Two unusual submarine earthquakes (Mw 5.8) occurred near volcanic islands, called Curtis and Cheeseman, in the Kermadec Arc in 2009 and 2017. Following both earthquakes, similar tsunamis with wave heights of about a meter, that are disproportionate to their moderate seismic magnitudes, were observed by coastal tide gauges. We investigate the source mechanism for both earthquakes by analyzing tsunami and seismic data of the 2017 event. Preliminary analysis of tsunami data indicates that the earthquake uplifted a submerged caldera around the islands. Source modeling using tsunami and seismic data reveals that a trapdoor faulting, involving ring-faulting and deformation of an underlying magma reservoir, occurred due to magma overpressure in the reservoir, possibly in association with caldera resurgence. The relationship between the maximum fault slip and the seismic magnitude for trapdoor faulting events found at global calderas is different from that for regular earthquakes, reflecting the peculiarity of the volcanic earthquakes.