A USGS webpage, linked here, is titled 'How the jello-shakes depends on the shape of the bowl..." The page begins with these words: "The Puget Sound is lined with a string of deep sediment-filled basins that will influence groundshaking during a large earthquake. Understanding the shapes of the basins is important to assessing the earthquake hazard potential of the region."
Looking further north, a page, linked here, speaks of the Georgia Basin which exists beneath Vancouver BC. The page states that "seismic waves are amplified as they pass through the Georgia Basin, the deposit of softer sedimentary rock that lies partly beneath Metro Vancouver." It states that "seismologists have known that sedimentary basins can increase shaking..."
The page, linked above, states further: "If a quake occurred within 100 kilometres of the city, such amplification could make the ground quake three to four times more than it would if the basin were not there." The page states this: "Essentially what the basin is doing is producing stronger shaking and producing longer-duration shaking." Now we have something to look at. This "three to four times" increase in shaking in the Georgia Basin beneath Vancouver BC is a good starting point for comparison to what happens in the Seattle Basin, beneath Seattle, Washington, during a seismic event.
A webpage, linked here, is titled 'The Influence of Sedimentary Basins on Ground Shaking in the Puget Lowland, Washington State." Regarding the Seattle Basin, the page declares: "Analyses of the shear-wave arrivals from teleseisms in the 0.1 to 1 Hz band (10 to 1 sec periods) consistently show large amplifications of seismic waves in the 0.2 to 0.7 Hz band by the Seattle basin. These amplifications reach a factor of 10 near the center of the basin. Potential causes of this amplification include resonance within the shallow (<1 km) basin sediments, focusing, and locally-induced basin surface waves."
For the Georgia Basin, beneath Vancouver BC, it is noted that seismic waves may be amplified and make the ground quake three or four times more than if the basin was not there. In the Seattle Basin, the seismic waves are amplified by a factor of 10, near the center of the basin, rather than just by a factor of 3 or 4, like in the Georgia Basin under Vancouver BC. So, it appears that in a major quake, Seattle, Washington could potentially get hammered badly -- potentially much worse than most other cities in the Cascadia region.
A 2001 New York Times article, linked here, notes the findings of USGS scientists of that time. It states that there is "as much as a five- to twelve-fold difference in shaking inside and outside the basin." One scientist declared: "It's as if all of a sudden you're 10 times closer to the earthquake." This does not sound good for Seattle, especially when the next massive earthquake breaks loose in the Cascadia region. But, there is more to this basin story.
The webpage, which is linked here, makes known an interesting concept. It appears that for a particular basin with a given shape, it depends on the direction from which the earthquake waves enter that basin, which ultimately may determine the degree of shaking and the potential destruction experienced at various locales within the basin. With this information in mind, let us now begin to examine the unique shape and various features of the Seattle Basin.
A LiveScience page, linked here, declares: "The basin under Seattle essentially holds complex layers of sediment within a bowl of rock. Such basins can trap and focus seismic energy within them." This is very important information to understand about the Seattle Basin and others like it. They can "trap and focus seismic energy within them." In a massive earthquake, it will be somewhat like the luck of the draw. Some may get hammered much worse than others, in a given area. It is hard to say beforehand where things will get hammered the worst. There are just too many variables involved.
The contour of the basin beneath Seattle is not just a uniformed bowl-shaped depression in the bedrock, whose bottom is situated miles below the surface. The LiveScience page indicates that there are "several sub-basins or pockets of sediment" beneath Seattle. These sub-basins can affect or alter the manner in which seismic waves "propagate through the subsurface." The sub-basins can have a great effect on the intensity and focusing, plus other characteristics of the earthquake event at any given locale on the surface.
Let us begin to look at the shape of the Seattle Basin. An image, linked here, shows generally what the Seattle Basin would look like if the sediments were removed all the way down to bedrock. This view is looking from the south toward the north. The darker-colored line on the near-side of the image would represent the Seattle Fault. Looking down into the bowl-like depression, the various sub-basins (dark-blue and purple) can readily be seen. These various sub-basins may tend to throw all kinds of variables into the seismic pattern, plus what could happen at various locales on the surface, during a massive seismic event.
An image, linked here, also shows what the Seattle Basin may look like if all the sediments were removed. The white dashed-lines in the lower portion of the image represent the Seattle Fault Zone. The colors used in the image represent depths, in kilometers, generally below sea level. Notice that some of the sub-basins are around 8 kilometers deep. That is a very deep pocket of softer, sedimentary materials which is "supporting" the area around downtown Seattle. In a massive earthquake, possibly Seattle is not one of the better places to be.
A map located on the USGS website, linked here, shows not only the Seattle Basin (in the lower, south portion of the map), but also the rather large Everett Basin to the north of the Seattle area. From the information already learned about basins and earthquakes, areas located on the surface above the Everett Basin would most likely not be ideal places to be in the next, massive Cascadia earthquake -- especially if the event were to trigger faults closer to the surface in basin regions.
If faults closer to the surface were to be triggered in a Cascadia event, this could result in a number of powerful, simultaneous earthquakes, which would be working together with the more deep-seated earthquake event. This could result in an event which is more severe and longer-lasting. In such an event, the devastation could possibly be stunning at certain locales in this region of geologic basins.
For further reading:
Current Expectations for a Cascadia Mega-Quake
https://cascadian.neocities.org/cascadiaquake.htm
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