Kapiti, Manawatu shakes confirmed as slow-slip earthquakes

Patches of the slow slip in the Hikurangi plate boundary. Source: Geonet

A quake which shook the Kapiti, Manawatu regions has been confirmed as another slow-slip, or silent, earthquake GNS says.

It joins the earlier large slow-slip quake deep below the earth's surface under Hawke's Bay and Gisborne which moved GPS stations 2-3cm.

On its blog, a GNS science spokesperson said the slow-slip events occur below the earth's surface where the Pacific Plate meets the Australian Plate, along the Hikurangi Subduction Zone.

"The Kapiti-Manawatu slow-slip event has involved movement across the Hikurangi subduction zone plate boundary of between 5-7cm, equivalent to a magnitude 6.8 earthquake in the last two weeks.

"The Gisborne-Hawke's Bay event has involved slip across the plate boundary up to about 15cm, equivalent to a magnitude 7.2 earthquake.

"The GPS data suggest that the Gisborne-Hawkes Bay slow slip event appears to be tapering off in the last day or two."

The spokesperson said although it has recorded multiple slow-slip events in all of these regions previously since they were first discovered in early 2002, they have never detected slow-slip simultaneously in multiple regions.

"We have also never monitored slow-slip following a central New Zealand earthquake as big as the M7.8, so this could be typical bahaviour in the aftermath of such a large earthquake.

"We are continuing to monitor the event closely as it unfolds, and we will provide regular updates."