Strong Earthquake, Southern Kermadecs

A strong earthquake struck the southern Kermadec Islands early this morning.

Tuesday, 19th April 2011

A magnitude 6.6 undersea earthquake struck 323 km south-south-west of L’Esperance Rock in the southern Kermadec Islands at 1:03 this morning. The quake, centred 527 km north-north-east of Gisborne (552 km east-north-east of Auckland) was 90 km deep according to the U.S. Geological Survey.

The USGS analysis places the quake beneath the Kermadec Ridge, near the western boundary of a structure known as the Tonga Microplate.

Geoscience Australia reports the quake as magnitude 6.7 at a depth of 39 km. Placing the quake just west of the Kermadec Trench, it estimates that the quake would have been felt up to 900 km from the epicentre.

The Pacific Tsunami Warning Centre issued one bulletin for the event at 1:11 this morning. Reporting the earthquake as magnitude 6.7 at a standard depth of 33 km it advised that, based on historical data, a widespread tsunami threat did not exist. GeoNet’s tsunami gauges showed no significant change in sea levels following the earthquake.

The Joint Australian Tsunami Warning Centre issued a bulletin at 1:19 this morning advising, “…no tsunami threat to the Australian mainland, islands or territories.”

New Zealand’s Ministry of Civil Defence ignored the event.

GeoNet’s Urewera seismic drum on 19 April 2011

[click for larger image] The trace for this morning’s southern Kermadec earthquake showed strongly on GeoNet’s Urewera seismometer.

GeoNet reported the quake as magnitude 6.4 at a depth of 33 km. Its analysis places the earthquake epicentre 330 km south of L’Esperance Rock, 440 km north-east of Te Araroa, 630 km north-east of Auckland, close to the Kermadec Trench about one degree east of the USGS location.

Whilst the depth and location calculated by the various agencies differs, the data indicates that the earthquake occurred on the descending slab of the Pacific Plate as it slides underneath the Australian Plate along the Kermadec Trench. It is known that it is difficult to accurately locate strong earthquakes in New Zealand territory using instruments based in New Zealand, and this will have been taken into account by GeoNet when analysing the quake. Instruments based here provide good north to south coverage, but the east-west baseline is narrow and some reliance on instruments based overseas is necessary. An accurate position for the earthquake should be known later today.

The earthquake was widely felt along the eastern side and in the southern part of the North Island, with reports being filed from Pukekohe in the North Island to Geraldine in the South Island.

The Kermadec Islands have experienced moderately strong earthquakes in recent weeks. A magnitude 5.7 earthquake struck 172 km south of L’Esperance Rock on Thursday 14th at 5:45 p.m. Three quakes with magnitudes between 5.0 and 5.2 struck the islands between April 5th and 11th.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST; the US Geological Survey, the U.S. National Oceanic & Atmospheric Administration’s Pacific Tsunami Warning Centre, Geoscience Australia, and the Joint Australian Tsunami Warning Centre.]

One Response to “Strong Earthquake, Southern Kermadecs”

  1. Lizzie from Gizzie says:

    Informative, as usual Ken

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