A slow-motion earthquake has pushed parts the east coast of New Zealand's North Island about four centimetres further east, scientists claim.
The slow slip event was so gradual most people would not have noticed - but the creeping tremor has so far released the same amount of energy as a 7.1-magnitude quake - just over a longer period.
GNS Science geophysicist Dr Laura Wallace said smaller 4.7-magnitude quakes around Waipukurau and Porangahau were probably linked to the recent slow quake.
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In a slow-slip quake, the Earth's crust moves up to tens of centimetres over weeks rather than rupturing suddenly in seconds as in a normal quake, Stuff New Zealand reports.
These events have been possible precursors for giant, "megathrust" quakes and appear to have occurred on the undersea plate boundary off Japan two months before the magnitude 9.0 quake in March 2011, which generated a deadly and destructive tsunami.
During the past week, southern Hawke's Bay GPS sites, such as those at Cape Kidnappers and Pawanui, had also started moving, she said.
"This suggests that the rupture of this slow-slip event is propagating south now too.
"Propagation of slow-slip events from offshore Gisborne into the Hawkes Bay region was also observed in 2016."
GeoNet GPS sites showed several centimetres of slip at the surface.
The event was probably near its end and was unlikely to carry on for more than another couple of weeks.
It was currently similar in size to the largest yet slow-slip quake recorded off the east coast, in March 2010.
