Manhattan Island is crisscrossed by earthquake faults, and twice in its history — 1737 and 1884 the nation's biggest city has been jolted by relatively mild quakes in the 5.0 range. Whenever the next one strikes, scientists worry that it could be far bigger. Much of Manhattan sits on a deep layer of soft, post-Ice Age sediment over extremely hard rock, a juxtaposition of geological extremes that bodes ominously. A 6.0 quake could shake the city's buildings with nearly the intensity of the 6.8 quake in Kobe. Inexplicably, the city dragged its feet about adding earthquake-mitigating requirements to its building codes until the mid-1990s. The generally well-designed towers in the Manhattan's skyline most likely would survive a 6.0, but the unreinforced masonry townhouses where most residents live might not fare so well. A 1989 study estimated that a quake would cause more than 130 simultaneous blazes, which could put the fire department under severe strain.