Singapore is not located on a plate boundary and is therefore not prone to earthquakes. But big earthquakes occurring on the nearest plate boundary (the Sunda Megathrust, offshore Sumatra) may affect the country: an earthquake powerful enough could swing the buildings located over reclaimed lands or sediments.
However, Singapore faces a more lurking threat: sea level could rise faster than the IPCC predicted after a big earthquake on the Sunda Megathrust. This is due to the overall tectonics of the region. After a big earthquake on the megathrust, the whole Sunda shelf will experience a subsidence. This phenomenon is observed nowadays in Thailand, where the sea level rises because of the subsidence caused by the 2004 Aceh-Andaman earthquake.