Document Actions
article: Nov 19 2009 - Palmerston North earthquake
This morning's magnitude 5.1 earthquake just after 7:00 am was centred south of Palmerston North at a depth of 40 km and was widely felt throughout the lower North Island. An aftershock of magnitude 4.3 occurred about an hour later.
The earthquake occurred near the top of the Pacific tectonic plate where it subducts beneath the North Island. It was a normal fault, meaning that extensional (spreading) faulting occurred in the top of the subducted plate. This extension is due to bending of the plate as it subducts. In this respect, the earthquake was similar to the Cook Strait earthquake earlier this year and to the Upper Hutt earthquakes of 2004.
The fact that this relatively small earthquake was felt so widely was due to its occurrence within the Pacific plate. The plate acts as a waveguide that causes seismic waves to spread more efficiently along the plate direction than if the plate was not present.
Scientists believe that the earlier Upper Hutt earthquakes were triggered by a slow-slip event (or slow earthquake) that occurred off the Kapiti Coast on the interface between the Australian and Pacific plates. The deeper slow slip event caused a change in stress at shallower depths, which in turn caused the Upper Hutt earthquakes to occur earlier, and perhaps more strongly, than they would otherwise have done. It is possible that this morning's earthquake was also triggered, in a similar manner, by the 2004-05 Manawatu slow-slip event that occurred in the same vicinity. More scientific interpretation will be needed to confirm or deny this possibility.
