They aren't frequent, only about once every 300-900 years, but it's been a while since the last megathrust earthquake. Smaller earthquakes near population centers also can cause devastation. It takes decades of attention, building, planning and practice to mitigate risks for large unusual events.
"This is an earthquake of the same type, with about the same magnitude and proximity that we face here in the Pacific Northwest from the Cascadia Subduction Zone. What you are seeing in Japan today is what you will also see in our future. Except they are better prepared than we are." Robert Yeats, Oregon State University