The Puget Sound, including the Seattle region, are seismically active. This is due to the dozen or so different fault lines in the region. If you’re visiting Seattle, the Seattle Fault is the one most local, but all provide some measure of risk, some greater than others.
The Seattle Fault was first identified in 1965,, but wasn’t classified as an actual, active fault until 1992. It runs east/west. Its eastern most terminus (where it starts/ends) is likely where it intersects with the Rattlesnake Mountain Fault, somewhere around Fall City, about 40 miles to the east. Its western terminus is still unknown for certain, but it is known that the fault line extends beyond the Puget Sound through Bremerton.
As for the specific location of the fault line? Take a look at the location of Interstate 90 from Seattle to Bellevue. The Seattle Fault line parallels that nicely. You’ll also note that the fault intersects through the stadium district, just south of downtown. All of that land there? Much of it is man-made, filler created from the dirt of the Denny Regrade. This is the city of Seattle’s biggest earthquake risk, a fault line that runs underneath a section of land that is likely to liquefy when the big one arrives.
However, the Seattle Fault is not the only fault line in the region. There’s the Tacoma Fault down by, well, the city of Tacoma. To the north, running north-west/south-east is the South Whidbey Island Fault, which also runs into the aforementioned Rattlesnake Mountain Fault. These are but a percentage of the amount of fault lines running in the Puget Sound area. Many, if not all, of these faults are actively seismic to some degree or another. The bulk of the earthquakes in the area occur either in the area between Mount Rainier and Mount Saint Helens, or in the immediate Puget Sound vicinity. The fact that these regions contain a great majority of the State of Washington’s population is a well-known risk.
Why are there so many faults in the region? It would be simple to point out the fact that Seattle, as well as the rest of the Western states in America are part of the Pacific Ring of Fire, that stretch of earth that starts many miles south of New Zealand, heads up north through Indonesia and then Japan, turns right north of Korea and the Kamchatka Peninsula, through the Aleutian Islands, and then turns south along the west coast of Canada and the United States, and then continuing south through Costa Rica, Peru, and Chile. But this tells us more of “where” rather than “why”.
One of the theory as to why, harkens back to glaciation and the last major ice age to hit the area. Yes, this would be the same glaciation that helped shaped the Sound as well as the many lakes in the region. The theory is that, if you have an ice sheet that is five times taller than the Space Needle, that amount of weight is bound to do some measure of damage to the tectonic plates.
Speaking of tectonic plates, the third major risk of earthquakes comes from the Juan De Fuca plate as it drives itself east under the Cascadian subduction zone. While technically not in the immediate Seattle vicinity (the Juan de Fuca plate is several dozen miles off of the Pacific coast, well into the Pacific Ocean), the sheer amount of potential energy that the plate can release will have an impact on all of the Puget Sound area to some degree or another. As a point of consideration, many experts believe that the Seattle Fault has the potential to produce a 7.0 magnitude earthquake. Any sudden movement in the Cascadia Subduction zone has the potential to release a 9.0 magnitude earthquake. In fact, that area had already demonstrated that level of destruction with the 1700 Cascadia earthquake.