Hinzen et al. (2016) documented extensive archaeoseismic damage at Qalʿat Nimrod, including dropped voussoirs and keystones in arches and vaults, collapsed walls, fractured lintels, and spalled corners of ashlar blocks. Their study examined a total of 95 semicircular arches, of which 90 were subjected to detailed Arch Damage Grade (ADG) analysis based on photographic and three-dimensional laser-scan data. A key result of this analysis is that, with the exception of the Gate Tower—where deformation may reflect structural constraints rather than ground-motion effects—Hinzen et al. (2016) identified no correlation between arch orientation and the severity of seismic damage. The lack of a preferred damage orientation argues against a single dominant direction of strong ground motion. Instead, the pattern is consistent with damage accumulated through multiple seismic events impacting Qalʿat Nimrod from differing directions over time, particularly since arch damage is generally sensitive to the directionality of intense ground shaking.

Establishing the precise timing of the seismic damage is difficult, as the site has not been fully or systematically excavated. The damage must post-date construction of the fortress in the early thirteenth century CE. On historical and seismological grounds, Hinzen et al. (2016) suggest that most of the observed damage was likely produced during the 1759 CE earthquake sequence, while explicitly noting that damage from other seismic events cannot be excluded. A further possible source of damage is the 1837 CE Safed earthquake.

Qalʿat Nimrod occupies a narrow ridge that is subparallel to, and only approximately 2.5 km from, the Rachaiya fault. This geometric setting is conducive to a ridge-amplification effect during strong ground motion. In this context, it is plausible that much of the archaeoseismic damage observed at Qalʿat Nimrod was produced by strong shaking during the earlier event of the 1759 doublet—the 30 October 1759 Safed earthquake, generally interpreted as rupturing the Rachaiya fault. The later 25 November 1759 Baalbek earthquake, thought to have ruptured the Serghaya Fault , may plausibly have contributed additional damage, but likely at a lower intensity than that associated with the earlier 30 October event.

Regardless of which earthquakes were responsible, at least one event must have produced very high seismic shaking at the site. Using Discontinuous Deformation Analysis (DDA), Kamai and Hatzor (2007) modeled the dropped voussoir stones at the Gate Tower entrance. The best-fitting model required an acceleration amplitude of ~1 g and a dominant frequency of 2 Hz. Only minor deformation occurred below 0.8 g, while partial arch collapse was simulated at amplitudes of 1.5 g or higher. The results show a strong frequency dependence, with 2 Hz clearly preferred over 1 or 3 Hz. Kamai and Hatzor (2007) further suggested that seismic amplification in the form of a ridge effect may have played a significant role, allowing for bedrock accelerations as low as ~0.4 g at 1 Hz. In this framework, a range of peak horizontal ground acceleration (PGA) values of approximately 0.4–1.0 g can be taken as a constraint on the level of bedrock shaking that struck the site of Qalʿat Nimrod. When translated using Equation 2 of Wald et al. (1999), this PGA range corresponds to macroseismic intensities of roughly 7.8–9.3, consistent with strong to very strong ground shaking capable of producing the observed archaeoseismic damage.

Entrance at the Gate Tower to Nimrod's Castle - click on tab to open a magnifiable image in a new tab - Photo by Jefferson Williams (2018)