Size matters with earthquakes, but the modest-sized temblor that struck Christchurch on the South Island of New Zealand on Monday dramatically demonstrated that there's much more to seismic destructiveness than just magnitude. Monday's magnitude-6.3 quake was just an aftershock to the magnitude-7.0 quake of 3 September 2010, which was 11 times more powerful. No one died in the big one, but it was about 50 kilometers west of the city. Monday's aftershock, however, was within only a few kilometers of the center of Christchurch and only half as far beneath the surface. That close to the rupturing fault, almost a third of a million people felt severe to violent ground shaking, the U.S. Geological Survey estimates. And because the smaller quake struck at midday, those residents were massed in larger buildings that were likely to kill victims if they collapsed. September's quake hit in the early morning when most people were at home.
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