The Cascadia fault, a 600-mile-long collision between two chunks of the earth's crust off the Pacific Northwest coast, has been quiet for a long time, and that is not a comforting fact. Major earthquakes occur somewhere in the world every year or two. Catastrophic tsunamis - giant waves generated by undersea earthquakes or landslides - strike less often, and some of the largest of tsunamis originate in places that do not, at first glance, appear particularly treacherous. The devastating tsunamis created Dec. 26 by a magnitude 9.0 earthquake that killed as many as 150,000 people on the shores of the...