Scenarios

It is not possible to predict when or where volcanic activity may take place again in the Auckland Volcanic Field. Only when the monitoring system shows ground movement taking place can geologists begin to get precise about a possible event.

All of the scenarios in the report, Volcanic Impact Assessment For the Auckland Volcanic Field, Auckland Regional Council, April 1997, have been chosen to give opportunities to carry out risk analysis under different situations. They are not necessarily locations where eruptions will take place.

The eruption centered on the Container Port/Railway Yards gives good analysis of the possible effects on the harbour, the port, the Central Business District (CBD) and on the hinterland. We will take a close look at this scenario which the Council has termed Scenario Three.

These potential effects of volcanic activity were one of the hazards studied as part of the Auckland Engineering Lifelines Project, the Stage 1 report of which was published in July 1997.

Knowledge of what a volcanic eruption may entail, helps organisations responsible for critical services to understand the consequences such an eruption may have on their operations. Once this has been done, they can begin to plan and prepare for the hazard, and try to mitigate against its effects.

What is a scenario

Working Out the Impact that a Volcanic Eruption might have on the Auckland region, for example, is known as carrying out a RISK ANALYSIS study. This is done by testing what we term 'scenarios'.

Imagine you have taken out your parent's new BMW Sports Coupe without permission and you have dinged it. All the way home you imagine different ways in which you are going to tell them what happened. For one approach you calculate their likely reaction - a massive blast! Grounding for 3 months! Cleaning the house for 6 months! The panel beater's bill! Maybe all of these. If you handle the situation a different way you might get away with a light sentence. Just the panel beater's bill and the cost of a rental car!

You have been working through different scenarios to calculate how this event is going to impact on you. You have been doing a 'risk analysis' on the problem.

When the Council carries out studies on the likely impacts of volcanic events it too works through several scenarios to assess the costs, both economic and social.

For example:

• What industries might be affected?
• How many people will have no work while clean up or rebuilding takes place?
• How will the elderly or young cope?

In 1997 the Council made a Volcanic Impact Assessment of the Auckland Volcanic Field with the Geological and Nuclear Sciences institute (GNS). Five scenarios were put forward that allowed for the consideration of a range of possible volcanic events. While the locations of each event are theoretical, they do describe a set of logical events that could be expected to take place. 

Scenario 1 - An offshore, explosive eruption, involving seawater flushing to streams, centred in the Rangitoto channel, producing a smaller version of Rangitoto volcano

Scenario 2 - An explosive eruption centred in the Tamaki Estuary, affecting residential and industrial areas.

Scenario 3 - A waterfront eruption centred in the rail yards, affecting central business district, port, and residential areas.

Scenario 4 - A fire-fountaining eruption from a vent at the top (south) end of Queen Street, Auckland City, affecting the central business district (CBD).

Scenario 5 - A fire-fountaining eruption from a vent at the intersection of Mt Albert/Mt Eden roads, affecting residential and commercial areas.

Scenario 3

Vent - Location is Auckland Railway Yards, downtown Auckland City Basalt Magma

Type - Basalt Eruption Volume

Tephra - 10 million cubic metres (equivalent to 6000 Olympic swimming pools)

Lava - 100 million cubic metres (equivilent to 60000 Olympic swimming pools)

Day 1	An Earthquake is felt weakly occurring in the Auckland Area.
Day 6	Throughout the next few days more tremors are felt, but they are thought to be aftershocks.
Day 9	Portable seismic recorders are located on the North Shore and a Level 1 alert is declared. Not thought there will be an eruption.
Day 10	More earthquakes take place, lightly felt in Auckland. Epicentre believed to be under Auckland City-Devonport. Depth around 30-40km. More portable seismographs are put out in the field.
Day 19	25 earthquakes are recorded. Getting closer to the surface, 10-20 km depth. Level 2 Alert is declared. There is a threat of volcanic eruption.
Day 20	More earthquakes, shallower epicenters, possible 15km deep.
Day 21	Earthquakes continue but seem to be focussed in a 3km diameter centered between Devonport and the Auckland Central Business District (CBD).
Day 22	150 earthquakes felt by people in Devonport and the CBD. Some Minor damage occurs to buildings, plaster cracks, items falling off Shelves. Level 2 Alert is maintained.
Day 23	Earthquakes continue. Tilt-level measuring sites at the Container Wharf indicate that some uplifting of the land may be taking place.
Day 24	Not so many earthquakes. Longer periods between tremors. No change to the land around the Container Wharf.
Day 25	A dramatic increase in earthquakes. Over 200 are felt as minor shaking in buildings in the CBD and Devonport. Volcanic tremors are now almost continuous. Sulphurous gas is smelt at Mechanics Bay. Level 3 Alert is declared. There is now a real danger of a hazardous volcanic eruption taking place. Earthquakes continue during the night.
Day 26 - Day of the eruption
4.00am	Seismic activity peaks; continuous volcanic tremors; shaking damage to buildings in CBD and Wharf. Ground cracking in the Railway-Container Wharf area.
6.00am	Ground shaking gets worse; severe damage to buildings; ground cracks widen; buildings and structures near them collapse. Steam appears from vents near cracks; volcanic gas emitted; strong smell of sulphur dioxide. SW wind takes gas plume towards Devonport
8.00am	Small eruptions throw volcanic material into the air. Violent eruptions take place along a 300 metre long fissure. Large earthquakes and large jetting eruptions. Some large windows are broken up to 2 km away. The column of erupted material is now 3 kilometre high. Level 4 Alert is declared. A hazardous local eruption is taking place. A large scale eruption is now possible.
8.40am	More frequent and larger explosions take place. Eruption column rises to 8km. Eruption fissure extends into the sea. Continues to expand. Surges of rock material and basaltic magma spread across low lying areas. Rocks 2m in size fall on buildings and star fires. Frequent lightning pulses are seen in the volcanic column.
9.00am	Explosions and eruptions intensify. Base surges flow into Parnell and CBD and into the Harbour. Surges destroy the Mechanics Bay area. The area to the 60 metre contour is filled with magma and flowing material. Strongly constructed buildings remain standing but are damaged by sand blasting and are partly buried. There are heavy tephra falls in Devonport. Large waves hit the Beaches on the North Shore. A Level 5 Alert is declared, a large Hazardous eruption is in progress.
1.00pm	Scoria and ash has fallen as far away as Newmarket, Remuera and Ellerslie and Mt Wellington. Northcote and Takapuna are also Covered. The eruption cloud has risen to 10 km. Eruptions continue during the night. Flat area in Parnell and Albert Park is completely buried in tuff. During the next 3 months the eruptions continue. Roads are blocked. More buildings collapse from shaking and tephra falls. Fire fountains occur. Scoria and ash is carried by the changing winds. Lava flows out to the Naval Base. The channel becomes blocked. Lava continues to flow forming a similar volcanic structure to Rangitoto. The previous Waitemata Harbour is now a lake dammed Behind the lava wall. A plan is made to cut a new channel through the Takapuna area.