On the Role of Weak-Motion Earthquakes Recorded on Earth Dams

Seismic accelerations recorded on earth dams through permanent monitoring systems could be very useful to handle the seismic safety assessment of earth dams. First, they allow to characterize the seismic loads, which in conjunction with the static ones, might induce critical changes in those physical quantities controlling dam safety, i.e. stresses, strains, permanent displacements, pore water pressures, seepage flows. In this way, control and quantification of all loads affecting dam response in its whole lifetime are assured. Second, seismic recordings could be useful to characterize some important features of dam mechanical behavior at relatively none or low cost. Focusing on this latter objective, the paper examines the basic procedures that could be adopted to interpret the seismic signals acquired on a zoned earth dam equipped with a permanent seismic network consisting of five accelerometer stations placed both on rock outcrops at the specific site and on the main cross section of the embankment (top, bank, base). As the dam is placed in a quite seismically active zone of Southern Italy, five years of continuous monitoring have delivered dozens of records associated to weak earthquakes having different source-site distance, magnitude and frequency content. Interpretation of these signals has yielded important aspects on the dynamic behaviour of the sample dam at low strain levels, such as natural frequencies, signal amplification, spatial variability of the motion. The recordings of weak-motion earthquakes, which surely are more frequent than strong motion events during the dam lifetime, may be easily interpreted and could be extremely convenient in completing - at relatively low cost - the huge amount of information needed to carry out the crucial task of seismic safety assessment of dams, especially in case of existing earth dams that have been in operation for several years.