Investigation of mass movement deposits within Lake Ohrid (FYR Macedonia/Albania) using high resolution acoustic methods

Lake Ohrid (FYR Macedonia/Albania) is situated within a tectonically active region on the Balkan Peninsula, and is most likely one of the oldest lakes in Europe (2-5 Ma). Its bathtub shaped basin is filled with over 700 m of partly undisturbed sediments. Lake Ohrid has been found as an important archive to study the sedimentary evolution of a graben system over several million years. Furthermore, with more than 210 endemic species, the lake is a unique aquatic ecosystem that is of worldwide importance. A drilling campaign within the SCOPSCO ICDP Project is scheduled for summer 2011. Here we present results from hydroacoustic surveys carried out in between 2004 and 2008 by means of sediment echosounder and multichannel seismic data proofing that the lake experiences several mass wasting events mainly in the southern area. Transparent units can be found in cross sections up to a depth of 0.8 sec TWT. In combination with new acquired bathymetric data from 2009 covering almost the entire lake we are able to characterize the most recent events in terms of their morphological structure. The main focus is on two individual slide events: the Udenisht slide and the Struga Slide. The Udenisht slide complex covers an area of about 27 sqkm in the southwestern part of the lake. First age estimation of the Udenisht slide revealed that it is most likely younger than 1000 years suggesting that it had an environmental impact on the populated areas along the coastlines. Deposits of the Struga slide located in the northern part of the lake cover the northern shelf area of Lake Ohrid. A prominent head scarp within this area is observable in bathymetric data. Additionally, several slides are located close to normal faults. By tracing selected horizons across the entire lake, it was possible to obtain a relative stratigraphy showing that mass wasting events cluster at specific horizons, and hence had been occurred at the same time. Such sliding events occurring at the same time but at different locations in the lake most likely had a common trigger. We assume earthquakes as such a trigger mechanism. Subsequently, ages for older events can be used to reconstruct the earthquake activity in the area or in other words, slides can be used as proxy for paleoseismicity.