New Zealand's
Institute of Geological and Nuclear Sciences (GNS Science) initially measured the earthquake at 6.6. The
United States Geological Survey's preliminary measurement was 8.0 before it was revised to 7.8. and equal in magnitude to the
2016 Kaikōura earthquake. The 2009 earthquake occurred after a sequence of seismic events in northern Fiordland since 1988—six earthquakes above magnitude 6.0 struck the region prior to 2009, including a
magnitude 7.2 earthquake in 2003. The
hypocentre was near
Resolution Island in
Dusky Sound, Fiordland, at a depth that was initially reported as . The earthquake was the result of shallow
reverse faulting at the
subduction zone of the
Indo-Australian plate and
Pacific plate. Shaking reportedly lasted for at least a minute, and was felt throughout the
South Island and in the lower
North Island as far away as
New Plymouth. GNS Science described the
ground motion as relatively slow with a "lower frequency shaking" as opposed to abrupt, "rolling" motion from higher frequency shaking. The ground motion characteristic resulted in relatively few landslides. Thousands of
aftershocks occurred, with most being too small to be felt. One of the larger aftershocks measured magnitude 6.1, and occurred 19 minutes after the
mainshock. Most of them were distributed at the two edges of the rupture. These aftershocks had
focal mechanisms indicating reverse and strike-slip faulting. == Effects ==