Seismic Hazard Assessment of the Historical Site of Jam and Stability Analysis of the Minaret

Student: Arun Menon
Supervisors: Prof. G. Macchi, Dr. C. Lai

 

Abstract

The Minaret of Jam in Afghanistan has been recently included in UNESCO's List of World's Endangered Monuments. It is the world's second tallest minaret (~60m) after the Qutb Minar in New Delhi and 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 and at an elevation of about 1900m above sea level. The minaret is in danger of collapse due to a 3.5° northward inclination inducing high stresses in its deteriorated brick masonry. The actual precarious conditions of the monument render it highly vulnerable to earthquakes. Therefore prior to any intervention to safeguard it, an assessment of the seismic hazard at the site followed by an evaluation of the tower's seismic vulnerability is of foremost importance.

Historical (non-instrumental) and instrumental seismicity data for the current study have been retrieved from the published catalogues compiled by Ambraseys and Bilham [2003] and Quittmeyer and Jacob [1979]. The earthquake catalogue has been processed to remove foreshocks and aftershocks by the Gardner and Knopoff [1974] "windowing" criteria and completeness intervals have been evaluated using the Visual Cumulative method of Mulargia et al. [1987]. The seismic hazard assessment of the archaeological site of Jam has been performed using both the probabilistic (PSHA) and the deterministic (DSHA) approaches. The Gutenberg-Richter recurrence law has been used to characterize the seismicity of the region and the Cornell-McGuire approach has been adopted for the probabilistic assessment. Uncertainty in PSHA has been handled using the logic-tree framework with attenuation relationship, seismogenic zoning and upper bound maximum magnitude as the controlling parameters. Horizontal and vertical uniform hazard response spectra have been computed for reference return periods of 72, 224, 475 and 975 years. The seismic input defined by the seismic hazard assessment of Jam has then been used to perform the dynamic analysis of the minaret. Structural analysis has been carried out using a lumped mass model and a 3D finite element model accounting in both cases for dynamic soil-structure interaction. Equivalent soil spring stiffness has been estimated using the Gazetas [1991] dynamic impedance approach for two cases of the foundation soil: stiff and soft soil.

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