Divergent Responses of the Cyclades to Large Earthquakes: Insights from Amorgos Island

Summary

On 9 July 1956, the southeastern Cyclades were struck by an earthquake doublet, consisting of an ∼Ms 7.4 mainshock, the largest recorded in Greece during the instrumental era, followed shortly afterward by a twin earthquake of magnitude greater than 6.0. The sequence caused severe damage on Santorini, leading Greek authorities to declare the island a disaster area, and triggered a major tsunami that affected much of the Aegean basin. Despite the historical significance of this catastrophe, ground-motion observations in the near-source region remain sparse and sometimes contradictory seventy years later, limiting our understanding of the rupture process and hindering reliable seismic hazard assessment. This is particularly true for Amorgos, located less than 30 km from the rupture zone, where available macroseismic evidence suggests unexpectedly low earthquake impacts. To address this knowledge gap, we conducted the first comprehensive geophysical investigation of Amorgos, combining passive and active seismic methods, including Ambient Vibration Arrays (AVAs) and Multichannel Analysis of Surface Waves (MASW). Taking advantage of the remarkable seismic sequence that occurred between Santorini and Amorgos in early 2025, we further compared the seismic responses of both islands to a set of local crustal earthquakes with magnitudes greater than 3.2. Our results provide a geophysical characterisation of the island’s main inhabited areas and reveal moderate to high near-surface shear- and compressional-wave velocities, consistent with the local geology and indicative of stiff soil conditions with limited amplification potential. Combined with the analysis of seismicity, these observations confirm the absence of site effects in Amorgos, in sharp contrast with the strong amplification and directivity observed in Santorini. Conversely, the permanent seismological station on Amorgos exhibits remarkably short-duration ground motions and a pronounced attenuation of high-frequency energy (> 6 Hz), corresponding to the frequency range of traditional Cycladic buildings. These findings offer a new perspective on the ground-motion pattern of the 1956 earthquake and suggest that the geological setting of Amorgos played a major role in mitigating earthquake impacts. However, the geophysical evidence does not account for the large spatial variability in the reported macroseismic intensities, raising questions about the reliability of the intensity assessments. More broadly, this study highlights the complexity of near-fault ground motions and demonstrates the relevance of integrated geophysical approaches for overcoming the limitations of macroseismic datasets and improving seismic hazard assessment.