Abstract
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.
Highlights
The western Sichuan region, along southeastern margin of the Tibetan Plateau, is located at the junction of three tectonic blocks— the Sichuan-Yunnan, Bayan Har and South China blocks (Fig. 1)
Historical records depict that the 1850 AD earthquake destroyed areas that mainly locate in the south of Xichang, for the 1536 AD earthquake, the destroyed areas most locate in the north of Xichang and even felt in Chengdu, and past researches have confirmed the historical 1850 AD earthquake produced surface ruptures with a length of more than 60 km and average left-lateral offset of 1.4–6 m along the Zemuhe fault (ZMHF) (Ren & Li 1993; Ren 1994; Feng et al 2000; Yu et al 2001; He & Ren 2003; Ren & Lin 2010)
Giving the timing and the fact that Xichang is just north of ZMHF (Fig. 3a), we suggest event Y is associated with the 814 AD M 7 earthquake
Summary
The western Sichuan region, along southeastern margin of the Tibetan Plateau, is located at the junction of three tectonic blocks— the Sichuan-Yunnan, Bayan Har and South China blocks (Fig. 1). Active tectonics of the region has been of interest to geoscientists, since the 2008 Mw 7.9 Wenchuan earthquake This event occurred along the eastern margin of the Tibetan Plateau where it bounds the Sichuan basin (Fig. 1). The active, north–west striking ZMHF is part of a larger, more complex left-lateral fault system located along the eastern and north boundaries of the Sichuan-Yunnan block in the southeastern area of the Tibetan Plateau. This fault system includes the Xianshuihe (XSHF), Anninghe (ANHF), Zemuhe (ZMHF), Daliangshan (DLSF) and Xiaojiang (XJF; Fig. 2). To better constrain the geological record of large earthquake activity along the ZMHF, we choose a small sag pond favourable to the continuous deposition of fine-grained strata that should offer a better record of palaeoseismic events along the Daqingliangzi section on the ZMHF (Figs 3 and S1)
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