This would be the tsunami which caused such devastation at Messina; Paul Voronov, who would later become the object of Grand Duchess Olga's affection, was decorated for his valor in saving lives during the aftermath of that tragedy. Here, in part, is what I found online:
The most deadly tsunami between 1900 and 2004 occurred in 1908 in Messina, Italy, on the Mediterranean Sea where the earthquake and tsunami killed 70,000.