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Earthquake-induced soft-sediment deformations and seismically amplified erosion rates recorded in varved sediments of Koycegiz Lake (SW Turkey)

2016-06-01
Avşar, Ulaş
Jonsson, Sigurjon
Avsar, Ozgur
Schmidt, Sabine
Earthquake-triggered landslides amplify erosion rates in catchments, i.e., catchment response (CR) to seismic shocks. In addition to historical eyewitness accounts of muddy rivers implying CRs after large earthquakes, several studies have quantitatively reported increased sediment concentrations in rivers after earthquakes. However, only a few paleolimnological studies could detect CRs within lacustrine sedimentary sequences as siliciclastic-enriched intercalations within background sedimentation. Since siliciclastic-enriched intercalations can easily be of nonseismic origin, their temporal correlation with nearby earthquakes is crucial to assign a seismic triggering mechanism. In most cases, either uncertainties in dating methods or the lack of recent seismic activity has prevented reliable temporal correlations, making the seismic origin of observed sedimentary events questionable. Here we attempt to remove this question mark by presenting sedimentary traces of CRs in the 370-year-long varved sequence of Koycegiz Lake (SW Turkey) that we compare with estimated peak ground acceleration (PGA) values of several nearby earthquakes. We find that earthquakes exceeding estimated PGA values of similar to 20 cm/s(2) can induce soft-sediment deformations, while CRs seem only to be triggered by PGA levels higher than 70 cm/s(2). In Koycegiz Lake, CRs produce Cr-and Ni-enriched sedimentation due to the seismically mobilized soils derived from ultramafic rocks in the catchment. Given the varve chronology, the residence time of the seismically mobilized material in the catchment is determined to be 5 to 10 years.