Multi-temporal assessment of landslide impacts on the Cathedral of Agrigento (Italy) imaged through Persistent Scatterer Interferometry

Exhaustive knowledge of both natural and anthropic factors represents the necessary background for planning effective mitigation strategies and preserving the cultural heritage exposed to landslide hazards. Identifying the areas potentially affected by such mechanisms contributes to focus consolidation works, proposing a priority criterion in relation to the estimated risk level. Methods suitable for the detection of land and structural deformation precursors are therefore fundamental to assess the damage at both short- and long-term, consequently guiding the decision-making. We present an integrated study carried out in the historic urban area of Agrigento (Italy), historically affected by retrogressive landslide processes involving the NW slope of Girgenti hill. Similarly to the slope instability threatening the UNESCO World Heritage site of the Valley of the Temples, located SE of Agrigento urban area, the geological and geomorphologic setting of the old town causes widespread forms of ground instability controlling the structural equilibrium of the overlying historical buildings, most of which are built at the edge of the NW slope of Girgenti hill. Major reactivations of these landslides date back to the last century (e.g. February 1944 and July 1966). Following a worsening of local instability observed in the NW sector of the hill in 2005, we carried out an integrated analysis based on the combination of conventional field investigations with a satellite PSI (Persistent Scatterer Interferometry) time series study. Such remote sensing technique exploits multi-temporal archives of radar imagery acquired by low-orbiting SAR (Synthetic Aperture Radar) satellites and it is widely employed thanks to the highly precise measurements of displacements induced by slow land processes (up to some tens of cm/yr). We used 1992-2007 PSI data, based on the processing of 66 ERS1/2 images in descending mode and 54 RADARSAT-1 images in ascending mode. As these archives are characterized by nominal revisiting time of 35 days and 24 days, respectively, such monitoring campaign provided a monthly sampling frequency for the investigated area. On the outside staircase of the Arab-Norman Cathedral of St. Gerlando (XI-XII century), the PSI time series analysis identifies an acceleration of ground deformation from 1-2 mm/yr in November 1992 – June 2006 to 13-15 mm/yr in July 2006 – May 2007. Already subject to many repairing and reinforcing interventions in 1981, 1998 and 2005, the Cathedral is now affected by widespread damages and cracks. Its ceiling, floor and columns are highly fractured, especially in the left aisle which faces the unstable slope. Two different main fracture planes with a sub-vertical dip and dip direction of about 185° (parallel to the slope edge) and 315° are also observed. The discontinuities visible on both the inner walls of the Cathedral and its façade also extend along the entire length of the outside staircase, on the W up to the Bishop’s Seminary (XVI-XVII century), and on the E up to St. Alphonsus Liguori’s Church (XIX century). Even though many provisional interventions have been already activated to reduce the risk of collapses, today the conditions of the Cathedral are becoming critical more than ever. Due to severe structural damages and the highly probability of collapse, the definitive closure of the left aisle turned out to be indispensable at the end of February 2011. The main success of the experimental study carried out on Agrigento’s Cathedral consisted in detecting a well-defined deformation pattern, confirming a real aggravation of the local instability since 2006 and providing a reliable risk scenario, as it actually happened. The results of such integrated investigation and the lack of appropriate countermeasures to the observed criticalities, demonstrate the need of responding – operatively and promptly – to increasing hazard and risk for cultural heritage.