Geological Summary for New Zealand Area, June 2009

Earthquake activity eased. White Island and Mt. Ruapehu remained at Alert Level 1.

GeoNet, the U.S. Geological Survey and GNS Science reported 43 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during June 2009.

The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (4), M4 to 4.9 (3) M3 to 3.9 (21).
An additional 15 events in the magnitude 2 range were deemed worthy of mention.

Earthquake activity during June 2009 eased from May levels, despite two intense earthquake swarms that struck the Taupo area.

Activity in the Kermadec Islands was also quieter, with only three events reported during May. A deep magnitude 4.6 quake struck to the north of Raoul Island on the 28th, and a deep magnitude 5.1 quake near L’Esperance Rock on the 6th. A shallow magnitude 5.4 event 115 km south-south-east of L’Esperance Rock (810 km north-east of Auckland) early on Tuesday the 30th was not reported by GeoNet and was not reported felt in New Zealand.

The largest mainland earthquake felt during June was a magnitude 5.1 event which struck 170 km beneath the city of Rotorua at dawn on the 18th. The quake was felt at some Bay of Plenty locations and at Wanganui, Hawke’s Bay and Wellington.

Residents of Turangi experienced two intense earthquake swarms during June. The first, centred on the northern shore of Lake Rotoaira, consisted of 34 distinct events with magnitudes between 1 and 3. The swarm commenced on the afternoon of Sunday May 31st and ceased at 5:17 the following morning. A summary of the Lake Rotoaira swarm can be found here.

Another intense swarm of earthquakes struck the southern shores of Lake Taupo during the latter part of June 2009. Activity was most concentrated near the town of Turangi, but was also spread underneath Lake Taupo and along the south-western shore. All of the quakes were shallow at depths of 8 km or less, and ranged between magnitude 2.2 and 4.4. A burst of activity on the 16th was mainly magnitude 2.9 events, with another burst on the 26th including two events of 3rd magnitude. Activity peaked on the 27th with eleven distinct events, including two quakes of magnitude 4.3 and 4.4. Activity then declined, ending with a magnitude 2.9 event on the 30th. A summary of the Lake Taupo swarm can be found here.

In all, 25 events from both swarms were likely to have been felt by the public and were reported on the GeoNet site during June.

Two quakes struck off the coast near Porirua on June 5th and 7th. The quakes, both magnitude 3.9, were 40 km deep and were widely felt in the Wellington region, along the Kapiti Coast and in the Marlborough Sounds.

An unusual magnitude 5.0 earthquake struck 120 km west of Greymouth just before dawn on June 28th. The quake, which was only 12 km deep, was located under the Tasman Sea in an area known as Lord Howe Rise. The quake was felt throughout Westland from Westport to Fox Glacier and inland at Reefton.

A pair of earthquakes struck 30 km north-west of Ashburton on the 12th and 14th. The magnitude 3.4 and 3.7 quakes were very shallow, at 6 and 4 km respectively, and were felt in the Methven area. The larger event was also felt in Christchurch, Ashburton and Temuka.

Regular reporting of the status of New Zealand’s volcanoes ceased at the end of June 2007, with the closure of the Hazard Watch service. GNS Science now only issues bulletins which record significant changes in volcanic behaviour.

No Alert Bulletins were issued by GNS Science during June.

At the end of June 2009, New Zealand’s active volcano status can be summarised as follows:
Raoul Island (Alert Level 0).
White Island (Alert Level 1).
Mt Ngauruhoe (Alert Level 0).
Mt Ruapehu (Alert Level 1).

[Compiled from data supplied by GNS Science, US Geological Survey, GeoNet, and their contributing agencies.]

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