Lake Ohrid Basin (FYROM/Albania): a seismogenic landscape

The Former Yugoslavian Republic of Macedonia and Albania share the Lake Ohrid Basin (40°54’ – 41°10’ N, 20°38’ – 20°48’ E) stretching over a length of c. 30 km and a width of c. 15 km. Clearly the strike of the basins (N-S) does not correspond to the strike of the major tectonic and geological units (NW-SE). This already gives evidence that the basin is formed due to a younger deformation stage. In addition, the area meets all criteria of an active, seismogenic landscape with linear step-like fault scarps on land and within the lake. In general, the faults and fault scarps are getting younger towards the basin centre, as depicted on seismic and hydroacoustic profiles. Post-glacial (or Late Pleistocene) bedrock fault scarps along the steep flanks of Mokra and Galicica Mountain chains are long-lived reflections of repeated surface faulting in tectonically active regions, where erosion cannot outpace the fault slip. Others like wind gaps, wineglass-shaped valleys and triangular facets, are accompanying morphological features of a tectonically active area. Additionally, mass movement bodies within the lake and also onshore (rockfalls, landslides, sub-aquatic slides, homogenites, turbidites) are likely to have been seismically triggered. Multichannel-seismic studies reveal evidence for wedge-like growth strata incorporating mass movement bodies, rather pointing to sudden earthquake-triggered events than to fault creep. Earthquakes larger than magnitude M 6.0 at Lake Ohrid may also be accompanied by secondary effects like liquefaction, seeps, dewatering structures, rock falls and landslides and others. These morphotectonic observations correspond to focal mechanisms of earthquakes in the greater Lake Ohrid area. An integrated multidisciplinary approach was chosen to investigate the neotectonic history of the basin, using tectonic morphology and a variety of geophysical and remote sensing methods.