Mention earthquake zones and most of us would think of Japan, Sumatra, Chile or Southern California; others might include western China, Pakistan and Iran, where the Indian and Arabian Plates are crunching into Eurasia. Few would include New Zealand but, as we learned yesterday, this is also earthquake country.
Deposited along the edge of future Antarctica throughout the Paleozoic Era, the sedimentary rocks of New Zealand rifted from the other Southern Continents about 85 million years ago, some 30 million years before Australia began its long isolation.
Lying along the boundary of the Australian and Pacific Plates, New Zealand straddles both of them; the North Island and the northwest section of the South Island lie on the Australian Plate while the remainder of the South Island lies on the Pacific Plate. Pressure and friction between the plates has lifted the "Southern Alps" of the South Island and subduction of the Pacific Plate beneath the Australian Plate produced the volcanic summits of the North Island.
Yesterday's 7.0 magnitude quake occurred on the South Island, west of Christchurch, where the Australian Plate is slipping northeastward along the Pacific Plate; this movement is triggered by sea floor spreading as the ocean between Australia and Antarctica continues to expand. Indeed, hundreds of earthquakes occur in New Zealand each year though most are too deep or too weak to feel at the surface. But, as in all earthquake zones, the next "big one" could strike anytime.
