Seismic Hazard Analysis for Qaraoun Dam in the Bekaa Valley, Lebanon

Qaraoun Dam is a 60 m high concrete-faced rockfill embankment built in 1960 and located at the southern end of the Bekaa valley in central Lebanon. Lebanon, which is situated near the tectonic boundary of the African and Arabian plates, is within a seismically-active region of the Dead Sea fault system that includes many complex fault structures with a history of producing high levels of earthquake shaking in the region. Large earthquake events that tend to occur in Lebanon, typically every few hundred years, have the potential to cause significant and possibly catastrophic damage to man-made structures not designed for such high levels of ground shaking. Therefore, it is important to update the evaluation of seismic hazard for structures, particularly ones whose integrity is critical to the prevention of loss of life and property, for these frequent (and less frequent, more damaging) earthquakes. A Probabilistic Seismic Hazard Assessment (PSHA) was conducted for the Qaraoun Dam including the active faults and background seismicity areal source zones in the surrounding region. The results of the PSHA are presented in terms of peak ground acceleration and response spectral values for various specified annual frequencies of exceedance, or return periods. The Yammouneh fault, located approximately 2.4 km west of Qaraoun Dam, was calculated to contribute the most to the seismic hazard due to its close proximity to the dam and its relatively high annual slip rate as part of the African-Arabian plate boundary. Uniform hazard response spectra, UHRS, were developed from the PSHA to represent the frequency content and level of shaking for the 100-year, 475-year, 975-year, 2475-year, and 10,000-year return periods. Deaggregation of the UHRS values was used to select appropriate earthquake scenarios which were used to develop two sets of vertical and horizontal scenario acceleration time histories for the 100-year and 10,000-year return period levels. The results of this study were compared with records of shaking from previous earthquakes in Lebanon and the Bekaa Valley. Previous studies on the seismic hazard of the region surrounding Qaraoun Dam, or for the dam itself, provide a comparison to the results of this study and highlight the importance of conducting a site-specific PSHA for critical structures. A comparison of methods used and documentation, or lack thereof, for previous studies also illustrates the evolution of PSHAs through time and how the incorporation of newly-available data can greatly affect the calculated results.