Earthquake-induced landslide hazard assessment in the Vrancea Seismic Region (Eastern Carpathians, Romania): Constraints and perspectives

In seismically-active regions, earthquake-induced landslides (EqIL) are likely to enhance slope denudation and sediment delivery both over short and longer terms, which might strongly condition landscape evolution in general. In mountain regions marked by a medium to high seismicity, co-seismic slope failures typically present a relatively low frequency but also high magnitude pattern which should be addressed accordingly within landslide hazard assessment, considering the already high frequency of precipitation-triggered landslide events. The Vrancea Seismic Region located in the curvature sector of the Eastern Carpathians (Romania) is the most active intermediate-depth seismic zones (focal depth > 70 km) in Europe. It represents the main seismic energy source throughout Romania with significant transboundary effects recorded as far as Ukraine and Bulgaria. During the last 300 years, the region featured 13 earthquakes with magnitudes (Mw) above 7, out of which seven events had Mw above 7.5 and three between 7.7 and 7.9. Apart from the direct damages, the Vrancea earthquakes are also responsible for causing numerous other geohazards, such as ground fracturing, groundwater level disturbances and deep-seated landslide occurrences (e.g. rock slumps, rock-block slides, rock falls, rock avalanches). The previous large earthquake-induced deep-seated landslides of the Vrancea region were found to affect the entire slope profile. They often formed landslide dams which strongly influenced the river morphology, posing a serious threat to human life and human facilities of the downstream rural communities through the imminent lake outburst floods. Despite the large potential of this research issue, the correlation between the region's seismotectonic context and landslide geomorphic predisposing factors has not been extensively documented and fully understood yet. Presently, the available geohazard inventories provide limited historical information to quantify the triggering role of seismic activity for observed slope failures across the Vrancea region. However, it is acknowledged that the morphology and geology of numerous large, deep-seated and dormant landslides of this region, which may be reactivated in future, with head scarps near mountain tops and located close to faults, in anti-dip slope conditions show significant similarities to the large mass movements with a proven seismic origin (such as in the Tien Shan, Pamir, Longmenshan, etc.). Thus, the relationship between landslide occurrences and the joint action of triggers and preparing factors (seismotectonic or climatic or both) needs to be investigated in more detail and further considered in the regional multi-hazard risk assessments. The purpose of this paper is to outline the current knowledge level of the landslide-earthquake relationship by accounting for the possible effects of the previous major earthquakes in the Vrancea region. The key findings contribute to the gain of the baseline knowledge for an improved assessment framework of multi-hazard (earthquake-landslide) risks, as required by the Sendai Framework for Disaster Risk Reduction (SFDRR).