Geological Summary for New Zealand Area, August 2007

Earthquake activity continued at a low level, with swarms reported near Rotorua and Matata. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.

Geonet, and the US Geological Survey (NEIC) reported 39 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during August 2007. The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (11), M4 to 4.9 (10) M3 to 3.9 (15).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.

A swarm of earthquakes between Rotorua and Atiamuri caused consternation early in August. The very shallow earthquakes were distinctly felt by people in the area when activity began just after 9 p.m. on Monday August 6th. By 9:45 p.m. eleven tremors had been felt, the largest of which was a magnitude 3.4 event at 9:32. Activity paused before another two tremors were felt at 11 o’clock and 11:46 p.m.

Two further quakes in the sequence were felt – a magnitude 3.4 at 6:51 the following morning, and a magnitude 3.1 event at 4 a.m. on the 8th. Geonet published details for six of the quakes, showing that the magnitudes ranged between 2.8 and 3.4 at depths of 2 or 3 km.

A further six quakes in the Matata swarm were reported during August 2007, with activity overlapping that occurring near Atiamuri and then being scattered throughout the month. The quakes ranged in magnitude between 2.6 and 3.5 at depths around 5 km. All were located within 10 km of the town of Matata.

There were four earthquakes within 155 km of Raoul Island during August. The quakes’ magnitudes were between 5.0 and 5.4 at depths of 55 km or less. Another 5.4 magnitude quake struck well to the north of Raoul on the 27th at a depth just under 500 km.

Six earthquakes were reported near L’Esperance Rock in the Kermadecs during the month, ranging in magnitude between 4.7 and 5.4. Four of the quakes were to the south of the rock, but not close enough to the Bay of Plenty to be felt.

A pair of shallow quakes struck near Hanmer Springs on the afternoon and evening of August 24th. The magnitude 4.1 and 3.7 events were located 10 km south and 10 km north of the township.

The largest quake to strike near the mainland part of New Zealand during August 2007 was a magnitude 5.1 event on the 25th. This earthquake was located in the Tasman Sea, 40 km west of Foxton at a depth of 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. Discussions with GNS Science indicate that regular reporting may resume soon.

At the end of August 2007, 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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