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Sci. Rep. et al: Progress in earthquake limnology on the eastern Tibetan Plateau

2018/7/6 16:31:14

Traditionally, paleoseismology incorporates geomorphology, trench data, and stratigraphic dating to reconstruct the history of fault activity, and to infer recurrence intervals and magnitudes of ancient earthquakes in any specific area. Tectonically active regions host continuous records of lacustrine sediments, which could possibly record many seismic events. Thus, increasing attention has been paid to paleoseismic records derived from lacustrine settings, which has led to the emergence of a new discipline: earthquake limnology.

Under the support of projects funded by the Director of the Institute and the State Key Laboratory of Earthquake Dynamics (SKLED), Professor Jiang Hanchao and his research team have conducted studies on the causes of dammed lakes, soft sediment deformation (SSD) probably related to earthquakes, and environmental events revealed by changes of high-resolution deposit indices on the eastern Tibetan Plateau.

With regard to the causes of dammed lakes, the lacustrine deposit base of the dammed lake in Diaolin, Maoxian, Sichuan Province, has no grading changes and the corresponding deposit bedding is the result of fluvial action. It is thought that the deposit could be poorly sorted and angular embedded gravels of different sizes from the opposite hill slopes. Coupled with Optically Stimulated Luminescence (OSL) and pollen concentrate dating, the suggestion is the dammed lake was probably caused by an earthquake in 638 AD (Xu et al., 2015).

In relation to SSD, six SSD structures including clastic dykes, ball-and-pillow structures, flame structures, clastic gravels, microfaults, and slump folds were found in the 23.4-m-thick lacustrine sequence in Lixian, Sichuan Province. Given that (1) the study area is seismically active, (2) the lacustrine sediments comprise potentially liquefiable silt and clay, (3) the deformed beds are separated by undeformed beds that can be traced laterally, (4) the partial SSD structures are comparable with those produced by recent earthquakes or experimental simulation, (5) some types of SSD structures are repeated vertically, and (6) factors such as gravity flows or collapses can be precluded, the SSD structures in the Lixian lacustrine sequence possibly reveal 24 seismic events (Jiang et al., 2016a).

With regard to high-resolution deposit indices, the research team studied the source of the lacustrine sediments, analyzed the rare earth elements, and used scanning electron microscopy to examine the grain-size features of lacustrine sediments from the Xinmocun section in Diexi Lake, Maoxian. Results suggested that the fine grains of the Xinmocun lacustrine sediments were transported by wind and trapped in the lake, reflecting an arid or semiarid climate (Jiang et al., 2014; Jiang et al., 2015). These findings are consistent both with detailed analyses on major elements and rare elements in the section (Liang and Jiang, 2017), and with results of geochemical evidence and grain-size records of the Late Cenozoic lacustrine sediments of East Asia (Jiang and Ding, 2010; Jiang et al., 2016b; Jiang et al., 2017a). Among them, the content of medium–coarse silt and sand (grain size: >16 μm) shows abrupt coarsening and upward fining, probably due to palaeoearthquake events. It has been proposed that lacustrine sediments in a tectonically active region have potential to record a continuous history of palaeoearthquakes and to provide a new perspective in studying the dust-producing process in tectonically active regions (Jiang et al., 2014).

To trace the longer history of dust events induced by earthquakes, compare regional dust events in different places, and reveal the relationships between high-resolution sedimentary index changes and SSD, Jiang et al. (2017b) studied the 23.4-m-thick Lixian lacustrine sedimentary sequence spanning from 19.3–6.0 ka. The interrelationships between high-resolution grain size, magnetic index changes, and SSD revealed that possibly 70 prehistoric seismic events might have taken place on the eastern Tibetan Plateau. Spectral results showed two strong periodicities of 810 and 378 yr, and two relatively weaker periodicities of 85 and 65 yr. The results of grouped magnetic susceptibility measurements indicated that medium-to-coarse silt (20–63 and 32–63 μm) made a greater contribution to the magnetic susceptibility values than other grain-size fractions, which suggests that increasing temperature and a wetter climate resulted in enhanced weathering that increased the supply of coarse-grained and magnetic particles to the study area (Jiang et al., 2017b). Provenance analysis based on U-Pb zircon ages showed the Xinmocun and Diaolin lacustrine sediments have similar material sources that differ from the Lixian sequence (Zhong et al., 2017). This means that dust particles generated by different earthquakes affect only limited loci and they are not comparable, even though the spatial distances between locations might be <100 km.


References and corresponding links:

Jiang, H.C., Wan, S.M., Ma, X.L., Zhong, N.,and Zhao, D.B., 2017a. End-member modeling of the grain-size record of Sikouzi fine sediments in Ningxia (China) and implications for temperature control of Neogene evolution of East Asian winter monsoon. PLoS ONE 12(10): e0186153.https://doi.org/10.1371/journal.pone.0186153

Jiang, H.C., Zhong, N., Li, Y.H., Ma, X.L.,Xu, H.Y., Shi, W., Zhang, S.Q., Nie, G.Z., 2017b. A continuous 13.3-ka record of seismogenic dust events in lacustrine sediments in the eastern Tibetan Plateau. Scientific Reports 7:15686, doi:10.1038/s41598-017-16027-8. http://www.nature.com/articles/s41598-017-16027-8

Zhong, N., Song, X.S., Xu, H.Y., Jiang,H.C., 2017. Influence of a tectonically active mountain belt on its foreland basin: Evidence from detrital zircon dating of bedrocks and sediments from the eastern Tibetan Plateau and Sichuan Basin, SW China. Journal of Asian Earth Sciences 146, 251-264. http://www.sciencedirect.com/science/article/pii/S1367912017302845

Liang, L.J. and Jiang, H.C., 2017.Geochemical composition of the last deglacial lacustrine sediments in East Tibet and implications for provenance, weathering and earthquake events.Quaternary International 430, 41-51. http://www.sciencedirect.com/science/article/pii/S1040618215007132

Jiang, H.C., Zhong, N., Li, Y.H., Xu, H.Y.,Yang, H.L., Peng, X.P., 2016a. Soft sediment deformation structures in the Lixian lacustrine sediments, Eastern Tibetan Plateau and implications for postglacial seismic activity. Sedimentary Geology 344, 123-134. http://www.sciencedirect.com/science/article/pii/S003707381630135X

Jiang, H.C., Guo, G.X., Cai, X.M., Thompson,J.A., Xu, H.Y., Zhong, N., 2016b. Geochemical evidence of windblown origin ofthe Late Cenozoic lacustrine sediments in Beijing and implications for weathering and climate change. Palaeogeography, Palaeoclimatology, Palaeoecology 446, 32-43. http://www.sciencedirect.com/science/article/pii/S0031018216000183

Jiang, H.C., Shevenell, A., Yu, S., Xu,H.Y., Mao, X., 2015. Decadal- to centennial-scale East Asian summer monsoon variability during the Medieval Climate Anomaly reconstructed from an eastern Tibet lacustrine sequence. Journal of Paleolimnology 54, 205-222. https://link.springer.com/article/10.1007/s10933-015-9847-1

Xu, H.Y., Jiang, H.C., Yu, S., Yang, H.L.,Chen, J., 2015. OSL and pollen concentrate 14C dating of dammed lake sediments at Maoxian, east Tibet, and implications for two historical earthquakes in AD 638 and 952. Quaternary International 371, 290-299. http://www.sciencedirect.com/science/article/pii/S1040618214006971

Jiang, H.C., Mao, X., Xu, H.Y., Yang, H.L.,Ma, X.L., Zhong, N., Li, Y.H., 2014. Provenance and earthquake signature of the last deglacial Xinmocun lacustrine sediments at Diexi, East Tibet.Geomorphology 204, 518-531. http://www.sciencedirect.com/science/article/pii/S0169555X13004376

Jiang, H.C. and Ding, Z.L., 2010.Eolian grain-size signature of the Sikouzi lacustrine sediments (Chinese Loess Plateau): Implications for Neogene evolution of the East Asian winter monsoon. Geological Society of America Bulletin 122(5/6), 843-854. https://pubs.geoscienceworld.org/gsabulletin/article-lookup/122/5-6/843