Overestimation of historical earthquake magnitudes in the Near East due to common site effects

Throughout historical times, human habitation in the Near East was limited by its arid climate to sites with strategic advantages. These sites underwent millennia of recurring destruction and rebuilding, resulting in the accumulation of artificial mounds, known as ‘Tells’, consisting of multiple archeological debris strata of ruined cities, which could pile up to a height of a few tens of meters. Intensity-based magnitude evaluation of Near East historical earthquakes often utilizes reported damage from cities built on earlier archeological mounds. Moreover, old residential quarters of many modern Near Eastern cities are founded on such mounds. In this study we measured the subsurface shear-wave velocity (Vs) profile on top of a typical mound structure as well as the Vs profile of the bedrock on which the mound was accumulated. The results suggest that the mound structure is likely to amplify seismic ground motions. These results imply that intensity-based magnitude evaluation of Near East historical earthquakes that are based on reported damage from cities built on archeological mounds, might be biased and could lead to over-estimations of past earthquake magnitudes. Furthermore, many old residential quarters of modern Near Eastern cities are commonly founded on such mounds and might be exposed to a higher seismic hazard than nearby modern neighborhoods, thus requiring special attention in seismic hazard mitigation programs and seismic design.