Slow-moving earthquakes lasting up to 30 minutes have been uncovered by scientists investigating the Alpine Fault in the South Island.

Stumped by the apparent absence of earthquakes in the middle section of the fault line, regarded as the country's most hazardous, researchers drilled holes up to 100 metres deep to insert sensors to monitor earthquakes.

They found the area, between Fox Glacier and Whataroa Valley in Westland, was seismically active but the small earthquakes were slow, creeping tremors each lasting up to 30 minutes.

The study, led by Victoria University geophysics professor Tim Stern, installed 11 stations along the fault and since late 2009 has registered around 2500 earthquakes.

It was not yet known what the seismic activity meant for the future risk of a sizeable earthquake.

"It could be that constant tremor builds up stress and may trigger a major fault movement or, alternatively, the activity may decrease the likelihood of a major quake by acting as a release valve for stress.

"What's important is that we find out more about these tremor events, such as where they happen and how often, so we can better predict the hazard the Alpine Fault poses," team member Dr Aaron Wech said.