Constraining regional paleo peak ground acceleration from back analysis of prehistoric landslides: Example from Sea of Galilee, Dead Sea transform

Accurate estimation of expected peak ground acceleration (PGA) in seismically active regions is a challenging task. The best way to estimate, quantitatively, expected PGA is by investigating instrumental data of past strong earthquakes in a given area. In some regions of the world however recorded data are scarce, and if they exist, they are typically available only since the late 19th century. As such they are hardly representative of the true seismicity in the studied region. We propose here an analytical approach to constrain the lower threshold of paleoseismic PGA on the basis of back analysis of old landslides. To perform the analysis we need a mapped landslide with geomorphic features that have been preserved in the field, the slip surface, a good reconstruction of the slope geometry and ground water level prior to failure, and the mechanical properties of the sheared material. We perform static and pseudo-static limit equilibrium analyses using standard solution procedures to obtain lower bounds of paleoseismic PGA. Back analyses of three different landslides around the Sea of Galilee (SOG) return similar results that range between 0.15 and 0.5 g, thus constraining the threshold paleoseismic PGA range for this region. The analytically inferred regional PGA is supported by results of an independent numerical analysis of toppled columns in a nearby Byzantine church. Using results from a recent paleoseismic trenching study performed on one of the studied landslides and a modified attenuation relationship for the study area we localize the loci of moment magnitude Mw = 7.0 earthquakes that can explain the studied failures along the boundaries of the SOG, and find that they coincide with traces of the Eastern and Western Margin faults of the Dead Sea transform. The temporal relationships between the observed failures are discussed on the basis of dated colluvial sediments, geomorphologic constraints, and archeological evidence.