Mar 18 2011 - February 2011 GeoNet roundup

The newly-commissioned National Network seismograph station at Inchbonnie. The vault containing two seismometers can be seen in the foreground. Data digitisers and communications equipment are in the shed in the background, and data telemetry back to our Data Centre is via satellite.

New website facilities

• A dedicated section for information about the Canterbury earthquakes was added to the website. This includes:

1. Timeline visualisations.
2. Extra seismic drums in the region.
3. A Canterbury aftershock forecast map for the next 24 hours.

• A link allowing 'felt reports' and their details to be viewed in Google Maps was added. The user can go on to save the KML file or instead view the details in the Google Earth application.

Additions and upgrades to the geophysical monitoring networks

• In response to the February 22 Christchurch earthquake we installed 4 temporary strong-motion accelerometers on the Port Hills and six temporary short-period seismographs to improve the network geometry in the region.
• A new National Seismic Network was completed at Inchbonnie (INZ) on the West Coast.
• Builds were completed at New continuous GPS sites were made operational at Raukawa (RAKW) in Hawke's Bay, Manuhara Road (MNHR), near Pongaroa and at Parikino (PKNO), north-east of Whanganui.
• The Wellington Regional Seismic Network seismograph at D'Urville Island (DUWZ) was upgraded with the addition of a strong-motion accelerometer and the replacement of the seismometer with a broadband model.

Earthquakes summary

• The most widely-felt earthquake of the month occurred on Tuesday, February 22, magnitude 6.3, 10 km south-east of Christchurch at a depth of 5 km. We have received 4,049 felt earthquake reports to date. Please read our news story about this earthquake for more information.
• The preliminary locations of January's earthquakes located by our analyst team are shown below.

Volcano surveillance update and field trips

Volcanic activity throughout New Zealand was low during February.

The heating cycle has continued at Ruapehu with the Crater Lake temperature reaching to 40 °C, the third highest temperature recorded since the re-establishment of the lake. The highest recorded temperature since 2002 is 42.5 °C recorded in May 2003. Associated with the current heating cycle there have been small changes in lake chemistry and minor seismic activity. These observations are consistent with the volcano-hydrothermal system now being more active and heating the Crater Lake water. There have been 8 heating cycles since 2002 that have each lasted between 9 to 20 months.

White Island also remains quiescent with minor declines in fumarole temperatures and a continued drop of water level in the Crater Lake.

Routine gas flights were completed at White Island and Ruapehu, along with ground visits to both.

Thursday February 17 - Gas flight over White Island

Conditions were good with a steady wind speed. SO₂ flux increased compared to the previous flight, whilst the CO₂ emission was lower than the previous survey. These variations are typical at White Island.

An upwelling was clearly seen in the northern part of the crater lake.

Friday February 18 - Gas flight over Ruapehu

It was cloudy at low altitude. All gases were detected by the instruments and levels are typical for Ruapehu. The crater lake was green in colour with no upwelling visible.

Tuesday February 22 - Spring and fumarole sampling, levelling and CO₂ soil gas survey at White Island

The water level of the Crater Lake continues to recede, being about 1.5 to 2 m lower than in January. The colour remains a khaki brown/green with abundant gas passing through it. Vigorous bubbling is seen in many places.

The fumaroles appear to have decreased in activity. The Main Crater floor appears to have dried out and areas of seep near the 1978/90 Crater Complex have dried up, consistent with the lower lake levels. The two streams continue to flow but are weaker.

The levelling survey was completed and continues to show minor subsidence of the Main Crater floor. The current subsidence follows a long period of uplift from 2003 to 2010.

The average soil gas flux measured was slightly higher than the last survey but is still within normal ranges for carbon dioxide output.

Wednesday February 23 - Ruapehu Crater Lake visit

Temperature at the lake outlet was 38.6 °C. Gas and water samples were taken. The lake was its normal uniform turquoise colour with no obvious upwellings, although the water suface was rough due to wind.

Reported landslides from media sources

Tuesday February 22

The magnitude 6.3 earthquake centred near Lyttelton produced very high ground accelerations that precipitated large numbers of cliff collapses and rock falls in the Port Hills. Very large numbers of boulders were dislodged from upper slopes, rolling down into valleys, most notably the Heathcote and Horotane Valleys, and Rapaki, where many houses were damaged by rolling rocks.

Three fatalities resulted from cliff collapses in the Sumner area, and two resulted from rock falls on the Bridle Path walkway between Heathcote Valley and Lyttelton. Significantly, most of the cliff collapses were in areas modified by past quarrying. The size of these collapses far exceeded those from the failures of the natural sea cliffs between Mount Pleasant and Godley Head.

Rockfalls and single boulders blocked the Sumner Road between Evans Pass and Lyttelton, trapping a fuel tanker, and also the Summit Road between The Tors and Mt Cavendish.

New rockfalls from Castle Rock occurred, but this time the debris runout was to the north and west rather than east as it was in September (when very large rocks landed very close to the Lyttelton Tunnel portal). However, large numbers of smaller boulders rolled eastwards onto the road leading to the Lyttelton road tunnel.

Significant rockfalls, landslides and boulder rolls occurred as far west as the Governors Bay area and as far east as Godley Head, but there was very little landsliding on the other side of Lyttelton Harbour.

There are concerns that further rockfalls could result from aftershocks or rainfall, and a number of ground fissures in Mount Pleasant are being monitored for further movement. Cliff-edge cracks at Scarbrough are also being monitored.

Felt earthquakes of
January 2011
(162 located).

Shallow earthquakes of
January 2011
(1,544 located).

Deep earthquakes of
January 2011
(414 located).