Welcome to Monday! Let us jump right in there and get things moving quickly for the week. Our subject today is the seismic vulnerability of Seattle, Washington, with some consideration of other areas in the surrounding region.
There is an informative document about the Seattle Fault, plus other faults in the Seattle area, which is linked here. This document is definitely worth a closer examination. On PDF page 15 of 58 (document page 9), there is information about that subterranean, rocky, geologic bowl which we commonly call the Seattle Basin. Why this subterranean basin is of such interest is because it is filled to a great depth with relatively soft sedimentary materials. This creates a serious earthquake hazard for those living in the area. From surface level, in the deeper areas, these sedimentary materials continue downward "nearly 24,000 feet" --- or about 4-1/2 miles --- before they reach the solid rock bottom of the geologic "bowl."
About this point, there appears to be some questions which should be raised --- especially since there is "nearly 24,000 feet" of sedimentary materials at the deepest points in the Seattle Basin, before genuine solid rock is reached. Does this mean that the tall buildings, plus other elevated structures in Seattle --- like bridges and freeway ramps --- which are located above the basin, do not have foundations which are actually secured into genuine solid rock? Does this mean that these buildings and structures are actually attached into something else altogether --- possibly attached to something which is seismically, much weaker?
Does the available information tend to indicate that these tall buildings and other elevated structures in the Seattle area may, in the event of a massive and longer-lasting earthquake event, be found as actually "floating" on top of pliable or rather weak sedimentary materials? Does available information indicate that these tall buildings and other elevated structures may be somewhat more vulnerable to the seismic effects existing during a massive earthquake? Bottom line: We will have to wait and see. But, it is guaranteed that we will all know the true answers to the above questions when the next, massive earthquake event strikes the region.
For a moment, let us consider just some of the seismic energy which it appears Seattle will be dealing with, potentially in the near future. Let us consider just the energy which it appears could be released by the Cascadia thrust fault. Information about an earlier study, linked here, declares: "If the new findings are accurate, the fault will rupture within 110 kilometers (68 miles) of downtown Seattle, pouring seismic energy into a densely populated urban area, threatening to knock down buildings both large and small..." Okay, so what if other faults around the city --- like the Seattle Fault, the South Whidbey Island Fault, the Tacoma Fault and the Olympia Fault, along with others --- were to "link up" during a Cascadia quake and create a much larger seismic event --- one which is located directly beneath Seattle and other urban areas in the region?
Let us, for a moment, consider Mexico City and the recent earthquakes in that region which have been in the news a lot. A page, which is linked here, notes that Mexico City "is built on a dried-up ancient lakebed." The page states further that "these soft sedimentary clay deposits amplified the seismic waves, or they liquefied, destroying the foundations of some buildings." Okay, so what happens to Seattle and its buildings and other elevated structures during a massive earthquake event which strongly affects the Seattle region? How much liquefaction will take place in this region? As the linked document about the Seattle Fault is examined, we find that serious liquefaction in the Seattle area during past earthquake events has happened at various locations. So it is not something unusual in this region which is surrounded by so much water.
Moving onward in the document about the Seattle Fault, which is linked here and above, on PDF page 16 of 58 (document page 10) is found a map which shows the Everett Basin, the Seattle Basin, and the Tacoma Basin, along with four of the more major, known faults in that area --- which faults are noted a couple of paragraphs above. Take a real good look at that map. At this point, there is something related to consider.
In recent times, because of those serious earthquakes which Mexico has been experiencing, once again, there has been talk about the devastating effects of quakes on Mexico City, because it is built on a bowl of sedimentary materials which are up to 300 feet deep. And now, back to the Pacific Northwest. It appears that, especially in the Seattle basin --- with its "nearly 24,000 feet" of sedimentary materials upon which much of the city is built --- the city of Seattle may be dealing with a potentially far worse situation, when it comes to serious earthquake events. The same may be true for cities in the Tacoma and Everett basins.
Looking further into the Seattle Fault document linked above, on PDF page 21 of 58 (document page 15) is a section titled 'Our Unique Setting.' Yes, it appears that Seattle is located in a very precarious setting. For those with an interest, read what this section of the document has to say. It does bring out just a few of the more obvious and yes, very important vulnerabilities of Seattle, in the event of a serious quake.
Moving onward, PDF page 22 of 58 (document page 16) contains the following words: "Finally, there are the recent [about 2010] revelations that our city sits atop one of the deepest non-marine basins in the world, the Seattle Basin. Filled with up to 24,000 feet of unconsolidated sediments, it acts as a world-class bowl full of jelly in an earthquake. The boundaries of the basin act to reflect waves back into this area, where coinciding waves can amplify the shaking. The greatest measure of amplification lies over the deepest part of the basin, just north of the downtown area." But, let us not stop here. There is more to this very important story.
In the linked document, beginning on PDF page 53 of 58 (document page 47) and continuing for a number of pages, it speaks about 20 feet of uplift occurring in a portion of the landmass during what is believed to have been the last rupture of the Seattle Fault --- which is said to have occurred about 900 years ago. Think about this for a bit, and what effect this 20 feet of uplift could have if it happened again in our day. Suppose that a tall building or other elevated structure was located directly above this fault and one side suddenly changed elevation by 20 feet. What would happen to this tall building or elevated structure, and what would most likely happen to people in that building or upon that elevated structure? But, there are more than just one tall building or other elevated structures which are located along the dangerous faults in this region.
At this point, let us for a moment consider another issue. Let us at least consider some of those vulnerable areas of fill --- especially vulnerable to loss of strength and liquefaction during a massive earthquake. The text in the middle-right of PDF page 55 of 58 (document page 49), in the linked Seattle Fault document, speaks about "the industrial section of the city, largely built on mudflats that were filled with assorted materials." The page continues: "This is a region prone to large-scale ground failure by liquefaction during major earthquake events." Considering what all in built on this seismically-weak fill material, this is something which is worthy of further consideration. There are a lot of lives at stake here!
In closing, the linked Seattle Fault page continues with these words: "In the end, it is difficult to conceive of an urban geography less accommodating to the prospects of major earthquakes." So, there you have it, folks. And you thought Mexico City was located in a very bad geologic setting, when it comes to serious and devastating earthquake events.
Once again, this sporadic blogger wishes you all well in the times ahead.
Further information about the seismic vulnerability of the Seattle region is found at the following links: (1) (2) (3) (4) (5) (6) (7) (8)
Looking northward, a bit of information about the seimic situation in the Vancouver, British Columbia, region is linked here. The page it titled 'Georgia Basin Could Amplify Ground Shaking In Next Vancouver Earthquake.' The Vancouver region in Canada does also warrant further investigation.
For further reading:
Current Expectations for a Cascadia Mega-Quake
https://cascadian.neocities.org/cascadiaquake.htm
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