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Quick DipsPutting on the SqueezeIn plate tectonic diagrams where continents are shown colliding, it looks like a narrow band, but the collision area between the Australian and Pacific plates underneath New Zealand has been found to be as wide as the South Island. Work by Dr Tim Stern and Dr Martha Savage, of Victoria University's School of Earth Sciences, shows that the earth in the central South Island deforms in a different way to most other plate boundaries of the world, indicating that plate tectonics has its limitations in explaining such movement. "Instead of movement between the two great Australian and Pacific plates being focussed along a fault that reaches to great depth, the study shows movement between the two plates is achieved over an increasingly wide zone as one goes down into the earth," says Stern. "Once in the upper mantle of the earth, at depths of 40 to 200 km, the width of the deforming region -- the fault zone' -- is as wide as the South Island itself, at about 200 km wide." Analysis of earthquake waves that have travelled through the crust and mantle beneath New Zealand forms the basis of their research. The work has important consequences for understanding earthquake occurrence at plate boundaries. Stern says great earthquakes, such as the 1960 Chile and 1964 Alaskan earthquakes, are typically produced on "subduction thrusts" where relative movement between two colliding plates is concentrated onto a single fault plane that persists to a depth of tens of kilometers. "This study suggests that because the relative movement between the Australian and Pacific plates is spread over a wide zone, there is no single subduction thrust beneath the central South Island. This provides an explanation for the lack of deep seismic activity in this area. "The rock is too hot and not strong and therefore can't build up large stresses." Scientists believe the shearing, or distortion, and squeezing action of plate movement over millions of years is one of the reasons for heating part of the earth to temperatures greater than normal, allowing deformation to be spread over a wide zone. Stern equates this to a stressed cold metal bar that can snap in a brittle fashion, yet if heated will bend more easily over a broader area. Savage, whose work compares the New Zealand scene to California, says the contrast between the amount of shearing in the central South Island and in California is striking, suggesting a fundamental difference in the interaction of plates at their boundaries. "In California, movement along the San Andreas Fault was mainly strike-slip in nature with no compression. In other words, the two plates slide past each other without being squeezed. In the central South Island, however, squeezing and sliding have both taken place, leading to distortion being spread over a wide area. "New instruments that have been installed temporarily in New Zealand enable a new technique of recording distant earthquakes to determine the direction of shearing directly beneath the recording station, for the first time allowing us to measure which way the mantle is moving beneath the surface," says Savage. |
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