Deadly quake: South Island fault sequence 'complex'

This morning's sequence of devastating quakes may involve more than one fault, Geonet duty seismologist Dr Anna Kaiser says.

The quake sequence was "looking complex", Kaiser said.

She did not have exact figures, but believed there had been at least 100 aftershocks on the East Coast of the South Island following the first, 7.5 magnitude, quake near the North Canterbury settlement of Culverden.

Three were over magnitude 6 and five over magnitude 5. The most recent quake was a 10 kilometre deep 4.6 that struck 30km south-west of Seddon at 4.16am.

Investigations were underway but there was a still a lot to learn about what had happened, Kaiser said.

"We're working very hard ... this is looking quite complex."

The epicentre was possibly just south of the Hope fault, but it did look like "something else was going on".

Aftershocks were occurring up the east coast of the South Island, which was a complex area of faults, Kaiser said.

"We don't know what we are dealing with right now but it may be ... it involves potentially more than one fault ... we need to figure out what was going on before and after the [7.5 magnitude] quake."

She did not know why some people may have reported seeing lights in the sky at the time of the quakes.