Deep Earthquake, North Island

Another moderately strong, deep earthquake woke residents of the North Island this morning, the fourth deep earthquake to have been widely felt in just over a month.

Friday 28th January 2011

A magnitude 5.9 earthquake struck 150 km below the Bay of Plenty town of Atiamuri at 4:02 this morning. GeoNet reports that the quake was centred 20 km east of Managakino, 20 km south-east of Tokoroa, 30 km north of Taupo.

The earthquake was felt from Auckland to Oamaru, resulting in more than 1600 reports from the public by 10:30 a.m. Instruments show that MM4 shaking was experienced from Tauranga to Wellington. MM4 shaking is defined as, “Generally noticed indoors, but not outside, as a moderate vibration or jolt. Light sleepers may be awakened. Walls may creak, and glassware, crockery, doors or windows rattle.”

Slightly stronger shaking was experienced along the North Island’s eastern coast from Gisborne to Hawke’s Bay and inland as far as Taupo where MM5 effects were observed. MM5 shaking is defined as, “Generally felt outside and by almost everyone indoors. Most sleepers are awakened and a few people alarmed. Small objects are shifted or overturned, and pictures knock against the wall. Some glassware and crockery may break, and loosely secured doors may swing open and shut.”

GeoNet’s seismic drum network on the morning of January 28, 2011

The earthquake left strong traces on GeoNet’s seismograph network.  [Click on image for a larger view]

Geoscience Australia reports the earthquake as magnitude 5.4 at the shallower depth of 87 km. It estimates that the quake would have caused damage within 20 km of the epicentre which it has placed nearer Mt Tarawera, and would have been felt up to 250 km away at Auckland and Palmerston North.

The U.S. Geological Survey reports the event as magnitude 5.3 at a depth of 154 km. Their epicentre is calculated to be 35 km south-west of Rotorua, 140 km north-west of Napier.

This morning’s earthquake is the fourth deep event to have been widely felt in the North Island in just over a month. On December 26th a magnitude 5.9 earthquake struck off-shore 80 km north of White Island at a depth of 230 km The following day a magnitude 5.4 earthquake struck 30 km north-east of Tauranga (30 km north-east of Te Puke) at a depth of 210 km. Both events occurred early in the morning, at 1 o’clock and 3 o’clock respectively.

On January 18th another deep quake struck the Bay of Plenty, this time at 9:45 p.m. The magnitude 5.5 earthquake was centred beneath the city of Rotorua at a depth of 150 km.

All four earthquakes were felt at many North Island locations but more strongly along the East Coast from Gisborne to Hawke’s Bay where the energy travelling up the descending slab of the Pacific Plate is transferred across to the Australian Plate.

Another deep magnitude five event on January 14th was less widely felt. The magnitude 5.5 quake was centred off-shore 140 km north-east of Te Araroa at a depth of 70 km. The 1 a.m. quake was felt from the Bay of Plenty to Gisborne.

To put the four deeper earthquakes that were felt into perspective, GeoNet’s database shows 15 events of magnitude 4 or greater struck at depths of 100 km or more over this period. As expected the quakes struck along a line from the Bay of Plenty to Nelson where the Pacific Plate survives to depths of hundreds of kilometres before melting.

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

One Response to “Deep Earthquake, North Island”

  1. Lizzie from Gizzie says:

    Both Neil and I felt this one.

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