Deep Quake, North Island

A moderately strong, deep earthquake struck the North Island last night.

Wednesday 19th January 2011

A deep magnitude 5.5 earthquake struck beneath the city of Rotorua at 9:45 last night. The quake, which was centred within 5 km of Rotorua at a depth of 150 km, was widely felt in the North Island according to GeoNet. 975 reports have been filed by the public showing that shaking was felt from Northland to Banks Peninsula.

The U.S. Geological Survey reports the earthquake as magnitude 5.0 at a depth of 159 km. Their report shows that the quake was centred beneath the Waikato River 40 km south of Rotorua, 125 km north-north-west of Napier.

Shaking was strongest along the eastern coast of the North Island from Gisborne to Hawke’s Bay, consistent with reports from GeoNet’s instruments. With such a deep quake in the Pacific Plate, the energy travels back up the slab of the Pacific Plate and is transmitted across to the Australian Plate (which carries the North Island) where the two plates are in contact just off the eastern coast of the North Island.

This explains why the strongest shaking from this event was felt in Gisborne and Hawke’s Bay rather than in Rotorua which was directly above the earthquake hypocentre. The shaking that was felt in the Bay of Plenty was transmitted through the Australian Plate from the east. The energy from the earthquake cannot pass directly upwards through the molten material that sits between the two tectonic plates directly under the city.

[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.]

One Response to “Deep Quake, North Island”

  1. Lizzie from Gizzie says:

    Well that’s interesting, I thought we were sitting right on it. Was gentle rocking motion, not abrupt the like last one.

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