Greetings from California, where "earth surfing" (aka, living with earthquakes) is the official state sport!
When a truly large one hits (6+), there's not much you can do. I will say I once read a post at The Krib from a guy whose marine reef tank went through the m5.9 Whittier Narrows quake in 1987 -- tumbling live rock hitting the interior of the glass and everything. Amazingly, the tank survived. So, anything can happen.
For more moderate quakes, my concern is the shock wave rocking the tank and stand so it tips over: the tank/stand combination tends to be top-heavy. (For that reason, btw, I only buy stand with a lip around the front and sides, to reduce the chance of a corner sliding off.) In the 1994 Northridge quake, a friend's 55g tank flew off the stand she had it on and exploded on impact in the living room. Don't know if a lip would have prevented that, but it might.
For stand stability itself, you might try sandbags in the cabinet, just to counter the weight of the aquarium.
Like I said, nothing will help in a truly big one, but the right kind of stand and counter-weighting might help in lesser events.
BTW, Portland is only now recognizing the city is in a seismic zone? Where do they think all those lovely mountains, including the volcano Mt. Hood, came from? The whole West Coast is an earthquake zone: it's just that the PNW has been less active in recent centuries.
