Pongaroa Earthquake Rattles New Zealand

MAGNITUDE 5.5, 15KM WEST OF PONGAROA, NEW ZEALAND. 23RD SEPTEMBER, 2:41 AM NZST.

Sizable earthquake strikes lower North Island, waking thousands in the early hours.

This earthquake is likely a triggered event from the Eketahuna earthquake on 20th January (Mw 6.2), on a parallel fault due east from the Eketahuna earthquake. This quake is unlikely to cause damage to the main towns (Eketahuna, Masterton, etc.), but may have caused minor damage in the a very localised region. Reports so far suggest that the quake caused no damage, which is quite lucky. The quake gained over 3300 felt reports and a maximum intensity of MM7 (lightly damaging shaking).

This earthquake had a foreshock of M3.8 at 12:06 am NZST (which gained 95 felt reports and a maximum intensity of MM4 (light shaking)); as of 6:30 pm NZST has had two aftershocks of significance; an M3.7 at 3:42 pm NZST, followed by an M3.0 at 3:47 pm NZST. The M3.7 gained 48 felt reports and a maximum intensity of MM5 (moderate shaking).

Recent research released by GNS suggests that the Kapiti slow-slip event which began in mid-2013 may have loaded stresses in the area of Eketahuna & today's Pongaroa quakes. This can be supported by recent seismic activity offshore of Paraparaumu which suggest the slow-slip event has still not fully subsided, or may have restarted with a very short recurrence interval.

NZ Herald Report: http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=11329696

GeoNet summary:http://geonet.org.nz/quakes/region/newzealand/2014p715167

GNS research on Kapiti slow-slip event & links with Eketahuna:http://info.geonet.org.nz/display/quake/2014/05/15/New+Zealand's+'silent+earthquakes'+of+2013

Written by J H Gurney on the 23rd September.