Geological Summary for New Zealand Area, September 2006

Earthquake activity continued at a low level during September 2006. White Island, Mt. Ngauruhoe, and Mt. Ruapehu remained at Alert Level 1, and Raoul Island was downgraded to Alert Level 0.

Geonet, the USGS (NEIC) and IGNS reported 32 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during September 2006. The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (6), M4 to 4.9 (12) M3 to 3.9 (13).
An additional event in the magnitude 2 range was deemed worthy of mention.

Earthquake activity in the Kermadec Islands continued at a rate similar to July and August, with magnitudes slightly lower. A pair of 5th magnitude earthquakes struck 130 km south of L’Esperance Rock on September 1st, with another pair with magnitudes of 4.5 and 5.2 striking near Raoul Island at the end of the month. In all, 8 earthquakes with magnitudes between 4.0 and 5.2 struck at depths between 30 km and 345 km near the Kermadecs during September.

The deep earthquake activity which has been occurring along a line from south of Rotorua to the southern Kermadecs eased still further. During August, this activity was notably reduced on previous months, and only two events of note occurred during September. They were a magnitude 3.7 event on the 20th, and a magnitude 4.7 quake the following day.

A pair of deep earthquakes struck near Wanganui on the 20th and 24th. The magnitude 4.3 and 4.0 quakes were at depths of 80 and 60 km.

Five earthquakes occurred near Wellington and the Nelson district between September 6th and 23rd. The quakes ranged between magnitude 3.4 and 5.2 at depths of 30 to 190 km. The 180 km deep magnitude 5.2 event of September 11th was felt from Wanganui to Christchurch, while the magnitude 5.0 quake of the 15th at the shallower depth of 47 km was felt throughout the southern North Island and in Marlborough causing some items to fall from shelves in the Wellington region and in Picton.

The magnitude 3.9 quake 40 km south of Seddon on the morning of the 15th was preceded by two smaller earthquakes which woke nearby residents during the early hours of the morning.

Three of the unusual swarm of earthquakes near Kerikeri in Northland were reported on the Geonet site as being felt. An examination of the earthquake database showed that there were 6 earthquakes in the swarm which ran from September 17th to 20th, with magnitudes between 2.4 and 3.5

Vulcanologists reported the nation’s volcanoes to be quiet during September. Their status at month’s end 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).

Volcanic activity at Raoul Island continued its decline during the early part of September, and its alert level was downgraded to 0 on the 18th, signifying that it had returned to its usual dormant or quiescent state.

Measurements at White Island at the end of August showed slightly elevated gas measurements but seismic activity was at a low level. By the end of September, the seismic activity was described as at a very low level with the crater lake having cooled 7° to 57°.

The small volcanic earthquakes near Mt. Ngauruhoe continued at the rate of 20 small events per day throughout September.

The collapse of ice cliffs into the crater lake at Mt Ruapehu at the end of August assisted the cooling of the lake which dropped to 13°C – its coolest since the eruption in 1996. The swarm of small tectonic earthquakes continued about 10-15 km beneath the southern flank of the mountain, but began to decline by September 8th. By the middle of the month, the earthquake activity has ceased, but the crater lake warmed slightly to 16°C. The volcano remained quiet for the rest of September apart from a brief burst of weak tremor on September 27th and 28th.

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

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