What is the Seattle Fault about? What kind of seismic hazard does it really present to Seattle and the surrounding area? A Wikipedia entry on the Seattle Fault, linked here, declares: "The Seattle Fault is a zone of multiple shallow east-west thrust faults that cross the Puget Sound Lowland and through Seattle (in the U.S. state of Washington) in the vicinity of Interstate Highway 90." The Wikipedia entry notes that the fault "was first recognized as a significant seismic hazard in 1992..." So, it has been about 24 years ago that this fault system was first "recognized as a significant seismic hazard." That really has not been that long ago. It is also very clear that there is still a lot more to learn about it and what it appears to be associated with.
There is something to note in the Wikipedia entry, linked above. The fact that there was a major earthquake on the Seattle Fault in earlier times is a part of Native American oral legend. Unfortunately, it appears that all too often, important pieces of information contained in "Native American oral legend" have been overlooked or ignored. Sometimes, this ignorance of "golden nuggets" contained within these oral "legends" can turn around and bite us, when we should have allowed these "golden nuggets" to help forewarn us about things which can readily happen in the Cascadia region and elsewhere.
The linked Wikipedia entry notes that since 1992, "extensive research has...shown the Seattle Fault to be part of a regional system of faults." Looking at its width, the Seattle Fault Zone is at least "a 4 to 7 km (2.5 to 4.3 miles) wide zone of complex faulting, with three or more main south-dipping thrust faults." But, there is something very important to note about this fault zone. Most of the faults in this system are classed as being "blind" or concealed faults. That means that these are faults which do not reach the surface of the ground, so they cannot be readily observed. It generally takes special equipment, plus a lot of work, to discover these potentially dangerous faults.
Looking further at the information in the Wikipedia entry, there is something else to note. Heading eastward from the Seattle area, the central section of the Seattle Fault Zone (SFZ) appears to cross the Olympic-Wallowa Lineament (OWL). Past the OWL, to the east, there are notable changes in the SFZ. But, it appears that scientists do not understand the significance of these changes, at least at this time. Once again, there truly is a lot to learn about this Seattle Fault Zone and how it relates to and interacts with other geologic features in the Cascadia region.
The Wikipedia entry, linked above, states that the Seattle Fault "extends for approximately 70 km (43 miles) from near Fall City on the east, ...to Hood Canal on the west..." This is the story commonly told about the Seattle Fault. But let us begin to consider the rest of the story, for it may have a great effect on what happens during the next major Seattle Fault quake or possibly the coming Cascadia megaquake event. It may also be a factor in deciding how well we are prepared for serious quakes in the region.
By researching a bit further, it appears that the Seattle Fault does not simply stop "near Fall city on the east," but continues possibly much further to the east. At this point, there is something important to note. Generally speaking, the longer the fault, the larger the earthquake it can potentially produce. Keep this in mind, as we consider a more realistic length of the Seattle Fault.
Scientists have produced a number of different models which express their various thoughts about the deeper configuration of the Seattle Fault Zone. But, let us put aside all hypothetical models and continue onward to consider one very important factor. Let us consider the potential length of this fault system, plus other of its attributes. Let us think about the earthquake potential for what appears to be a relatively long fault system. It is also time to consider a more realistic picture for the Seattle Fault and the system to which it appears to be connected.
The Wikipedia entry, noted further above, indicates that the Seattle Fault extends eastward to near Fall City. A webpage from the 1990s, linked here, states that the Seattle Fault follows "the route of Interstate 90 toward, and possibly beyond, the Cascade Mountains." This possibility of the Seattle Fault extending beyond the Cascade Mountains is very important, for it means that this fault could possibly produce a much larger earthquake than currently expected. This also means that Seattle could potentially get hit much harder in a major event on this fault, than people now commonly realize.
What makes the whole scenario about a major quake on the east-west aligned Seattle Fault even more concerning is that the fault runs "just south of downtown [Seattle] (link)". This puts a major part of the city on very shaky ground (those miles deep sedimentary layers). It appears that Seattle could be pounded hard during the next major quake event. It is therefore important for us to really consider the implications of that statement which indicates that the Seattle Fault extends beyond the Cascade Mountains. We must also consider other associates of this fault.
A 2014 webpage, linked here, declares: "The region's most dangerous surface fault is believed to be the South Whidbey Island Fault." The page notes that "the Whidbey fault crosses through the Cascade Range, reaching as far as the Tri-Cities in southeast Washington." Now comes the kicker. The page states that "the Seattle Fault...now is understood to be part of the South Whidbey Island Fault. Together, they form a system that extends across the Cascade Range to the Hanford Reservation." This indicates that the total length of this fault system may be "around 77 miles to 124 miles." This extra length means a much larger earthquake could potentially be produced by this system, than has commonly been expected. But, there is actually much more to this story.
About this time, there is something to note in another post in this blog -- the one from Friday, September 2, 2016. It is titled 'Cracks in the Ground from the 1872 Earthquake.' During the largest earthquake in Washington's recorded history -- the earthquake of 1872 -- fissures split the ground south of Seattle (link). In 1872, Seattle was a relatively small town. In 1870, two years before the big quake, Seattle had a population of only 1,107 people (link). In 1872, the year of the quake, Seattle had "a population of about 1,500 (link)." It was not the huge metropolis it is today, which sprawls out across the countryside for miles and miles.
Cracks which formed in the ground south of the small, 1872 town of Seattle appear to have formed in the Seattle Fault Zone. Then, on the other side of the Cascade Mountains, two cracks formed on a "hogback east of the Columbia (link)." This hogback is across the river from the Entiat area. A satellite photo, linked here, has Seattle marked, which is to the west of the Cascade Mountains. The Cascade Mountains run north and south (up and down) through the center of the photo. On the east side of the Cascade Mountains is marked the location of the Hogback on which the two cracks formed in the 1872 quake. With cracks forming in the ground on both sides of the Cascades, it appears that the major earthquake of 1872 was located on that network of faults which pass beneath the Cascades.
Regarding faults in the Puget Sound region, a source, linked here, states: "The faults don't just end in Puget Sound." It appears "that many big faults in eastern Washington go through the Cascades." Note the use of the words "many big faults...[which] go through the Cascades." The page indicates further "that the active faults west of the Cascades actually extend 250 to 300 miles from the Olympic Peninsula and through the Cascade Range, where they merge with the basalt formations of Eastern Washington, at least as far as Pasco..." Furthermore the source indicates that the faults under Central Washington are not shallow, but appear "to extend more than 12 miles below the surface." Let us consider this information further, plus its implications for major Washington earthquakes, even in the Seattle area.
For a moment, let us consider just a little more about those "many big faults in eastern Washington [which] go through the Cascades." It is time for some food for thought. A geologic road sign by the Montana Department of Transportation, linked here, in its lower-left corner, speaks about the Lewis and Clark Fault Zone as being "a series of faults that stretch between northwest Washington State and the Helena area." If this is true, this would tend to indicate that the Lewis and Clark Fault Zone is possibly deeper in the earth in Washington, and passes under the Cascade Mountains.
Looking further, a page from Idaho, linked here, declares: "The Lewis and Clark Zone is a megashear in the earth's crust, up to 30 miles wide..." At this point, there is something else to note. Many have speculated that the Olympic-Wallowa Lineament (OWL) is "a strike-slip fault or megashear (1) (2). The OWL is also believed to pass under the Cascade Mountains.
At this point, let us consider yet another thing. There are some lessons to be learned from information about another region of the country. Let us consider how the information presented could apply to fault systems in Washington. A 42 page study on the Owens Valley in California, linked here, notes that "a more shallow seismogenic zone would of course reduce the maximum magnitude that a given fault could generate."
The other side of the principle noted above would be this -- a deeper extending seismogenic zone could potentially increase the magnitude of earthquake which a given fault could generate. Once again, let us note that "the faults under Central Washington are not shallow," as was formerly believed. Now it is believed that these faults "extend more than 12 miles below the surface." This means that much larger earthquakes can potentially be produced on the eastern portion of the fault network, than was formerly believed. But, again, there is still more to this story.
A 250 to 300 mile long fault system, spanning across Washington, is noted further above. This long fault system could potentially produce a very large earthquake -- much larger than what could be produced by just a 70 km (43 miles) fault system, as the Seattle Fault was declared to be in Wikipedia. But there is still more to consider. The post in this blog for September 18, 2016, ends with information about linked faults and simultaneous earthquakes. Let us now look, once again, to information about the California region for possibly another important lesson.
A page, linked here, notes that "in Eastern California, big earthquakes seem to link up several smaller faults when they strike, instead of staying on one fault." It is a general principle that earthquakes working together on a number of faults at the same time can produce a much larger earthquake event. So, how could this information apply in Washington, especially when considering the complex fault system which passes under the Cascade Mountains and links fault systems in the west to those in the east?
It appears that when things start to seriously move, seismically, in Washington -- whether initiated to the east or the west of the Cascades -- the potential exists for simultaneous earthquakes to occur. These earthquakes can occur in the complex fault network, which passes under the Cascade Mountains. Once again, it appears that this complex network joins faults on both sides of the Cascade Mountains. This means that much larger earthquakes could potentially be produced across the state. The major earthquake of 1872 was just one example.
A massive, interconnected earthquake could negatively affect a much larger portion of Washington on both sides of the Cascade Mountains, than is commonly believed. It could also negatively affect areas outside of the state of Washington. Areas closer to the activated fault network, such as Seattle and the surrounding territory, could also experience much greater devastation than formerly believed. Furthermore, if major movement on one of the deep-seated rifts were to occur, the resulting seismic activity could include a number of the shallower faults. This would greatly complicate the seismic picture, plus the resulting devastation would be increased.
For further reading:
Current Expectations for a Cascadia Mega-Quake
https://cascadian.neocities.org/cascadiaquake.htm
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