–4.8 m along the Daqingliangzi section where we excavated several trenches in a small triangular-shaped sag pond. We analyse the depositional sequences and deformation evidence in the trenches to recreate the sequence of earthquakes that occurred from the early Holocene until the present. From trenching and 14C ages, five palaeoseismic events are identified and named Z through V from youngest to oldest at 1370 AD–present, 690–940 AD, 5900–1400 BC, 6590–6350 BC and 7380–7110 BC. We associate two young events Y and Z with the historical records of 814 AD M 7 and 1850 AD M 7.5, respectively. Moreover, integrated palaeoearthquake results on ages of one event occurred slightly younger than 1700–1100 BC and 1540–1270 BC from previous palaeoseismic studies, we further constrain the age of event X at 1700–1400 BC. The average recurrence interval of earthquakes at the Daqingliangzi section on the ZMHF is about 2300 yr. The recurrence interval of palaeoseismic events during the early Holocene and late Holocene is shorter than that of the middle-Holocene. The interval between event V and W is 1030–520 yr, 2514–2214 yr between X and Y and 1036 yr between Y and Z; however, the recurrence interval between W and X is much longer at 5190–4650 yr. Holocene surface-faulting events along the ZMHF appear to be unevenly spaced in time. The irregular recurrence intervals of earthquakes on the ZMHF might be related to complexity of fault interactions in the left-lateral fault system located along the eastern and northern boundaries of the Sichuan-Yunnan block and may indicate regional irregularities of large earthquakes within the southeastern region of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Trenching is a primary technique on paleoseismology to reveal evidence of surface deformation produced by large earthquakes. A good trenching site requires completeness of geologic recording on paleoseismic events and corresponding reliable dating from radiocarbon samples. Based on three-dimension trenching, we show a structure of a small triangular pull-apart basin at the Daqingliangzi section on the Zemuhe fault, then explore interrelation between paleoseismic surface rupturing and evolution of the pull-apart basin, and give a corresponding identification model. Sedimentary boundary of the pull-apart basin is tightly bounded by two branch faults, which produced multiple paleoseismic events with deformation of some large fissures in sequence. Strata are thinner at north of the pull-apart basin, however thicker at south. These above characteristics show that evolution of the pull-apart basin is a continuous sedimentation process accompanying extensional deformation produced by multiple paleoseismic events. Small pull-apart basins are favorable sites for trenching and paleoseismic study on active strike-slip faults. [ABSTRACT FROM AUTHOR]