If you’ve been following the Mercury’s occasional stories about our region's coming earthquake disaster, then you will no doubt know the term Cascadia subduction zone. If not, here’s an oversimplified refresher: it’s a fault just off the West Coast stretching from Vancouver Island to Northern California. It’s pretty quiet, as far as faults go, but approximately every 200-500 years the Cascadia subduction zone produces a magnitude 8.0 or greater earthquake. And right now, scientists say we are overdue for one of these “big ones.” It could happen later today, or 100 years from now. But it will happen, and when it does, every town and city east of the fault line, including Portland, is screwed. However, if you think the Mercury is just playing Cassandra over this whole Cascadia thing, well, we’re not, and we’re not alone.

Canadian journalist and documentary filmmaker Jerry Thompson is the king of the Cascadia doomsayers. He’s covered our impending disaster for over a quarter century. The long-time British Columbia resident was one of the first to report on the fault. And, more recently, he wrote a very readable book, Cascadia’s Fault, documenting how our understanding of the subduction zone has evolved over the years. Tonight, at 7:00 pm at the Mercy Corps building (28 Southwest First Ave), Thompson will be giving a free lecture about his book and all things Cascadia. The Mercury caught up with him for a quick Q & A.

Mercury: Let’s start with the basics. What is the Cascadia subduction zone and why should people on the West Coast care?

Thompson: It's an enormous offshore fault. It's a place where two of the earth's plates crash into each other, the North American plate, which is the continent we live on, and the Pacific plate, which is drifting east and beneath it. When the plates grind together, they get stuck and sometimes they stay stuck for hundreds of years before enough stress builds up in the rocks to where it finally fails and snaps. When that happens, you get these huge earthquakes. And the reason it's a worry to us on the West Coast is that when the next Cascadia earthquake comes, it has the potential to be bigger than anything we have ever experienced in the lower 48 states and Canada. There are 312 years of stress in those rocks just waiting to be released.

Why did you start investigating the fault?

In 1985, when that really bad earthquake happened in Mexico City, I was sent to a government laboratory on Vancouver Island to find out what scientists learned from the Mexico City quake. Much to my surprise, I heard them say the same thing could happen up here [in the Pacific Northwest].

I asked them what they meant. They said the Northwest had a fault similar to the one that caused the Mexico City earthquake. They said it stretched from the middle of Vancouver Island all the way to Cape Mendocino. They said it could do the same kind of thing that happened in Mexico City. So I asked if that meant a magnitude 8.0 earthquake, and they said, “yes, or worse.” And so I asked myself, “why have we never heard of this before?” They told me, the reason was there had never been an earthquake on the fault in all of recorded history. They were pretty sure big earthquakes had happened before [on the fault] and would again, but others weren't so sure.

One theory had been that the stress between the plates was bleeding off slowly over the years, never being enough to build up to a major earthquake. And that seemed like a pretty plausible theory, at least for a while. But a bunch of guys here in Oregon at OSU [Oregon State University] were the real trailblazers, and they turned that idea around. They took core samples from the ocean floor and found evidence of massive offshore landslides in the layers of sand and mud. And these landslides, they believed, came from huge earthquakes that had been going on every so many hundred years. Finally, they made a convincing case that turned the whole scientific community around to the fact that these big earthquakes will happen again and that they are a definite threat.

You write, when you tell people about the West Coast’s coming disaster, they go through a period of shock, which you write, “quickly wears off,” and, "after that, nothing much changes." How do we get over this self-imposed helplessness?

I wish I knew. The book was written in the aftermath of the Sumatra tsunami [in 2004]. And we were in the process of putting it together when the Japan earthquake happened [2011]. So we rushed it to print. And I thought this was going to be a great opportunity to get people's attention, but I don't think that is what happened. I think news editors became distracted by the nuclear disaster in Japan. And then the royal wedding happened, and the news cycle changed and people just weren't interested in it any more.

One thing I found really interesting in your book was the connection between the San Andreas fault and the Cascadia subduction zone. You write that a Cascadia rupture could trigger an earthquake on the San Andreas.

That is generally the theory. It's new research, and controversial, but basically what Chris Goldfinger and the OSU team that were drilling core samples did was go further down past the Cascadia fault further off the California coast to see if they would continue to see the same offshore landslides associated with the quakes. And they did find them on areas that were directly adjacent to the San Andreas. So they started to do carbon dating and they found that within 40 or 50 years of every Cascadia event, there was also a San Andreas event. With carbon dating, you can’t know exactly which year it was. So they didn't know which event came first. Did San Andreas trigger Cascadia, or the other way around? But I think their theory is now more refined, and they now believe Cascadia is more likely to trigger San Andreas. But this is still very much a work in progress.

Since covering Cascadia, you have been accused of fear mongering. In your book you ask yourself: "Do I make things better or worse by warning people about an event that may not happen in their lifetimes?" What keeps you pursing this story?

Going back to the beginning, there is still this interesting detective work going on with the science and that keeps me interested. Every now and then you see some progress. I walk around in cities now and I can see seismic retrofitting begin done. And I think, “good, things are happening.” And you just hope that enough of that happens before our time runs out.