Seismic hazard assessment of the historical site of Jam in Afghanistan and stability analysis of the minaret

The Minaret of Jam in Afghanistan was recently included in UNESCO's List of World's Endangered Monuments. The minaret is the world's second tallest (60 m) after the Qutub Minar in New Delhi and it is also one of the oldest (800 years). It is situated at the centre of the Hindukush range in the Ghor Province at the junction of Rivers Hari-Rud and Jam-Rud, at an elevation of 1900 m. The Minaret of Jam is in danger of collapse due to a 3.4° northward inclination inducing high stresses in its deteriorated brick masonry. The precarious conditions of the monument render it highly vulnerable to earthquakes. Therefore prior to any intervention to protect it, an assessment of the seismic hazard at the site, followed by an evaluation of the tower's seismic vulnerability, is of foremost importance. This article illustrates the results of the seismic hazard assessment of the archaeological site of Jam using both the probabilistic explanation (PSHA) and the deterministic explanation (DSHA) approaches. Uncertainty in PSHA has been handled within a logic-tree framework. Uniform hazard response spectra have been computed for return periods of 72, 224, 475 and 975 years. The earthquake input defined by the seismic hazard assessment of Jam has then been used to perform the dynamic analysis of the minaret. The latter has been carried out using a lumped mass approach and a 3D finite element model, accounting in both cases for dynamic soil-structure interaction.