Neotectonics and slope stabilization at the Alhambra, Granada, Spain

Over 600 years, the Alhambra Palace of Granada, Spain, (a World Heritage site) has been damaged by earthquakes and slope instability. The western part of San Pedro Cliff, on the northern slope of the palace is a compound fault scarp — fault-line scarp, modified by river erosion and latterly by successive slab falls. The plane of the fault with the largest throw (c. 7 m) outcrops in the innermost part of the escarpment, and is a normal fault with a NW–SE strike and steep SE dip. It is part of a set outcropping along the Alhambra hill. Fault activity may be very recent, perhaps related to historical earthquakes. Seismic risk at the Alhambra is considered to be moderate: there is earthquake damage of the Arab walls and barrier. The most significant historical damage occurred in 1431 and partially collapsed the Arab barrier. Extension associated with the faults loosens the ground and contributes to slab falls. The faults are also preferential water paths. Both the many cracks of the walls and collapses of the Alhambra barrier appear concentrated and aligned with the fault set.Stability analyses suggest that the factor of safety of the San Pedro slope under 1000-yr-return-period earthquake loading may drop below 1.0 and the critical slip surface could penetrate the Alhambra walls. To raise the safety factor above 1.0 and to counteract extensional stress in the cliff, an apparently environmentally acceptable solution with minimal visual impact is proposed. It consists of high-yield-stress wire mesh, post-tensioned by anchors, and coloured to blend with the cliff.