Why is there a concentration of earthquakes between the Taranaki and Ruapehu volcanoes? And why is a mountain range sinking into the South Taranaki Bight while coastal cliffs around Hawera are rising?

Victoria University geophysics professor Tim Stern is coming to speak in Whanganui on those and other mysteries of the earth's crust. It will be the first Whanganui Science Forum talk of the year, and starts at 7.30pm on February 28.

The venue is a new one - the St John ambulance rooms at 25 Tawa St - and the cost is $4 for members and $5 for nonmembers.

Professor Stern wants to talk about what happens under the earth in this part of New Zealand. But he will start with an overview of the world and this country, and an explanation of plate tectonics.

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"I really think New Zealanders need to understand a little bit about plate tectonics, because then they would have more idea about what causes earthquakes," he said.

There are some geophysical oddities in this part of the earth's crust.

In most of New Zealand earth movement centres around the alpine fault, which runs in a northeast/southwest direction. The country's only east-west concentration of earthquakes happens between Mount Ruapehu and Mount Taranaki.

Last year there was a 5.1 magnitude quake near Opunake, on that line. A study has shown the origin of the movement is 52km below the surface - unusually deep for that type of activity.

"There's a very interesting process going on underneath that land. In simple terms part of the earth's upper mantle is actually starting to peel off. It's delaminating, like a layer of plywood."

Meanwhile the coastal terraces between Whanganui and Hawera are rising "quite fast" and a mountain range that was as high as the Tararuas five million years ago is sinking at its southern end near Marlborough.

Professor Stern hopes to unravel these and other mysteries, through his talk and the graphics that go with it.