Segmentation and millennium recurrence interval of morphogenic earthquakes of the Longmen Shan Thrust Belt, central China (Invited)

The magnitude (Mw) 7.9 Wenchuan earthquake occurred on 12 May 2008 in the Longmen Shan region of China, the transition zone between the Tibetan Plateau and the Sichuan Basin, rupturing ~285-300 km segment along the active fault zone of the Longmen Shan Thrust Belt (LSTB), which resulted in widespread damage throughout central and western China. Although many studies undertaken since the 2008 Wenchuan earthquake have made substantial progress in terms of understanding the co-seismic ground deformation features along the major active faults of the southwestern and central segments of the LSTB, and the current activity, only a few studies have examined northeast- and southwest-most segments and the paleoseismicity along the 500-km-long LSTB. The structural features, therefore, including its slip sense and paleoseismicity, are still unclear due to a lack of geologic data. In this study, based on the geological structures, tectonic landforms, slip sense and paleoseismic data, co-seismic surface deformation features of the 2008 Wenchuan earthquake, the active fault zone of the LSTB can be divided into four segments: Qingchuan, (the northeast-most), Beichuan (northeast-central), Dujiangyan (southwest-central) and Lingguan (the southwest-most). The northeast-most Qingchuan segment is dominated by right-lateral strike-slip with a little thrust component. The slip rate of this segment is estimated to be ca.4-8 mm/yr with an average rate of 6 mm/yr. In contrast, the central Beichuan and Dujiangyan segments are dominated by folding and thrusting with a little right-lateral and left-lateral displacement component, respectively. The vertical slip rate of these two segments is estimated to be ca.1-3 mm/yr with an average amount of 2 mm/yr. While the southwest-most Lingguan segment is dominated by folding and thrusting with a distinct left-lateral displacement component. Recently, the Mw 6.6 Ya-An earthquake occurred on 20 April 2013, which ruptured ~60 km along this segment. Field investigation, paleoseismic and archaeological evidence, and radiocarbon dating results reveal that at least four large morphogenic earthquakes including the 2008 Mw7.9 Wenchuan earthquakes occurred in the Sichuan region during the late Tang-Song (AD 800-1000), Han (206 BC to AD 220), and late Shang-Chunqiu (1200-800 BC) dynasties, comparable with the 2008 Wenchuan earthquake in its seismic intensity, suggesting an average millennium recurrence interval for Wenchuan-magnitude (M = ∼8) earthquakes in the late Holocene within the LSTB. This finding is in contrast with previous estimates of 2,000-10,000 years for the recurrence interval of morphogenic earthquakes within the LSTB, as obtained from long-term slip rates based on the Global Positioning System and geological data, thereby necessitating substantial modifications to existing seismic-hazard models for the densely populated region at the eastern marginal zone of the Tibetan Plateau. Our results confirm that the present-day shortening strain upon the eastern margin of the Tibetan Plateau is mostly released by seismic thrusting and folding with strike-slip partitioning along the active faults within the LSTB. References Lin et al., 2009, Tectonophysics, 471, 203-215; 2) Lin et al., 2010, J. Seismol., 14, 615-628; 3) Lin et al., 2010, Tectonophysics, 491, 21-34; 4) Lin, A., 2011, Geology, 39, 547-550; 5) Lin et al., 2012. Tectonophysics, 522-523, 243-252; 6) Lin et al., 2013, Tectonophysics, submitted.