By Catarina Demony and Pedro Nunes
SAO JORGE, Portugal (Reuters) -Volcano experts warned on Tuesday that the earthquake-hit island of Sao Jorge in Portugal's Azores archipelago could experience an initial eruption similar to Spain's La Palma last year, which destroyed thousands of properties and crops over 85 days.
The lush mid-Atlantic island has been rattled by more than 20,000 small earthquakes in the past 11 days, with the strongest tremor since the beginning of the "seismic crisis" recorded on Tuesday evening with a magnitude of 3.8, said the region's CIVISA seismo-volcanic surveillance centre.
Portugal's meteorology agency IPMA said the earthquake reached a magnitude of 4.
There are fears the tremors, which have caused no damage so far, could herald a volcanic eruption for the first time since 1808, or a powerful earthquake.
Tuesday's quake was felt in Sao Jorge as well as on the nearby islands of Terceira, Pico and Faial, which are also volcanic.
Elderly people and those with reduced mobility have been evacuated from Velas, the municipality where most of the seismic activity has been recorded, and many locals have left the island in fear.
"The magnitude of the earthquakes has reduced slightly ... but the population must stay alert, they should not relax," said Eduardo Farias, head of Azores' Civil Protection authority.
At a school in Velas, classes were suspended and some of the school's rooms will be turned into a health centre.
Francisco Fonseca, president of Sao Jorge's health service, said health workers with experience working in natural disasters have been deployed to the island.
"This whole situation is a challenge," he said. "It is distressing and out of the ordinary."
Sao Jorge's sudden increase in seismic activity is reminiscent of the earthquakes detected before the first eruption of the Cumbre Vieja volcano on Spain's La Palma island last September, some 1,400 km (870 miles) southeast of the Azores.
Experts at the Canary Islands Volcanology Institute, Involcan, which monitored the La Palma eruption, said on Tuesday about 20 million cubic meters of lava could be spewed out on Sao Jorge if an eruption takes place.
Luca D'Auria, director of Involcan's volcanic surveillance department, told Reuters a soil deformation caused by the volcanic activity had been detected in Sao Jorge.
"It could either end in nothing (...) or it could end in a volcanic eruption," he said.
Farias said CIVISA would analyse Involcan data but asked the roughly 8,400 residents of Sao Jorge not to panic.
(Reporting by Catarina Demony, Guillermo Martinez and Pedro Nunes in Sao Jorge and Emma Pinedo Gonzalez in Madrid; Editing by Alexandra Hudson and Grant McCool)