There are things that the residents of the Cascadia region of North America, plus those especially in southeast California (around the Salton Sea and Salton Trough region, and even northward from there), can learn from the earthquakes which have occurred recently in Mexico. Some of the lessons relate to buildings and structures located on sedimentary materials --- especially if this sedimentary material resides within a bowl-shaped pocket of harder rock.
A New York Times page, linked here, is titled 'Mexico City Was Built on an Ancient Lake Bed. That Makes Earthquakes Much Worse.' The page notes that much of Mexico City resides "on layers of sand and clay --- up to 100 yards deep --- that used to be under the lake." Yes, much of Mexico City is built, as the page states, on "soft, water-laden sediments [which] make the city uniquely vulnerable to earthquakes and other problems."
The page linked above does a reasonable job of explaining why earthquake waves are amplified in this sedimentary material upon which Mexico City is built --- yes, waves which can be amplified to "a hundred times stronger than they would be otherwise." It also notes that the way things are geologically configured beneath the city causes "the amplified shaking [to] last longer." Furthermore, this sedimentary material in a bowl of harder rock can amplify and be seriously affected by earthquakes which are centered a great distance away, while surrounding areas on firmer ground are hardly affected.
The following linked pages contain information about the dangers of building on soft, sedimentary materials --- especially when it comes to massive earthquakes.
A page, linked here, notes that Mexico City "is built on a dried-up ancient lake bed." The page states further that "these soft sedimentary clay deposits amplified the seismic waves, or they liquefied, destroying the foundations of some buildings."
A Los Angeles Times page, linked here, has a video which shows what can happen to even a concrete building in these sedimentary-type conditions just mentioned. The building in the video starts wobbling and concrete starts falling. Then, suddenly, things just crash to the ground. The linked page speaks about the dangers of these concrete buildings --- buildings of a type which, many like them are found in the Cascadia and southern California regions, plus elsewhere in "earthquake country."
A Smithsonian page, linked here, is titled 'How Mexico City's Unique Geology Makes Deadly Earthquakes Even Worse.' The page states: "The geology of Mexico --- and particularly that of Mexico City --- makes it a perfect storm for seismic catastrophe." And, why is this? It is because "Ancient sediments that underlie the city trap and magnify the vibrations that ripple through the region." A good explanation is given in the linked page for why this happens.
A page from Phys.org, linked here, is titled 'Soft soil makes Mexico City shake like it was built on jelly.' The page states this soft soil "amplified the shaking from Tuesday's earthquake and increased its destructive force." The page indicates that seismic waves which enter the softer soil area via the hard rocks below are then "amplified by the soil and sediments above," which makes things on the surface "shake longer and more intensely." The page states further: "The same deep soft soil effect worsened the deadly 2015 Nepal earthquake because Katmandu is also built on a dry lake bed."
The Phys.org page linked above states that "Los Angeles, Seattle and the San Francisco Bay area have soft soil that can amplify seismic waves," plus "New Zealand has been affected by similar issues in past quakes." Regarding Seattle, Washington, a King5.com page, linked here, is titled 'Why Seattle geology makes earthquakes worse.' A caption states: "Mexico City and Seattle have a lot in common when it comes to earthquakes. One example, the soil the cities sit on can actually amplify the effects and length of the shaking." The page also states: "Both Seattle and Mexico City are built on top of soil that's surrounded by rocks." That means that Seattle can take a real beating in a massive quake. Definitely not something to look forward to.
So, let us take a closer look at those earthquake-amplifying, sedimentary filled, "bowls-of-rock" which are beneath Seattle and other cities in the region. For those who have a relatively good grasp on maps, an image, linked here, shows the general contour of the rock-bowls which reside beneath the Seattle and Everett areas. Then, looking at the "much bigger picture" in the Cascadia region, a page linked here, shows the 40 to 60 kilometer deep Georgia Basin of northwest Washington and southwest British Columbia. This is a huge earthquake-amplifying basin which is filled with "soft glacial sediments and sedimentary rocks." An image and text, which are linked here, agree with this conclusion.
In all of this, there is something to consider. In the Cascadia region, there are many areas where buildings and other structures are built on sedimentary-type fill materials, which reside above harder bedrock. California also has many places where sedimentary-type materials are all that is supporting buildings and other structures. These materials can lose their strength and even experience serious liquefaction in a massive earthquake. This type of situation can allow even strongly built buildings and structures --- including bridges --- to tip sideways, fall over or simply collapse.
Looking into the Portland, Oregon area, regarding another matter, a KGW.com page, linked here, speaks of damage which likely could happen to the Interstate Bridge over the Columbia River, between Portland and Vancouver. It declares: "In a major quake, the drawbridge towers holding the counterweights would buckle, sending giant blocks of concrete into a free fall. That counterweight would come down just like a hammer, smashing through the bridge deck." But, there is more to this story.
Those falling counterweights would thoroughly smash any vehicles and people which happen to be on the roadway beneath them. Other vehicles may also slam into the concrete weights at freeway speed, as the large weights hit the roadway. The carnage in just this one area could be stunning, especially if it happened during a time of heavy traffic. Similar things could also happen in other cities in the Cascadia region, because of falling structures, etc. Seattle and other cities up and down the Pacific Coast and the inland Interstate-5 Freeway corridor all have their problems which make them rather vulnerable to great earthquakes.
Now, back to building on sedimentary materials and the Portland, Oregon area. Along the Willamette River, on a thick layer of sedimentary fill, is the six mile long, Critical Energy Infrastructure Hub (CEI). This is a six-mile stretch of fuel storage tanks and refineries, located right next to the Willamette River. What is worse, all this infrastructure is built on top of soft, sedimentary materials (link). The CEI holds a 3 to 5 day supply of petroleum products for the state of Oregon. In a massive earthquake, storage tanks may rupture and a large share of this stored fuel may be spread downstream in the Willamette and Columbia river systems. Furthermore, this fuel could be on fire. Up in the Puget Sound region of Washington state exists similar vulnerabilities, in the event of a massive quake.
Along the rivers and waters in many areas of the Pacific Northwest, and even in British Columbia, Canada, are many residential, commercial and industrial structures built on soft, earthquake-vulnerable, sedimentary materials. Research indicates that possibly the region around Seattle, Washington and Vancouver, British Columbia is no better off than Mexico City, in this regards (1)(2). The same holds true for may other cities of the Pacific Northwest region, and even in California, which are built near and along waters, or even on filled, swampy areas.
Let us now begin comparing who may actually have things worse. The recent Mexico earthquake was only an M7.1. A Cascadia earthquake, on the other hand, if it unleashes its full potential, could possibly be greater than an M9.2. That means a Cascadia quake at its full potential could be about 1000 times stronger than the recent quake in Mexico, and that quake in Mexico did a lot of damage. Therefore, in Cascadia, it may be wise to prepare for unimaginable damage and destruction --- along with numerous injuries and deaths, more than you would ever like to think about.
Looking further, after a massive West Coast seismic event, it may be wise to plan on surviving basically on your own for an extended period of time --- possibly even much longer than just two weeks. Those who have not properly prepared may be of little or virtually no use to anyone, especially to themselves, in this type of situation.
Let us now look at one particular location in southeastern California, to see their predicament. Let us consider the vulnerability of people and structures around the Salton Sea and surrounding region. Let us begin, by once again stating that much of Mexico City is built on layers of sand and clay which are up to 100 yards (or, up to 300 feet) deep. Remember this depth of sedimentary material, as we continue with this discussion. That depth of sedimentary material beneath Mexico City is very minor compared to the depth of sedimentary materials which exists in the Salton Trough --- in that area from the tip of the Gulf of California, northward, through the Colorado River Delta and Mexicali Valley in Mexico, plus the Imperial and Coachella valleys in California.
Almost one-third of the way down a page, linked here, is information on the Salton Trough. It states: "This formation, sediment nearly 9 miles deep, can trap earthquake energy and amplify seismic waves, resulting in longer, more intense shaking." Furthermore, in this area, people "live a couple of hundred feet below sea level with a 30 foot high pile of sand between you and the ocean." And yes, there is something else to think about.
If there is truly anywhere remotely near 9 miles deep of silt and sand beneath this area in which people live, it could potentially become rather unstable and possibly even somewhat movable, during a massive earthquake in the trough. In a state of severe and extended shaking, could this rather weak, sandy and sedimentary material still hold back the waters of the Gulf of California, which are a couple of hundred feet above the surface of the Imperial and Coachella valleys?
It is good to have at least a second or third opinion when looking at things like what is being dealt with, geologically, in a particular region and the depth of sedimentary materials which are presently all that is holding up your house or other structures --- especially when it is seismically-vulnerable sedimentary materials. A page on the San Diego State University (SDSU) website, linked here, states the following:
"The Gulf of California and the Salton Trough are components of a single geologic structure that averages less than 100 miles in width yet is more than 1,000 miles long. In this context, the Salton Trough is considered the landward extension of the Gulf. This point becomes clear when one considers that the Salton Valley exists only because the Colorado River delta is a natural earthen dam that completely excludes the waters of the Gulf. If it was not for that dam, the Valley would be submerged as far north as Indio."
The SDSU page also states: "The basement rocks under the north end of the Gulf are covered by as much as 25,000 to 30,000 feet of these deltaic sediments [from the Colorado River]." Well, 25,000 to 30,000 feet of sediments is only about 5 to 5-1/2 miles of seismically-shaky sedimentary materials --- which is far less that the 9 mile thickness of sediments noted in the page linked further above. But, even 5 to 5-1/2 miles of seismically-shaky sedimentary materials in much more than the mere "up to 300 feet" of sedimentary materials beneath Mexico City. So, it appears that people in the southeastern region of California could potentially be in a worse predicament.
A page associated with the University of California, Santa Barbara, linked here, states that "at the head of the Gulf of California, including the Salton Trough," there is "a vertical thickness of about 6,000 m (20,000 ft) of young sediments, mainly derived from ancestral Colorado rivers, has accumulated on spreading quasi-oceanic floor as the gulf has opened." Okay, so 20,000 feet of sediments in the Salton Trough is still about 66 times deeper in those seismically-shaky materials than that mere 300 feet of unstable sedimentary materials beneath Mexico City. And, just look at what has happened to Mexico city during serious earthquakes, over the years. Yes, there has been a lot of destruction and many people have died. So, what could it by like in southeastern California, if a massive earthquake were to hit in that region?
A Southern California Earthquake Center page, linked here, presents information relating to "the sedimentary basin in the Salton Trough, southern California." An earthquake simulation was done on a structural model of the Salton Trough. What did it show? The page states: "These simulations indicate that great amplification will occur during large earthquakes in the region due to the low seismic velocity of the sediments and the basin shape and depth." Further information relating to the seismic hazard in the Salton Trough region is found in a page linked here.
The writer wishes you all the best in the days ahead. Do yourself a favor and get properly prepared as quickly as you can. Nature waits for no one!
For further reading:
Current Expectations for a Cascadia Mega-Quake
https://cascadian.neocities.org/cascadiaquake.htm
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