"L.A. is gone in a flash, but San Diego is also obliterated by a series of apocalyptic events. First, only 12 seconds after the L.A. impact, a fireball of hot vapor reaches the city 125 miles to the south, igniting clothing, plywood, even green grass. Thirty seconds later, a huge earthquake levels most structures. Rock fragments then shower down on the region, burying everything in debris as high as a 40-story building.

An airblast with violent winds and deafening noise then sweeps in, scattering all that remains of San Diego. Over a thousand miles away, Denver likely escapes being roasted by the fireball, but wooden buildings topple and a foot of rocky debris falls. In New York City, windows rattle from the tremors, and a half inch of debris blankets much of the East coast."

A catastrophe calculator lets you try out some "what if" scenarios.

Simulate a 130-foot diameter Iron based asteroid, an 11 mile wide "planet killer" and an Apophis-sized (1050 foot wide) impact - then read what some of the possible results would be.