While these rates are rather small, comparable to the rate your fingernails grow, the plates are about 100km thick, so a large volume of rock is deformed as the plates collide.  This rock can deform in two ways: either through straining elastically and eventually fracturing, which produces earthquakes, or by flowing.  This situation is analogus to pushing the two ends of a Mars bar together.  While the brittle chocolate layer deforms by fracturing in tiny earthquakes, the caramel interior deforms by flowing.

The convergence of the plates is taken up in differing ways along the plate boundary.  In the North Island and the northern South Island, the Pacific Plate dips below the Australian Plate.  This process is known as subduction, and earthquakes originating within the subducted Pacific Plate occur as deep as 600 km beneath Taranaki.  In Fiordland and the region to the south, the situation is reversed, with the Australain Plate subducting beneath the Pacific Plate.  Between these two subduction zones, the crust of both plates is too buoyant to subduct, so the convergence is accomodated by a combination of uplift, which creates the Southern Alps, and movement along the Alpine Fault.

Convergence of plates is largely driven by the sinking of the relatively dense deeper part of the subducted plates into the earth's mantle.  This phenomenon of "slab pull" often leads to tensional stresss in the subducted plate, and many of the earthquakes within New Zealand's subducted plates results from such stresses.  Earthquakes also result from bending and tearing stresses which occur as the plates subduct.

If the interface between the subducted and overlying plates were smooth and well lubricated, such earthquakes within the subducted plate would be all that we would experience at our subduction zones.  However, this is nearly always not the case, and the plate interface periodically locks up.  This has two main consequences.  First, the overlying plate deforms as convergence continues, resulting in faulting in the brittle crust.  Good examples of such faulting are the Wellington, Wairarapapa and Mohaka faults in the overlying plate.  And secondly, the plate interface itself may eventually rupture in a large earthquake.

Because of these major tectonic processes operating at our plate boundary, New Zealand will continue to experience many earthquakes.  The level of activity is comparable to that in California, but somewhat less than that in Japan.  Living in New Zealand thus means living with earthquakes.  The studies described in this Tephra are all aimed at quantifying the hazard that these earthquakes pose, so that engineers and planners, and the public at large, can prepare for them.