Police said all but one of the people known to be trapped were responding to rescuers and were expected to be extracted alive.
One person, however, wasn't responding, raising worries the death toll could rise, said Giovanni Salerno of the financial police.
Italy's national volcanology institute said the temblor struck a few minutes before 9 pm yesterday, just as many people were having dinner. The hardest-hit area was Casamicciola, on the northern part of the island.
While such discrepancies are common, Italian officials complained that the Italian agency's initially low 3.6- magnitude estimate greatly undervalued the power of the temblor.
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At least one hotel and parts of a hospital were evacuated. A doctor at the Rizzoli hospital, Roberto Calloca, told Sky TG24 that some 20 people were being treated for minor injuries at a makeshift emergency room set up on the hospital grounds. Calloca said the situation was calm and under control.
Civil protection crews, already on the island in force to fight the forest fires that have been ravaging southern Italy, were checking the status of the buildings that suffered damage.
Together with the nearby island of Capri, Ischia is a favorite island getaway for the European jet set, famed in particular for its thermal waters. Casamicciola was the epicentre of an 1883 earthquake that killed more than 2,000 people.