Geological Summary for New Zealand Area, August 2006

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

Geonet, the USGS (NEIC) and GNS Science reported on 33 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during August 2006. The magnitude distributions were as follows:
M6 to 6.9 (2), M5 to 5.9 (6), M4 to 4.9 (6) M3 to 3.9 (19).
An additional 2 events in the magnitude 2 range were deemed worthy of mention.

Earthquake activity in the Kermadec Islands continued at a rate similar to July, but magnitudes increased, and activity was concentrated within 250 km of L’Esperance Rock, south-west of Raoul Island. Six earthquakes with magnitudes between 4.8 and 6.5 were reported in the area at depths between 10 and 520 km.

The deep earthquake activity that has been occurring along a line north-east of Rotorua to the southern Kermadecs virtually ceased during the month. Deep activity under the central North Island continued at a lower rate with two events near Mangakino. The first was a magnitude 3.8 event at 184 km depth on the 5th, the second was a magnitude 5.2 event at a depth of 170 km on the 22nd.

Three shallow events in the Bay of Plenty area near Kawerau and Matata were felt on the 2nd, 11th and 16th of August. Magnitudes ranged between 2.5 and 3.0, at depths of 4-5 km.

A pair of shallow 3rd magnitude earthquakes which struck 10 km north-east of Ohakune on the 26th and 29th appear to be part of a swarm of tectonic quakes which were detected on the southern flank of Mt Ruapehu. The swarm commenced activity overnight on the 23rd/24th August and continued through to the month’s end. The quakes occurred in an area between Ohakune and the Turoa skifield, at depths between 10 and 15 km, the largest member being magnitude 3.1 (see volcanic notes).

A swarm of 5 earthquakes within 20 km of Hastings commenced activity on August 4th. Magnitudes ranged between 3.5 and 3.9 at depths of 40 to 50 km. A magnitude 3.7, 50 km deep quake struck further south near Porangahau on the 28th, 4 days after the Hastings swarm appears to have ended.

A magnitude 5.9 earthquake 30 km east of Murchison at a depth of 100 km on August 13th threw items from shelves at various locations and was widely felt in the upper South Island and lower North Island. A magnitude 3.5 quake struck nearby at the much shallower depth of 5 km on the 21st.

Canterbury’s unusual run of shallow earthquakes continued with a mainshock – aftershock pair early on the 19th. The magnitude 3.7 main shock struck 20 km east of Akaroa at a depth of 5 km, throwing items from shelves in Akaroa. The magnitude 3.1 aftershock, also 5 km deep was felt in Akaroa 17 minutes later at 4:09 a.m.

Vulcanologists reported the nation’s volcanoes to be quiet during August. Their status can be summarised as follows:
Raoul Island (Alert Level 1).
White Island (Alert Level 1).
Mt Ngauruhoe (Alert Level 1).
Mt Ruapehu (Alert Level 1).

Activity at Raoul Island continued to ease during August 2006. The level of Green Lake fell slowly during the month and steam vent activity was low, but the upwelling of gas and hot water in the lake continues.

White Island exhibited low levels of seismic activity during August, with the crater lake temperature at 49 °C during the second week of the month, and the lake’s level 2.5 metres below overflow. Rain during the following week caused the lake level to rise and the barrier preventing an overflow to erode further. By the 24th, the lake temperature had increased to 63 °C, with slightly elevated gas levels being detected during the last week of the month.

Mt Ngauruhoe experienced low frequency earthquakes at a rate of 15-30 per day at the beginning of August, and this increased slightly to 20-30 per day by mid-month before dropping back to about 10-20 per day by month’s end. No other changes were observed.

Mt Ruapehu’s crater lake continued a cooling trend during August, starting the month at about 19 °C and ending the month at 16 °C, the coldest temperature since late 2004. However, low levels of volcanic tremor were detected throughout the month, and a swarm of tectonic earthquakes on the southern flank of the volcano between Ohakune and Turoa skifield commenced activity on the 23rd of August. The swarm remained active to the end of the month, with the largest quakes having magnitudes of about 3. Vulcanologists advise that such events are not unusual for Ruapehu.

A spectacular collapse of snow and ice dropped small icebergs into the crater lake on Wednesday 30th August, when about 25,000 cubic metres of material fell from Paretetaitonga Peak. The lake level is currently about 1.5 metres above the level of the tephra dam raising questions about the likelihood of a lahar during the spring snowmelt.

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

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