|A 2005 photograph of downtown Seattle (Source)|
From the point of view of prepping and disaster planning, I am always struck by how the terrain would prevent the evacuation or relocation of much of the population in the event of a disaster: especially along the edge of the bay and in and around the downtown area. As you may know, the primary north-south artery through Seattle is Interstate 5 (I-5) which bisects the peninsula on which the City is located, swinging south-west below the harbor. (I-5 also continues across the narrow channel on the north of the peninsula and continues northward). The primary east-west artery is I-90 which crosses the Lacey V. Murrow Bridge to Mercer Island, and then across another narrow channel to Bellevue. At the north end of the peninsula is another bridge over which runs Hwy 520 that is also provides east-west access from the tip of the peninsula to Hunts Point north of Bellevue. To the west of Seattle is Puget Sound. While there are other surface streets running north south and east west, movement is ultimately limited by the waters of the Sound, which means that you will have to cross a bridge going any direction but south.
Notwithstanding that Seattle is in-line with the Cascade line of volcanoes (part of the Ring of Fire around the Pacific), it is not very geologically active. The most likely disasters in the Seattle area would be from flooding--coastal flooding from waves and tides (including King tides) and riverine flooding from high runoff or, potentially, a dam failure--or landslides, which because of the steep terrain, soil, and high rainfall, is not uncommon.
However, there is always the risk of a major earthquake from shifting within the Cascadia subduction zone. The New Yorker published an article in 2015 explaining why such an earthquake would be so devastating. According to the article, the Cascadia subduction zone "runs for seven hundred miles off the coast of the Pacific Northwest, beginning near Cape Mendocino, California, continuing along Oregon and Washington, and terminating around Vancouver Island, Canada," and is capable of producing extraordinarily powerful earthquakes--much more powerful than could be produced by the San Andreas Fault. If only the southern portion of the fault slipped, it would produce an earthquake of magnitude 8.0 to 8.6. But "[i]f the entire zone gives way at once, an event that seismologists call a full-margin rupture, the magnitude will be somewhere between 8.7 and 9.2."
... When the next very big earthquake hits, the northwest edge of the continent, from California to Canada and the continental shelf to the Cascades, will drop by as much as six feet and rebound thirty to a hundred feet to the west—losing, within minutes, all the elevation and compression it has gained over centuries. Some of that shift will take place beneath the ocean, displacing a colossal quantity of seawater. ... The water will surge upward into a huge hill, then promptly collapse. One side will rush west, toward Japan. The other side will rush east, in a seven-hundred-mile liquid wall that will reach the Northwest coast, on average, fifteen minutes after the earthquake begins. By the time the shaking has ceased and the tsunami has receded, the region will be unrecognizable. Kenneth Murphy, who directs fema’s Region X, the division responsible for Oregon, Washington, Idaho, and Alaska, says, “Our operating assumption is that everything west of Interstate 5 will be toast.”
In the Pacific Northwest, the area of impact will cover some hundred and forty thousand square miles, including Seattle, Tacoma, Portland, Eugene, Salem (the capital city of Oregon), Olympia (the capital of Washington), and some seven million people. When the next full-margin rupture happens, that region will suffer the worst natural disaster in the history of North America. ... FEMA projects that nearly thirteen thousand people will die in the Cascadia earthquake and tsunami. Another twenty-seven thousand will be injured, and the agency expects that it will need to provide shelter for a million displaced people, and food and water for another two and a half million.
According to the article, the Pacific Northwest has experienced 41 subduction-zone earthquakes in the past 10,000 years, giving an average recurrence interval of 243 years. The last earthquake was in early 1700--317 years ago. Scientists believe that there is a 1:3 chance of such an earthquake within the next 50 years.
If and when such a quake hits, it will be a significant event. The New Yorker article describes it thus*:
The first sign that the Cascadia earthquake has begun will be a compressional wave, radiating outward from the fault line. Compressional waves are fast-moving, high-frequency waves, audible to dogs and certain other animals but experienced by humans only as a sudden jolt. They are not very harmful, but they are potentially very useful, since they travel fast enough to be detected by sensors thirty to ninety seconds ahead of other seismic waves. That is enough time for earthquake early-warning systems, such as those in use throughout Japan, to automatically perform a variety of lifesaving functions: shutting down railways and power plants, opening elevators and firehouse doors, alerting hospitals to halt surgeries, and triggering alarms so that the general public can take cover. The Pacific Northwest has no early-warning system. When the Cascadia earthquake begins, there will be, instead, a cacophony of barking dogs and a long, suspended, what-was-that moment before the surface waves arrive. Surface waves are slower, lower-frequency waves that move the ground both up and down and side to side: the shaking, starting in earnest.
Soon after that shaking begins, the electrical grid will fail, likely everywhere west of the Cascades and possibly well beyond. If it happens at night, the ensuing catastrophe will unfold in darkness. In theory, those who are at home when it hits should be safest; it is easy and relatively inexpensive to seismically safeguard a private dwelling. But, lulled into nonchalance by their seemingly benign environment, most people in the Pacific Northwest have not done so. That nonchalance will shatter instantly. So will everything made of glass. Anything indoors and unsecured will lurch across the floor or come crashing down: bookshelves, lamps, computers, cannisters of flour in the pantry. Refrigerators will walk out of kitchens, unplugging themselves and toppling over. Water heaters will fall and smash interior gas lines. Houses that are not bolted to their foundations will slide off—or, rather, they will stay put, obeying inertia, while the foundations, together with the rest of the Northwest, jolt westward. Unmoored on the undulating ground, the homes will begin to collapse.
Across the region, other, larger structures will also start to fail. Until 1974, the state of Oregon had no seismic code, and few places in the Pacific Northwest had one appropriate to a magnitude-9.0 earthquake until 1994. The vast majority of buildings in the region were constructed before then. ...
... FEMA calculates that, across the region, something on the order of a million buildings—more than three thousand of them schools—will collapse or be compromised in the earthquake. ....Bridges will also collapse: half of the highway bridges, and the greater number of the bridges crossing rivers and estuaries. The article continues:
... The shaking from the Cascadia quake will set off landslides throughout the region—up to thirty thousand of them in Seattle alone, the city’s emergency-management office estimates. It will also induce a process called liquefaction, whereby seemingly solid ground starts behaving like a liquid, to the detriment of anything on top of it. Fifteen per cent of Seattle is built on liquefiable land, including seventeen day-care centers and the homes of some thirty-four thousand five hundred people. ...The earthquake will also generate a tsunami. Those living within the zone danger will have only 10 to 30 minutes to evacuate following the earthquake--along roads that will largely be impassible because of the earthquake.
... Its height will vary with the contours of the coast, from twenty feet to more than a hundred feet. It will not look like a Hokusai-style wave, rising up from the surface of the sea and breaking from above. It will look like the whole ocean, elevated, overtaking land. Nor will it be made only of water—not once it reaches the shore. It will be a five-story deluge of pickup trucks and doorframes and cinder blocks and fishing boats and utility poles and everything else that once constituted the coastal towns of the Pacific Northwest.According to estimates given in the article, "it will take between one and three months after the earthquake to restore electricity, a month to a year to restore drinking water and sewer service, six months to a year to restore major highways, and eighteen months to restore health-care facilities."
The City of Seattle has a web-site called the "Seattle Hazard Explorer" that shows in a map format some of the principle risk area in the event of various natural disasters: tsunamis/seiches, liquefaction of the soil, landslides and flooding. It also has a map showing unreinforced masonry buildings which are particularly prone to failure in an earthquake. There are a large number of these buildings between I-90 on the south and Hwy 520 on the north. If you live in the area, I would advise consulting those maps.
None of the foregoing considers the impact if Mr. Rainier should erupt.
Mount Rainier's next eruption ... could produce volcanic ash, lava flows, and avalanches of intensely hot rock and volcanic gases, called "pyroclastic flows."
Some of these events swiftly melt snow and ice and could produce torrents of meltwater that pick up loose rock and become rapidly flowing slurries of mud and boulders known as "lahars." In contrast to lava flows and pyroclastic flows that are unlikely to extend farther than 10 miles from the volcano's summit and remain within Mount Rainier National Park, the largest lahars can travel for tens of miles and reach Puget Sound.
Volcanic ash will be distributed downwind, most often toward the east, away from Puget Sound's large population centers. Airborne plumes of volcanic ash can greatly endanger aircraft in flight and seriously disrupt aviation operations. Although seldom life threatening, volcanic ash fallout on the ground can be a nuisance to residents, affect utility and transportation systems, and entail substantial clean-up costs.
At Mount Rainier, the risk from lahars is greater than from lava flows, volcanic ash fall, or other volcanic phenomena because some pathways for future lahars are densely populated and contain important infrastructure such as highways, bridges, ports, and pipelines. Lahars look and behave like flowing concrete, and they destroy or bury most manmade structures in their paths. Past lahars probably traveled 45 to 50 miles per hour and were as much as 100 feet or more thick where confined in valleys near the volcano. They thinned and spread out in the wide valleys downstream, slowing to 15 to 25 miles per hour. Deposits of past lahars are found in all of the valleys that start on Mount Rainier's flanks.However, if does not appear that Seattle or its environs would be at much risk from an eruption at Mr. Rainier, although lahars might follows some of the rivers and valleys to cities and towns south-west of Seattle, including Tacoma.
* NOTE: The New Yorker article also has some information regarding Oregon generally, and Portland in particular, which I have omitted.