• The most significant earthquake of the month occurred on September 1, magnitude 5.8, 10 km south-east of Turangi, 80 km deep. It was felt from the Bay of Plenty to Dunedin, with a maximum intensity of MM 5 in the central North Island.
• A new continuous GPS site near Hunterville, Ohingaiti (OHIN) was made fully operational.

• The most significant earthquake of the month occurred on August 25, magnitude 5.9, 10 km south-west of Hastings, 30 km deep. It was felt throughout the North Island (south of Auckland) and the top of the South Island, with damaging intensities of MM 7 and 8 in Napier and Hastings.
• The GNS Science terrestrial laser scanner was again taken down to the Shotover River landslide (upstream of Arthur’s Point) to carry out a second scan of the landslide.
• A new seismic site near Wimbledon, Hawke's Bay, Angora Road (ANWZ) was made operational.
• A new strong-motion recorder was installed at Eastbourne, near Wellington, Point Howard (PHHS).

Mainshock details

Monday, August 25 at 11:25 pm, magnitude 5.9

Major aftershocks to date

Only a weak aftershock sequence is expected for this type of earthquake, not lasting for too many days and with few large aftershocks.

Monday, August 25 at 11:35 pm, magnitude 3.5
Wednesday, August 27 at 6:23 am, magnitude 3.4
Thursday, August 28 at 10:29 am, magnitude 3.4

No tsunami

Due the earthquake's magnitude of 5.9 and its depth at 30 km, and as this earthquake occurred under the New Zealand landmass, there was no tsunami generated.

Reported damage

Reports of damage sent in through the online felt questionnaire are consistent with an estimated maximum Modified Mercalli Intensity of MM 7 in the Bay cities; one report of MM 8 has been received. In both Napier and Hastings we have reports of damaged chimneys, broken windows and some cracks in walls. Household contents have taken a battering with items falling from shelves and furniture being displaced. An office building in Taradale was not safe to enter until cleared by authorities later in the morning.

The earthquake was widely felt in the North Island from Waikato through to Wellington and weakly in the upper South Island.

Earthquake characteristics

The focal mechanism (see accompanying figure) is an almost pure normal faulting mechanism where one side of the fault slides downwards relative to the other side. The red area of the mechanism has moved downward relative to the white area. There are two possible choices for the fault plane; however, without additional information it is not possible to determine which the actual fault plane is. This type of faulting generally occurs in regions that are being stretched or pulled apart. The depth and the normal faulting mechanism are consistent with the earthquake being located within the Pacific Plate to the east as it pushes beneath the Australian Plate to the west and is pulled apart. A good description of fault plane solution representations (or beachballs) and the faulting type are to be found at the United States Geological Survey.

There has been no observable signal at GPS sites within the vicinity of Hastings, consistent with the moderate magnitude of the earthquake.

Similar past earthquakes

Since 1980, there have been two earthquakes causing similar levels of damage in Hawke's Bay:

• Monday, October 6 1980, magnitude 5.6
  
  
• Sunday, May 13 1990, magnitude 6.4

Last updated August 29, 4:00 pm.

• The generation of the New Zealand earthquake catalogue was restructured to accommodate a "near real-time" catalogue. GeoNet Data Centre Technicians are now processing earthquake locations one week behind real time. This allows for more timely finalised earthquake hypocentres to be generated for scientific use.
• The most significant earthquake of the month occurred on July 12, magnitude 6.0 (revised), 80 km south-west of Te Anau, 100 km deep. It was felt throughout Fiordland, Otago and Southland, with maximum intensities MM 5 in Queenstown and around Invercargill.
• The Utiku landslide monitoring system was installed, just south of Taihape. This consists of 4 stations spread across the landslide, each with a continuous GPS receiver and a ground water level sensor, and one site with a rain gauge.
• The Tauranga tsunami gauge was brought on line with the upgrade of the radio repeater site at nearby Papamoa.
• A new continuous GPS site near Levin, Moutere No.3 (LEVN) was made fully operational.
• A landslide rapid response was undertaken, which involved taking the GNS Science terrestrial laser scanner down to the Shotover River to scan the landslide upstream of Arthur’s Point. Preliminary results indicate that the landslide is still moving. A repeat scan is planned during August.
• Links to web services for access to hypocentres and felt reports were added to the Resources section of the website.

• Improvements to the popular Quake Search application were released. It features a more straightforward interface and a wider variety of output formats, as well as being more robust "under the bonnet".
• The regional seismic site at Highcliff Hill (HHSZ) near Dunedin was upgraded to full National Network specifications, and has been renamed Otago Peninsula (OPZ). The short-period seismometer has been replaced with a broadband seismometer, capable of measuring seismic waves at a wider range of frequencies, and an accelerometer has been added to measure the amplitude of strong shaking should a large earthquake occur.
• A combined seismic and CGPS site was made operational at Black Hill Station (BHHZ) east of Taihape.
• A down-hole strong-motion array (site code EDAS) was installed and made operational at the Fonterra factory near Edgecumbe in the Bay of Plenty. It features three seismometers at varying depths and a pressure sensor to determine the water-table depth.

• Additional volcano monitoring information was made available on the website. A new section called Current Activity shows a network map, recent earthquakes and links to a representative seismograph for each volcanic centre, plus webcam photographs and movie sequences where these are available.
• Two new combined seismic and CGPS sites were made operational at Arahi (near Wairoa) and Kereru (in southern Hawke's Bay).
• Three more strong-motion recorders, part of the CanNet network, were installed in Christchurch city.

A new section called Current Activity has been added. For each volcanic centre, it shows a network map, recent earthquakes and links to a representative seismograph. For those volcanoes with cameras, we are now regularly constructing short movies from the still photographs.

Images

A big, new gallery of stunning photos.

Monitoring Methods

Lots of detailed information here about how we perform volcano monitoring.

• One new seismic station was completed and brought on line at Post Office Road, on the eastern side of the northern Tararua ranges.
• The web camera watching Ngauruhoe was upgraded; the previous camera had given faithful service since April 2002.
• A VSAT satellite connection was installed on Great Barrier Island and is now transmitting data from a hydrophone array. This will also serve a tsunami gauge and a seismic station that are planned to be built.
• Hazards information is now available in mobile and PDA-friendly form on the "mobile" GeoNet website.
• Sea level data recorded by the New Zealand Tsunami Monitoring Network is now available on the website, and a plot showing the recordings of the last 36 hours can be viewed.

On the website you can find:

• Plots of the relative sea levels on the Tsunami Gauges page.
• Raw data from the gauges.

To get there from the main website, a link has been placed on the home page (the phone icon next to the RSS icon at the top right). There is also user agent detection, so if you attempt to go to www.geonet.org.nz from a mobile device you should get redirected to the mobile content.

The release of this website will allow emergency responders to view geohazard maps and images as well as text information even if they are away from their computers without expensive download costs and difficult site navigation.

Photograph looking north towards the dam crest along the outflow channel, March 4 2008.

Photograph looking north of the Young River landslide, dam and lake, March 4 2008.

In November 2007 GeoNet installed radio communications to the lake-level sensor, allowing data from the dam to be accessed from GNS Science in Avalon near Wellington. These data are proving very useful for both GNS assessing changes in lake level due to rainfall and Otago Regional Council for their management of the hazard.

A second laser-scan survey of the landslide (by GNS on November 1 2007) covered a much larger portion of the landslide and was combined with a control survey using satellite global positioning system (GPS) equipment. This allowed the landslide and lake volumes to be more accurately estimated.

Dam height (maximum) above the original river bed:	70 metres
Landslide volume:	11 million cubic metres
Lake volume (at dam crest level):	21 million cubic metres

Lake water continues to flow over the dam crest and has caused some erosion of material from along the outflow channel during periods of high rainfall. A laser-scan survey of the outflow channel was repeated on March 4 2008 to enable assessment of the amount of this erosion.

When we hear about an earthquake, we often want to know who is going to have been affected by it. ShakeNZ on the GeoNet home page tells us the shaking intensities at individual locations within minutes. What if we want a broad view of the zone of impact? This is where the new isoseismal map can help. Isoseismal contours join up points of equal Modified Mercalli intensity, much like an isobaric map shows the high and low pressure areas on a weather map. A model¹ is used to predict the likely level of shaking caused by an earthquake of a certain magnitude, depth, region and fault rupture type.

A model is necessarily a simple view of the more complex reality, and the contours shown will not exactly correspond to the spot intensities reported by instruments and people. Nevertheless the contours give a quick visual appreciation of the anticipated zone of impact.

The model has been derived from the contours of "felt" reports of many past New Zealand earthquakes. The reports you make after feeling an earthquake add to the data that can be used to improve this model in the future. The model also allows planners to set up "what if?" scenarios and be better prepared for actual earthquakes when they happen.

The accompanying images show two recent examples of isoseismal maps: the top one is a "felt only" earthquake, with damage unlikely; the bottom one is potentially serious, but the structural damage zone is out to sea, and the lesser damage zone is largely in wilderness. We have used three coloured zones to help understand the overall seriousness of the earthquake: green for areas likely to have felt the earthquake, yellow for areas where household contents and fittings may be damaged, and red for areas that may experience damage to buildings.

In the past two years there have been three earthquakes that caused shaking in the red zone: the Fiordland earthquake (shown) and one of its aftershocks, and the December 2007 earthquake off the coast of Gisborne. By using colour to highlight the expected seriousness of shaking, we can better understand whether an earthquake is significant or not.

¹Dowrick, D.J.; Rhoades, D.A. 2005 Revised models for attenuation of Modified Mercalli intensity in New Zealand earthquakes. Bulletin of the New Zealand Society for Earthquake Engineering, 38(4): 185-214.

The twelve active volcanoes in New Zealand are monitored by the GeoNet project using a variety of techniques. The most common and useful is volcano-earthquake monitoring, along with direct observation and water and gas chemistry. Ground deformation surveys are also conducted at some of the volcanoes.

A moderate to large eruption through the active Crater Lake at Mt Ruapehu on September 25 generated a blast and ballistic fall field to the north of the lake, across much of the summit area. It also generated lahars into two catchments draining off the summit of the volcano. The lahar to the north down the Whakapapa Valley reached into the skifield, while the floods down the Whangaehu Valley reached to just beyond the National Park Boundary. The Crater Lake started the year warm, ranging around 23 - 28 ºC cooling through the middle part of the year. The eruption occurred from a cool lake, but the lake has been heating since the eruption, reaching 37 ºC in December. Volcanic tremor and volcanic earthquakes are often recorded at Ruapehu, and were recorded through 2007. Following the September eruption the level of volcanic tremor increased and remained high at year's end.

In March, the tephra dam blocking the natural outlet of the Crater Lake failed during a storm and the much anticipated break-out lahar occurred. This created a unique opportunity for scientists to study a volcanic lahar and test contingency plans.

In late May 2006, a period of seismic unrest began at Ngauruhoe with a several-fold increase in the rate of volcanic earthquake activity. On June 6 2006, in response to the volcanic earthquakes, the Scientific Alert Level was raised from 0 to 1, signifying that there had been a departure from typical background activity and that there were consequently signs of volcano unrest. The signs of volcanic unrest have continued this year; however the program of volcanic gas and temperature monitoring has shown no changes to date. There have been no eruptions at Ngauruhoe since 1977. Volcanic earthquakes are considered a common precursor to volcanic eruptions, particularly when there is a significant increase in the number and/or size of events.

Also since 2000 we have also observed significant numbers of shallow volcanic earthquakes at Tongariro volcano without any other changes in monitored parameters and without an eruption.

New Zealand’s other active volcano is White Island, which has not erupted since 2000, when a scoria eruption occurred, marking the end of an eruption episode that ran from December 1975. Following the eruptions in 2000 the active crater has cooled, allowing a crater lake to form and fill the crater. The water level rose in this lake to reach within about 1 m of overflow in February 2006. Later that year the lake temperature rose to over 60 ºC and the lake level started to fall and by March 2007 the water level had receded 25 m. The fall is due to evaporation of the hot lake water. Through the remainder of the year the water level in the lake remained low.

Only two small hydrothermal eruptions occurred in Rotorua during 2007. The first on May 27 at the mouth of the Puarenga Stream, and a smaller one at the former site of Malfroy's geyser in the Government Gardens in late June. These were smaller than events last year.