Not until 28 February 1990, however, did the structure experience a large quake, when one of 5.5 magnitude struck just 10 kilometres away.

So far, his improvements have held through quakes large and small.

The most recent large quakes along it occurred around 4,000 and 7,000 years ago.

The largest quake occurred in 1872 and measured 7.4 on the Richter scale.

He said a much larger quake along that fault was therfore unlikely.

After a large quake, strain can build rapidly for several decades and then slow way down, he said.

If, they say, every large quake in the Seattle area produces such a wave, there has been only one in the past 2,000 years.

Also unknown were many features that protect present-day structures from large quakes.

The largest quake to have taken place in this Zone was a 5.4 earthquake in 1968.

We haven't had a large quake for many years, but small ones are always happening.