March 13, 2006
Wanganui Chronicle - Wanganui, New Zealand

A SWARM of five small earthquakes buzzed and rattled into Wanganui on Saturday morning. The five all originated in the same place, 30km south west of Wanganui and off the coast approximately due west of Marton.

New Zealand’s earthquake website, www.geonet.org.nz, says they ranged from 4.5 to 3.8 on the Richter Scale, and were all 30km deep. They regularly interrupted the sleep of Wanganui and Marton residents – at 55 minutes past midnight, at 1.04am, at 3.17am, at 5.27am and finally at 10 minutes past eight in the morning.

GNS Science duty seismologist Martin Reyners said the swarm was related to another that happened in 1982 and originated at the same depth.

There were six earthquakes on one day that year, ranging from 4.6 to 4.9 on the Richter Scale.

“Every now and then we get these reasonably persistent earthquakes about 30km deep. They’re usually quite small.” The earth’s crust under Wanganui was about 40km thick, he said, with earthquakes happening at its base. They were basically a result of the Pacific tectonic plate, 80km to 100km deep underground, progressively sliding under the Indo-Australian plate.

Scientists thought that as it slid under, the Pacific plate heated up. Fluid - mainly water - evaporated out of it and moved toward the earth’s surface. When this fluid broke through a small fault 30km below the earth’s surface, it caused an earthquake.

The quakes would keep happening until the fluid reached a stronger bit of the fault which did not break. GNS was not expecting even minor damage from Saturday’s earthquakes.

“They’re not nearly as big as the 5.2 one on February 17 last year, which happened on the same spot but on a relatively bigger fault. It was the biggest for the region in 20 years.”

Mr Reyners couldn’t say whether the quakes would continue. That was possible, if they changed stress conditions deep under the earth’s surface.

Saturday’s shakes typically had two peaks, because their origin was deep enough that the waves of movement reached the surface at slightly different times.