LISBON—A series of multiple small earthquakes that have been rattling a mid-Atlantic Portuguese island for three days could trigger a stronger tremor or a volcanic eruption, experts said on Tuesday, as authorities urged people not to travel there. The number of earthquakes recorded on the volcanic island of Sao Jorge, in the Azores archipelago, since Saturday afternoon has increased to around 1,800 from an earlier figure of 1,329, said Rui Marques, head of the region’s CIVISA seismo-volcanic surveillance centre. Only 94 of the 1,800 earthquakes, with a magnitude of between 1.7 to 3.3, registered so far have been felt by the population, Marques told Lusa news agency. Sao Jorge, one of nine islands which make up the Azores, is home to around 8,400 people and is part of the archipelago’s central group, which includes the popular tourist destinations of Faial and Pico, which are also volcanic. The series of small …