Seismic experts credit the development and enforcement of seismic provisions of national building codes with dramatically reducing the impact of the most damaging earthquake to hit New Zealand in nearly 70 years. The magnitude-7.1 temblor that struck Christchurch, the second- largest city, at 4:36 a.m. on Sept. 4 caused no significant damage to major buildings. Low-rise unreinforced masonry buildings, not engineered to resist quakes, sustained the most extensive damage. Water and sewer lines bedded in soft alluvium were stressed and pipe-joint displacement occurred, disrupting service. By nightfall, two-thirds of the city had water.