Why the tsunami warning was withdrawn, and why the long term threat remains

The relatively shallow magnitude 7 quake struck about 70 kilometers off the coast of Northern California, in a sparsely populated area. Roads in the states of California and Oregon became clogged with traffic as people attempted to flee further inland. The National Weather Service initially issued a tsunami warning that covered almost 5 million people -- including the San Francisco area -- before announcing that the coast was clear.