The first slide showed rocks under the seabed to the south of Whanganui which clearly showed a mountain range 2000m high. This range sank and was buried under by sedimentary rocks about 4 million years ago.
This area may again be thrust upwards and we could add ski resort to our long list of local attractions. Tim dubbed the peak "Mount Whanganui".
Fresh memories of the Kaikoura earthquake in November, 2016 may have been a factor in such a great turnout and the question everybody wanted to ask was: "Can we predict earthquakes?"
Professor Stern's answer was an unequivocal "No". We only have about 120 years of reliable data -- by contrast, weather forecasters have many recorded climate cycles to work from.
A graph showing major earthquake activity over the past 120 years showed a possible cycle on a period of 50 years. However, the fit is very rough and we only have two cycles which is not statistically significant.
More important to most people is "Where?" and "How big?" and on these aspects scientists are nowhere near an answer.
Since 1843, the cost to the New Zealand economy from earthquakes far exceeds any other disasters. Thus we need to work on disaster relief planning and building regulations. On a positive note, Professor Stern felt it unlikely that Whanganui would be hit by huge earthquakes. Mainly as a result of much improved instrumentation, great strides have been made in the physics of earthquakes in the past 10 years and this is Professor Stern's area of interest.
Ice is less dense than water and so it floats on the surface. As rock cools and solidifies it becomes about 1 per cent more dense than liquid rock and tends to sink.
The mantle is the layer of rocks from a depth of about 30km to about 1000km -- the upper 100km of the mantle, being cooler, is largely solid and tends to sink into the lower liquid regions.
In the lower regions of the mantle, heat from the Earth's core causes slow but powerful convection currents.
Convection currents in a heated pan of water circulate on a time scale of seconds. In the mantle, the time scale is millions of years.
These convection currents drag continents across the surface of the Earth. New crust is dragged up from within the Earth in places such as the mid-Atlantic.
Where sections of the moving crust move against each other, the heavier solid top layers of the mantle tend to sink and dive under the opposing section of crust in what is called a subduction zone.
The energy of this "subduction" gives the heat of volcanic activity and the release of mechanical energy of earthquakes.
Most earthquakes in New Zealand occur along the line running roughly northeast through Fiordland, the Southern Alps, up through the Nelson area and then the east of the North Island.
On this line the Australian and the Pacific Plates meet.
However, the line of earthquakes between Ruapehu and Taranaki, which runs northwesterly, and the Whanganui region itself which are exceptions to this rule. Taranaki is way off the line of volcanoes running down from Fiji through Taupo and to the south. Professor Stern has put a lot of work into studying these areas.
Whanganui sits on soft, lightweight rocks which require less energy than heavier harder rocks to get shaking. A quake may occur some distance away from Whanganui but we feel it more because the lighter local rocks vibrate more readily. There is no evidence that Whanganui has yet had a large earthquake in its own area.
What Whanganui has had are swarms of smaller deep earthquakes -- and Professor Stern regards this as a good thing.
Most quakes are at a depth of 15km to 20km but the swarms are much deeper -- at about 50km -- and at this depth, water which has been carried down with the subducting plate is driven off and returns toward the surface and in the process sets off numerous small earthquakes.
This is much better than one big one.
Because of the softness and lightness of the rocks under Whanganui these quakes feel larger but, fortunately -- as mentioned earlier -- it means we are unlikely to get the damage-causing magnitude 7 quakes.
I have only covered about half of the talk and will finish with one point -- among many -- that amazed me.
We know the Whanganui River flows south to the sea -- but this was not always so. Until about 1 million years ago, the Whanganui flowed northwest into the sea somewhere north of New Plymouth but with the lifting and tilting of the land its direction has changed to what we now see.
In the next week or so I intend to put up a video on YouTube of the full talk.
�Frank Gibson is a semi-retired teacher of mathematics and physics who has lived in the Whanganui region since 1989.
