| This video clip animation (which is silent by the way) made by GeoNet NZ is a helpful illustration of the way the New Zealand land mass has been formed between the interaction or collision between the Pacific Plate and the Indian Australian Plate. It's all interesting but what is especially interesting for the South Island is that the Southern Alps were formed by the two plates rubbing together and forcing the edges to buckle and bend and form the huge upthrusts that are now the Southern Alps. |
 This graphic shows the situation today. What is of particular interest is the fact that the Pacific Plate is "subducting" under the Indian Australian Plate to the north of New Zealand. ie it is diving under it. The arrow heads show where that is happening. But along the Southern Alps the edge of the Pacific Plate is apparently sliding along the edge of the Indian Australian Plate.
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 This difference in interaction is further illustrated in this graphic. The transverse movement/fault known as a "strike slip region" it appears. Even though the Plates are sliding along each other in the South Island, along the Southern Alpine fault, it's not as if this movement is well oiled. Any movement is in huge and violent jerks. Enormous pressures will still be exerted in an East-West direction (because the plates still press together), the edges will bind, causing a shearing stress on the Pacific Plate that lies under the Canterbury Plains. As a result a number of smaller faults have developed from the top of the South Island, parallel to the Southern Alps fault, that run down into the Canterbury Plains.
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 Some of these smaller fault lines are shown in this recent graphic prepared by NIWA which is now mapping new faultlines. You can just make out other red fault lines under the earthquake dots on the land around in Christchurch. It is movement in these smaller faults that give rise to smaller earthquakes (compared to those released in and by the Southern Alp fault.). But these faultlines are close to the surface and close to a city. So they can be much more destructive than a big distant earthquake.
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 This is an East/West cross section of Banks Peninsula (looking North). It shows the volcanic rock that formed Banks Peninsula after volcanic activity a million or more years ago. These volcanoes are extinct. The underlying rock is known to be faulted. (You can see the faultlines in the graphic.) This base rock was once flat but has been pushed up out of shape, and it has been squeezed by East-West tectonic plate pressure, and must have been cracked allowing volcanic magma to push through. One scenario is that this underlying rock is folding up slowly now. It may be a weak point in the plate. New faults may be forming. But this is speculation.
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Intensive monitoring is the duty of authorities now. |
