When Tim Stern last did a talk for Whanganui Science Forum, the place was so full, people were arriving and not able to get in.

On this occasion we had a capacity crowd and a great publicity stunt with a 6.2 quake earlier in the day.

Because the Science Forum is at the cutting edge, we got preliminary findings on this quake a matter of hours after it happened.

The main content of the talk concerned possible effects of tectonic events on Whanganui which Tim classified as earthquakes, volcanoes and sea level change. Recent findings in each of these areas have been made possible by the astounding accuracy of modern GPS. A device similar to the thing in your car can measure the distance of the surface of the sea or land from the centre of the Earth within a millimetre allowing data to be gathered that would have been impossible even 20 years ago.


The financial cost of these problems is put in perspective by totalling the costs of earthquakes, tsunami, eruptions and floods since 1843. Adjusted for modern monetary values, earthquakes total well over $60 billion wheile the others, in total, barely top $10 billion.

Our small population and relatively small gross domestic product means this earthquake cost puts us second only to Bangladesh, with its regular huge floods, in terms of annual expected losses as a percentage of GDP. We sit on about .66 per cent with Bangladesh at .83 per cent.

This explains our increasing insurance costs especially in areas perceived as high risk such as Wellington.

The first question that people ask about earthquakes is "Can we predict them?". The answer from the seismologists of the world is a resounding NO! We can make forecasts which are like weather forecasts in that they give the likelihood of an event, but we cannot tell people to be ready at a certain place and time. The problem is that geological processes have a very long-time scale and we only have reliable data back to about 1900. If hints of a cycle seen over the past 118 years repeat over the next 1000 years we may have enough data.

If we do not have enough data to see patterns, we must understand the physical processes involved.

The basic cause of earthquakes is simple. When the Earth was formed it was molten. Over many years the surface cooled, solidified and shrank slightly. This shrinkage makes the average density of the top 130km (the lithosphere) about 1 per cent more dense than the underlying mantle (asthenosphere). If a crack appears in the lithosphere, one edge will tend to sink into the less dense and more fluid mantle beneath. This slow sinking of one plate of the lithosphere beneath another causes earthquakes. Convection currents in the mantle keep the process going by pushing up new crust (lithosphere) at the mid oceanic ridges. Within this, essentially simple, process the complications are immense and probably impossible to predict.

The processes causing the shallow (20 - 30km deep) earthquakes under Wellington are pretty well understood. At a depth of 200km the pressure is significantly greater, and the mechanisms associated with these deep quakes are not well understood. The Taumarunui quake was of this deep type.

We tend to associate earthquakes with fault lines but the correlation between fault lines and major earthquakes is poor. Of the last 19 quakes over magnitude 6 only three happened on known fault lines. This may suggest that fault lines are an effect and not a cause of major quakes.

On a clear day from the high point of my land I can see the tops of both Ruapehu and Taranaki. Measurements using GPS tell us that if I wait about 3 million years I will meet their children more closely. What we call a volcano is, in geological terms, a temporary structure. It is the current indicator of where great heat is being generated by tectonic plates grinding together 20 or so kilometers below our feet. The hotter, less dense material gradually floats upwards forming the mountain while at the same time water and wind are eroding the mountain. You can see the bones of the ancestors of Taranaki in the Sugar Loaves off New Plymouth. With the movement of the plates the heat source moves. Taranaki is moving towards Whanganui at 25mm per year and Ruapehu is coming this way at 33mm per year. A bit of maths tells you their children will arrive about 3 million years from now. We will put on a big welcome.

Whanganui is rising at about 1mm per year giving a net sea level rise of about 2.3mm per year. It is this rapid rise that gives us some of our local geological features.


Of more immediate concern are sea levels. Accurate GPS measurements show that, worldwide, mean sea level is rising at about 3.3 mm per year due to atmospheric warming. These same GPS measurements of land surface show that New Plymouth is sinking at a rate of about 1mm per year. This means that New Plymouth has an effective sea level rise of about 4.3 mm per year. This may not sound much but it means things like faster cliff erosion.

Whanganui is rising at about 1mm per year giving a net sea level rise of about 2.3mm per year. It is this rapid rise that gives us some of our local geological features.

The upper reaches of the Whanganui river often have steep gorges instead of banks. This is because the rise of the land is faster than the rate of erosion that would give shallower banks. It is this rise over the past million years that also built the terraces along the local coastline. The land is rising more slowly than the sea. This is one cause of the likely increase in frequency of flooding along the river.

The sensitivity of the GPS measurements has led to the discovery of "slow earthquakes" which occur over a period of days or even months.

These can be equivalent to magnitude 7 or 8 quakes but because they happen so slowly the rate of energy release is so slow as to be almost undetectable. Several have been detected along area of sea to the west of Kapiti. There is hope that this slow release could act as a safety valve for the "big one" expected one day for the Wellington region.

The Whanganui Science Forum talks give much more information than can be crammed into this column.

You can see videos of most of the talks at totallyfrank.org

Frank Gibson is a semi-retired teacher of mathematics and physics who has lived in the Whanganui region since 1989