Scientists from the National Institute of Water and Atmospheric Research are on an expedition to discover more about fault lines off the Canterbury coast.
NIWA says the information will be used as part of the preparation work for rebuilding Christchurch.
The scientists are travelling on the research vessel Kaharoa and have started a seismic survey of an area in Pegasus Bay.
Voyage leader Philip Barnes says the earthquakes in September and last month ruptured a fault system stretching about 70 km to the coast.
He says scientists do not know if there are faults extending further offshore that might pose a risk to Christchurch.
Dr Barnes says their equipment allows them to look up to 1.5 km deeper than the sea floor.
He says there is very little information available about the area.