Nine earthquakes in just over seven hours awakened and shook up the town of Whakatane, on the east coast of New Zealand's North Island on Thursday.
The swarm began at 2.38 am with a shake measuring 4.3 on the Richter scale, and the area was still shaking just before 10 am with a series of smaller quakes at regular intervals.
Seismologists at the national earthquake monitoring organisation GeoNet said they could not predict what would happen with swarms.
"They may carry on for hours or even days, or the centre of the earthquakes might move around," a statement said.
Whakatane, a coastal town on the Bay of Plenty, lies close to large-scale faults, where two land masses are gradually pulling apart at the rate of about one cm a year, GeoNet said.