Sequences of lake sediments often form long and continuous records that may be sensitive recorders of seismic shaking. A multi-proxy analysis of Lake Bohinj sediments associated with a well-constrained chronology was conducted to reconstruct Holocene seismic activity in the Julian Alps (Slovenia). A seismic reflection survey and sedimentological analyses identified 29 homogenite-type deposits related to mass-wasting deposits. The most recent homogenites can be linked to historical regional earthquakes (i.e. 1348 AD, 1511 AD and 1690 AD) with strong epicentral intensity [greater than 'damaging' (VIII) on the Medvedev-Sponheuer-Karnik scale]. The correlation between the historical earthquake data set and the homogenites identified in a core isolated from local stream inputs, allows interpretation of all similar deposits as earthquake related. This work extends the earthquake chronicle of the last 6600 years in this area with a total of 29 events recorded. The early Holocene sedimentary record is disturbed by a seismic event (6617 AE 94 cal yr BP) that reworked previously deposited sediment and led to a thick sediment deposit identified in the seismic survey. The period between 3500 cal yr BP and 2000 cal yr BP is characterized by a major destabilization in the watershed by human activities that led to increases in erosion and sedimentation rates. This change increased the lake's sensitivity to recording an earthquake (earthquake-sensitivity threshold index) with the occurrence of 72 turbidite-type deposits over this period. The high turbidite frequency identified could be the consequence of this change in lake earthquake sensitivity and thus these turbidites could be triggered by earthquake shaking, as other origins are discarded. This study illustrates why it is not acceptable to propose a return period for seismic activity recorded in lake sediment if the sedimentation rate varies significantly. 1777
Develle, et al.. Holocene-long record of flood frequency in the Southern Alps (Lake Iseo, Italy) under human and climate forcing. Global and Planetary Change, Elsevier, 2019, pp.Abstract A high-resolution sedimentological and geochemical analysis of a 21 m sediment sequence of Lake Iseo (Southern Alps, Italy) allowed for the reconstruction of the long-term flood frequency by visual identification of the event layers over the last 12 kyr cal BP. In a previous study that was undertaken on another sediment core from Lake Iseo, these layers were attributed to extreme surface runoff events. However, in this former core, large mass-wasting deposits that induce significant hiatuses did not permit a continuous record of flood events to be established. Such disturbances were absent in the core studied in the present paper. This permitted to establish a high-resolution continuous Holocene record. Based on the flood chronicle and sedimentological and XRF geochemical analyses, ACCEPTED MANUSCRIPT A C C E P T E D M A N U S C R I P Twe found evidence of a major palaeohydrological transition at approximately 3.8 kyr cal BP, which was previously described as occurring in the western Mediterranean region. The oldest part of the record indeed presents a very low frequency of flood events (< 1 flood/century), while after 4 kyr cal BP, the flood frequency increased. This pattern appears to be in agreement with other Southern Alpine paleo flood records. The transition is interpreted as a nonlinear climate response to the orbital-driven gradual decrease in summer insolation at 60°N, which together with the influence of the Mediterranean mesoscale precipitation events, is typical for the Mediterranean climate. However, the comparison of the flood record with the archaeological and historical data from the watershed suggests that human activity during the Roman period in the vicinity of the main tributaries also influenced the flood frequency. Even in a large Alpine lake and more than 2000 years ago, extreme precipitation events that were recorded through the sedimentation process can hence be impacted by the anthropization of the catchment area pointing the requirement of deeper studies of Earth surface critical zone pluri-millennial dynamics.
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