gary griggs director of the institute of marine sciences at the university of california at santa cruz has studied subduction zones. >> one of those plates is diving down beneath the continent and as it goes down, it's actually dragging on the overlying layer. it's actually hung up, it's stuck. every 100 or 200 years, that'll break loose. and what typically happens is the upper layer will sort of rebound. it'll bounce back up and that displaces a huge amount of water. and that happens all around the pacific intermittently. if the quake happened close to shore, the tsunami would arrive very quickly, as it did in japan and cause enormous devastation. >> reporter: 10% of the damage was caused by the earthquake. but the tsunami that followed did 90% of the damage, and killed 99% of the people. >> reporter: but a close-in tsunami is not the only threat worrying scientists. in santa cruz, california, the japanese tsunami was felt when a five or six foot wave roared into the harbor, sunk 20 boats damaged a hundred, and broke docks, nearly 6,000 miles from japan. and that underscored another t