Tectonic Ping-Pong

Last night’s magnitude 5.3 earthquake south of the Kermadec Islands provoked a quick response from New Zealand’s south, in an effect that I call tectonic ping-pong.

The three main islands that make up New Zealand sit along the edge of the collision zone between the Pacific and Australian tectonic plates. The North Island sits on the edge of the Australian Plate and, off the coast to the east, the Pacific Plate is sliding under (subducting) the Australian Plate to be recycled as the rock heats up at increasing depths.

To the south-west of the South Island, the Australian Plate is sliding underneath the Pacific Plate. In between the two zones, the plate edges grind past each other forming the Southern Alps and the dramatic Alpine Fault. Near Cook Strait, the plates are locked together, and the land surface buckles as it absorbs the relative motion of the two gigantic plates.

Periodically, one of the many earthquakes that occur along the plate boundary strikes at a time when the accumulated strain causes a reaction elsewhere in the collision zone. Usually this is illustrated by a large earthquake triggering smaller events nearby as the stress in the area is redistributed.

One peculiar phenomena that I have observed in recent years, and dubbed tectonic ping-pong, occurs when an earthquake in the subduction zone between the Bay of Plenty and the southern Kermadec Islands triggers a similar-sized event in the subduction zone between Fjordland and Macquarie Island – usually within a few hours, but sometimes up to 24 hours later.

This was certainly the case last night when a magnitude 5.3 quake struck at a depth of 19 km, 190 km SSW of L’Esperance Rock at 9:42 p.m. on Tuesday September 4th 2007. Two hours later, a magnitude 4.7 quake struck in the southern Tasman Sea at a depth of 74 km. The 11:45 p.m. quake was located about 200 km south-west of Stewart Island, near the Puysegar Trench. This completed the ping-pong sequence, and was followed by a magnitude 3.2 quake at a depth of about 5 km located half-way between Milford Sound and Wanaka at 1:07 this morning.

Obviously, the conditions need to be right for a southern Kermadec – Bay of Plenty earthquake to trigger a ping-pong response from New Zealand’s south-west, and these events are not common. However, similar tectonic ping-pong sequences were observed in January, February, April, June, and October of 2005.

[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST; and the US Geological Survey and its contributing agencies.]

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