Geological Summary for New Zealand Area, June 2008

Earthquake activity continued at a low level. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.

GeoNet, the U.S. Geological Survey and GNS Science detailed 33 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during June 2008.

The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (4), M4 to 4.9 (7) M3 to 3.9 (18).
An additional 4 events in the magnitude 2 range were deemed worthy of mention.

Earthquake activity in the Kermadec islands was at a low level during June. One very deep magnitude 4.6 earthquake was reported north-west of Raoul Island on the 28th and two deep magnitude 5 events south of L’Esperance Rock on the 11th and 12th.

A burst of activity in the Taupo Volcanic Zone which stretches from Lake Taupo to the Bay of Plenty attracted considerable attention during June 2008. A swarm of several hundred shallow earthquakes followed a magnitude 5.4 earthquake located 10 km south-west of White Island at 9:06 a.m. on June 13th. Four of the aftershocks, with magnitudes between 3.7 and 4.3 were reported by GeoNet and triggered enhanced monitoring of the nearby volcano.

Further south, a swarm of 5 earthquakes struck beneath Lake Taupo, the world’s most productive rhyolite volcano. The quakes, with magnitudes between 2.2 and 3.3 began with a pair of earthquakes near Turangi on the 5th. Activity drifted northeast underneath the lake toward Taupo township and ceased on 7th. All of the events were at depths of 3 to 5 km.

Activity continued drifting toward the Bay of Plenty coast with a further two very shallow events north-west of Taupo on the 10th and 16th.

A cluster of four off-shore quakes struck near Wanganui between the 9th and 24th of June. Depths of the quakes, with magnitudes between 3.0 and 4.1, were between 30 and 50 km.

Further south, a burst of earthquake activity occurred near Seddon between the 20th and 29th. Four quakes, with magnitudes between 3.3 and 3.6, were located between 10 and 40 km eastward of Seddon at depths of 3 to 50 km.

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.

Three Alert Bulletins were issued by GNS Science during June 2008, two for White Island volcano and one for Mt Ruapehu.

An alert was issued for White Island after the magnitude 5.4 earthquake on the morning of the 13th. While no immediate response had been noted from the volcano, a ban on visits to the off-shore island, which is popular with tourists, was recommended until scientists could carry out a monitoring flight.

Sufficient information on post-quake behaviour had been collected to issue another bulletin for White Island on June 17th. Whilst bursts of volcanic tremor and earthquakes had been noted, these are common at White Island and were not thought to be out of the ordinary. Volcanic gas levels were typical for the volcano so it was concluded that the volcano would return to a normal quiescent state.

However, the active crater at White Island now contains a lake and the vent that erupted in 2000 is now submerged. Whilst no historical connection between moderate-sized tectonic earthquakes and eruptions has been noticed in the past, it was pointed out that White Island is an active volcano and there is always a hazard associated with it.

Elevated gas output and high crater lake temperatures at the start of the ski season prompted the issuing of an alert bulletin for Mt. Ruapehu on June 17th. Since the volcano’s last eruption on the 25th of September 2007, sulphur dioxide gas levels had gradually increased, with both sulphur dioxide and carbon dioxide gases showing significant increases since April. Measurements taken on June 12th showed both gases were at levels ten times the usual background.

The volcano’s crater lake continued to hover at temperatures between 34 ºC and 37 ºC, sitting at 35.2 ºC on the 12th of June. It was thought that the source of the heat was magma (molten rock) within the volcano conduit. The volcano remained in a state of unrest and it was pointed out that further eruptive activity could occur without warning.

At the end of June 2008, 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 1).
Mt Ruapehu (Alert Level 1).

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

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