Shallow Earthquakes, Central New Zealand

Residents of the lower North Island and upper South Island have reported feeling several earthquakes since February 27th. These earthquakes have been unusually shallow events, the latest of which occurred near Foxton last evening.

The shallow earthquake activity has been concentrated along the western edge of the Australian Plate where it carries the lower part of the North Island and the western part of the upper South Island. This part of New Zealand picks up much of the strain caused by the two tectonic plates colliding, with the Pacific Plate sliding under the Australian plate off the eastern coast of the North Island.

However, the two plates are locked together near Wellington and, to the south, they grind past each other to form the Southern Alps before the Australian Plate takes its turn to slide under the Pacific Plate to the southwest of Fjordland.

As the two plates push into each other, their edges become crumpled forming mountain ranges, deep valleys and fractures (earthquake faults) in the land to the north and south of the locked zone. In addition, where they grind past each other, the opposing edges form the Southern Alps – a long segment of plate boundary where the land stores the energy of the collison in the structure called the Alpine Fault.

Earthquakes are regular events to the north and south of the locked zone, but less frequent along the Alpine Fault where they occur during abrupt releases of the stored energy.

The shallow earthquakes of the past week have been occurring in the “crumple zone” in the upper South Island and its northern extension in the western North Island. The earthquakes reported by Geonet since the 27th of February 2007 are:

February 27th, magnitude 3.5, 7 km deep, centred 20 km north of Ohakune
February 28th, magnitude 3.5, 12 km deep, 20 km south of Wanganui
March 4th, magnitude 4.0, 12 km deep, 20 km north of Karamea
March 6th, magnitude 4.7, 5 km deep, 30 km south-east of Karamea
March 6th, magnitude 3.2, 5 km deep, 30 km south-east of Karamea
March 7th, magnitude 4.6, 7 km deep, 10 km north-east of Foxton

Last night’s earthquake near Foxton showed a remarkable trace on Geonet’s seismograph page, indicating that the earthquake had first registered in the upper part of the South Island, gathering strength as it propagated north through the southern and western part of the North Island, to begin losing its force as it reached the Urewera Range.

However, closer analysis revealed that a smaller seismic event in northern Westland or the Nelson region seems to have preceded the Foxton quake by a few moments (perhaps 10 seconds). As the seimographs recorded the South Island event, they also began tracing the bigger event near Foxton. This produced overlapping traces that made the event appear to have originated in the South Island when in fact they were two separate seismic events, even though one may have triggered the other.

At the time of writing, the South Island quake has not appeared in Geonet’s database, but the Foxton event, which was widely felt in Manawatu, Kapiti and the southern part of the North Island has been detailed as magnitude 4.6 at a depth of 7 km.

[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST.]

One Response to “Shallow Earthquakes, Central New Zealand”

  1. Chris says:

    Well, I’ve just learnt something there about the trace and what it means. Will have to research that a bit further.

    I’ve been impressed at the number of shallow earthquakes too. These are really shallow.

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