Publication Date:
2015-01-29
Description:
Seismically-induced event deposits embedded in the sedimentary infill of lacustrine basins are highly useful for palaeoseismic reconstructions. Recent, well-documented, great megathrust earthquakes provide an ideal opportunity to calibrate seismically-induced event deposits for lakes with different characteristics and located in different settings. This study used 107 short sediment cores to investigate the sedimentary impact of the 1960 M w 9.5 Valdivia and the 2010 M w 8.8 Maule earthquakes in 17 lakes in South-Central Chile (i.e. lakes Negra, Lo Encañado, Aculeo, Vichuquén, Laja, Villarrica, Calafquén, Pullinque, Pellaifa, Panguipulli, Neltume, Riñihue, Ranco, Maihue, Puyehue, Rupanco and Llanquihue). A combination of image analysis, magnetic susceptibility and grain-size analysis allows identification of five types of seismically-induced event deposits: (i) mass-transport deposits; (ii) in situ deformations; (iii) lacustrine turbidites with a composition similar to the hemipelagic background sediments (lacustrine turbidites type 1); (iv) lacustrine turbidites with a composition different from the background sediments (Lacustrine turbidites type 2); and (v) megaturbidites. These seismically-induced event deposits were compared to local seismic intensities of the causative earthquakes, eyewitness reports, post-earthquake observations, and vegetation and geomorphology of the catchment and the lake. Megaturbidites occur where lake seiches took place. Lacustrine turbidites type 2 can be the result of: (i) local near-shore mass wasting; (ii) delta collapse; (iii) onshore landslides; (iv) debris-flows or mudflows; or (v) fluvial reworking of landslide debris. On the contrary, lacustrine turbidites type 1 are the result of shallow mass wasting on sublacustrine slopes covered by hemipelagic sediments. Due to their more constrained origin, lacustrine turbidites type 1 are the most reliable type of seismically-induced event deposits in quantitative palaeoseismology, because they are almost exclusively triggered by earthquake shaking. Moreover, they most sensitively record varying seismic shaking intensities. The number of lacustrine turbidites type 1 linearly increases with increasing seismic intensity, starting with no lacustrine turbidites type 1 at intensities between V½ and VI and reaching 100% when intensities are higher than VII½. Combining different types of seismically-induced event deposits allows the reconstruction of the complete impact of an earthquake. This article is protected by copyright. All rights reserved.
Print ISSN:
0037-0746
Electronic ISSN:
1365-3091
Topics:
Geosciences
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