The Faults in the Puget Sound Area

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.

The 20 highest summits of the United States

Rank    Peak            State        Elevation
1    Mount McKinley        Alaska        20,236 ft./ 6168 m
2    Mount Saint Elias    Alaska        18,009 ft./ 5489 m
3    Mount Foraker        Alaska        17,400 ft./ 5304 m
4    Mount Bona        Alaska        16,550 ft./ 5044 m
5    Mount Blackburn        Alaska        16,390 ft./ 4996 m
6    Mount Sanford        Alaska        16,237 ft./ 4949 m
7    Mount Fairweather    Alaska        15,299 ft./ 4663 m
8    Mount Hubbard        Alaska        15,016 ft./ 4577 m
9    Mount Bear        Alaska        14,831 ft./ 4520 m
10    Mount Hunter        Alaska        14,573 ft./ 4442 m
11    Mount Alverstone    Alaska        14,564 ft./ 4439 m
12    Mount Whitney        California    14,505 ft./ 4421 m
13    University Peak        Alaska        14,470 ft./ 4410 m
14    Mount Elbert        Colorado    14,440 ft./ 4401 m
15    Mount Massive        Colorado    14,428 ft./ 4398 m
16    Mount Harvard        Colorado    14,421 ft./ 4396 m
17    Mount Rainier        Washington    14,417 ft./ 4393 m
18    Mount Williamson    California    14,379 ft./ 4383 m
19    La Plata Peak        Colorado    14,343 ft./ 4372 m
20    Blanca Peak        Colorado    14,357 ft./ 4376 m

The Mountains and Volcanoes of the Seattle Area

Thanks to the Kitsap and Olympic peninsulas that help shape the Puget Sound, Seattle ends up situated between two mountain ranges.  There’s the Olympic Mountains located on the Olympic Peninsula approximately fifty miles to the west.  To the east, also roughly fifty miles away, is the Cascade Range.  Within the Cascade Range, located about 80-ish miles to the south east is Mt. Rainier.

Looking from Seattle across the Puget Sound, one can easily see the Olympic Mountain Range. Compared to the height of many of the peaks of the Cascade Range, the Olympics are smaller. The highest peak is Mount Olympus, clocking in at around 7900 feet.

However, what makes the Olympics notable is the fact that the range is part of the Pacific temperate rain-forest ecoregion, noted by the World Wildlife Fund as the largest temperate rain-forest ecoregion on the planet. This region runs from Prince William Sound in Alaska, all the way south through the Pacific Northwest, down to northern California. The forests that this massive, interconnected region contains are larger than any other ecosystem on the planet.  
However, to visit the Olympics from Seattle takes some planning, due to the Puget Sound. Either one has to wait in a car to pick up a ferry in order to cross the Sound, or the other option is to drive an hour or so to Tacoma, and cross the Tacoma Narrows Bridge.  From a purely logistical perspective, it is far easier to access the Cascade Range to the east.  Due to this, when a Seattle-ite talks about the mountains, they primarily mean the Cascades.

Specifically, the Central Cascades, which have loads of hiking, climbing, and skiing.  A quick 30 minute jaunt to Issaquah, WA via Interstate 90 is all that’s needed to get to the foothills of the Cascades.

The Cascade Range is considered part of the Ring of Fire, a region of the planet that contains about 90% of the world’s earthquakes, and contains about 75% of the world’s volcanoes, both active and dormant. It runs from the west coast of Chile, up north through the western coast of South America, then through the west coast of Mexico and the United States, up through the Pacific Northwest and into Canada. Around Alaska, it turns west, and heads over to Russia, before turning south to Japan, and the Philippines. It runs east-west around Malaysia, Indonesia, and New Guinea, before turning south again around Fiji and Samoa. It ends up just east of New Zealand.  Or, to put it another way, the Ring of Fire encompasses the border of the Pacific Tectonic Plate.  What this means is that Seattle shares similar risks to volcanoes and earthquakes as Japan and even New Zealand.

The mountain range of the Cascades run from northern California, to the Fraser River in British Columbia, which separates the Cascade Range from the Coast Mountains.  Included in this range are some of the more well-known mountains in the United States, including Mount Shasta in California, Mount Hood in Oregon, Mount St. Helens in southwest Washington, Mount Baker near the Canadian border, and Mount Rainier, about 85 miles south/southeast of Seattle.

The Duwamuish River

At the southern portion of Elliot Bay, beyond the man-made Harbor Island, is the mouth of the Duwamish River is the name of the lower 12 miles of the Green River.

The river has had a history of man-made upheaval and…well…Industrial contamination/poisoning. Until 1906, the White and Green Rivers joined around present-day Auburn, about twenty-eight miles to the south of Seattle, and then joined the Black River near Tukwila to form the Duwamish River. In 1906, after a massive flood, the White River changed course with the result being it emptying into the Puyallup River as it does today. With the confluence now gone, the lower portion of the White River is now simply considered part of the Green River.

In 1911 the Cedar River was diverted to empty into Lake Washington instead of into the Black River, the result of which included less water flowing in to the Duwamish. Finally, in 1916, with the opening of the Lake Washington Ship Canal, the lake's level dropped nearly nine feet and the Black River dried up. From that time forward, the point of the name change from Green to Duwamish is no longer the confluence of the Green and Black Rivers, though it has not changed location.

In other words, today the Duwamish River is part of Green River, but it used to be the result of the confluence of several other rivers that have since dried up, or have been diverted elsewhere.

Seattle’s economy used to be primarily driven by manufacturing, and all of the industrial aspects of that in which it entails. While the Seattle of today has a more diversified economy, what with the technology, financial, and bio-genetic companies in the area, manufacturing still exists.  This can be seen in the area of south Seattle, including the region bordering the Duwamish River.

The result of over one hundred years of industry in this region has resulted in a heavily polluted river. So much so that the Environmental Protection Agency has labeled the river a Superfund site, the environmental program established to address hazardous waste sites.

These pollutants have affected the local wildlife, including the chinook, coho, chum and steelhead, salmon that inhabit the waterways.  Many of the animal species found in or around the river contain an unhealthy amount of contaminants. In a memo from the EPA in 2008, they stated, “any type of fish or shellfish found in the river is unfit for human consumption. It was found that PCB levels in fish and crab that live in the waterway most of their lives are 35 to 110 times higher than in Puget Sound salmon.” Their Ecological Risk Assessment also found that river otters from the Lower Duwamish River might be exposed to such high levels of PCBs that the growth or survival of their offspring may be reduced.

Efforts are underway to clean up the area, with current estimates on completing the project pointing to sometime between 2016 -2020.

Lake Union

Lake Union from Gasworks Park

While Lake Washington is the de facto eastern border of Seattle, and technically outside of the city limits, Lake Union is a part of the Emerald City, as the city has developed around it.

It, like Lake Washington, was formed by the retreating Vashon Glacier, although it is certainly not as deep, and at five-hundred and thirty-eight acres, it is neither as wide nor as long as the lake to the east.

Native Americans of the Duwamish tribe lived on the shores of Lake Union for more than 5,000 years. They called it by two names that compared it with Lake Washington.  In Lushootseed (the Duwamish tribe’s primary language), it was XáXucHoo or Ha-AH-Chu (“littlest lake”).  In Chinook it was “Tenas Chuck” which means “small water”.

In addition to the lake’s year-round residents, there are also stories of other extended members of the Duwamish tribe coming from the local area to spend their winters in a seasonal camp at the south end of the lake.  The coastline of the lake provided wild roses, red currant and squaw berry bushes for the gatherers, as well as deer, elk, bear, and cougar for the hunters.

When pioneers arrived in Seattle in the 1850s, the Duwamish camp was in the southwest corner of the lake, near where Thomas Street and 8th Avenue North meet just north of Denny Park. The native inhabitants remained there, albeit with dwindling numbers, until 1875.

Lake Union also received its present name from Thomas Mercer, who predicted that canals would join Lake Washington to Puget Sound in a "union of water", with Lake Union being the body of water that binds Lake Washington to the Sound.

If you look at Lake Union on a map, it looks sort of like a deformed head of a rabbit, with the canal to the north-west forming its left ear, and the right ear being the water way to Portage Bay. The northern shore of the lake is home to Gas Works Part, and southern end of the lake is called the South Lake Union area. Eastlake and Westlake are the neighborhoods found on each respective shore. The names surrounding the lake are far more underwhelming than the use and impact of the lake upon Seattle’s history.

Seattle - How Now Puget Sound

 

The location of Seattle is unique insofar as it is one of the largest cities in America to be situated upon a sound. It’s not the largest city in America to have this distinction – that’d be New York City, who finds itself on the west end of the Long Island Sound. 

What defines a sound is dependent upon which geological definition you use; either the Scandinavian definition, which means a strait, or the narrowest part of a straight, or the 18th century English definition, which means a sea inlet that contains islands. This is the definition used for the Puget Sound.

This definition helps explain the fact that Seattle is a coastal city located upon the Pacific Ocean, even if it is around one hundred miles or so from open sea. This is a fact that is easy to ignore at times due to looking across the sound and seeing various aspects of the Olympic Peninsula.

Puget Sound is defined as all waters of the Hood Canal, the Admiralty Inlet, the Possession Sound, and the Saratoga Passage, among others. Essentially, if one were to draw a line from Mount Vernon, to Port Townsend, Puget Sound would include all waters of the inlet from that line, down to Budd Inlet in the south by Olympia.  That’s a body of water that stretches approximately one hundred miles.

Seattle is located on the Eastern Edge of the Puget Sound, and this aspect of the city is the primary, but certainly not only, defining geologic feature of the region.

All of the Puget Sound region was shaped in larger part due to the Fraser Glaciation. Evidence of glaciation is easily seen today all along the Puget Sound area. The Fraser glaciation lasted about 10,000 years and consisted of 3 periods of ice expansion (called “stades”) and 2 ice recessions (called “interstades”). The Puget Sound was shaped by the Puget Lobe portion of the Cordilleran Ice Sheet that was part of the Fraser Glaciation that shaped much of the Montana, Idaho, Washington, British Columbia, the Yukon Territory, and Alaska.  The Puget Lobe portion of this ice sheet made it as far south as modern-day Olympia.  As it retreated over the course of many thousands of years, it geologically shaped many of the water ways of the region, including Puget Sound, as well as Lake Union, Lake Washington, and Lake Sammamish to the east.

The retreat also helps explain the hilly nature of the region, though not so much the mountainous portions, whose shapes were created under different forces.