September 21st, 2017

Two Mexico earthquakes

The terrible earthquake that occurred on September 19 in Mexico brings to mind the one that happened exactly 32 years—to the day—earlier. The 1985 earthquake had a large death toll, variously estimated to have been between 5,000 and 10,000 people, and over four hundred buildings collapsed.

I certainly hope the recent one doesn’t amass anywhere near that amount of destruction when all is said and done.

I remember the older one so vividly because I had a relative who was in Mexico City at the time, and it took us a while to find out she was okay. But, looking that earthquake up now, I found some information that may be relevant to what has happened there 32 years later.

The key thing is that Mexico City is built on an old lakebed:

Mexico City’s downtown area mostly lies on the silt and volcanic clay sediments of the bed of the historic Lake Texcoco, which are between seven and thirty-seven meters deep and have a high water content. Above this is a layer of sand and above this is a layer of sand and rock. The western and northwestern parts of the city are outside the old lakeshores and are located on sands from eroding volcanic cones that surround the Valley of Mexico. The southern part of the city rests on hardened basalt lava flows. The old lakebed, with its high water content, is easily moved or compressed. The old lakeshore area also has a fairly high water content, allowing movement, though not as much as the lakebed. The old lava flows have little water content or movement in comparison and are therefore stable.

Another factor is that the old lakebed resonates with certain seismic waves and low frequency signals. This lakebed has a natural “pitch” of one cycle every 2.5 seconds making everything built on the bed vibrate at the same frequency. This is the same “pitch” as a number of shallow earthquake waves. This resonance amplifies the effects of the shock waves coming from an earthquake far away.

However, only certain types of structures are vulnerable to this resonance effect. Taller buildings have their own frequencies of vibration. Those that are six to fifteen stories tall also vibrate at the 2.5-second cycle, making them act like tuning forks in the event of an earthquake. The low-frequency waves of an earthquake are amplified by the mud of the lakebed, which in turn, is amplified by the building itself. This causes these buildings to shake more violently than the earthquake proper as the earthquake progresses

I picture the phenomenon as being something like the famous Tacoma Narrows Bridge collapse. In that case it was wind that was the culprit:

After the 1985 Mexico earthquake:

A survey by the government of the damage done found that few buildings from one to five stories suffered serious damage; the same was true for buildings over fifteen stories. When the buildings were built seemed to have an effect as well. Before the 1957 earthquake, there were no building codes with respect to earthquake resistance. Some regulations were passed in that year and more in 1976 after another, stronger earthquake shook the city. However, none of these regulations had an event like 1985’s in mind when passed. Most of the seriously damaged buildings were built between 1957 and 1976, when the city was starting to build upwards, in the six-to-fifteen floor range. In second place were buildings from before 1957, possibly because they were weakened by the earlier earthquakes. Structures built between 1976 and 1985 suffered the least damage…

At the time of the earthquake, Mexico City had one of the most stringent building codes, based on experience gained from earthquakes in 1957 and 1979. However, the codes were not designed for seismic activity of the intensity experienced in 1985. The event was one of the most intense of any recorded in the world, allowing for macroseismic waves to arrive in the Valley of Mexico with unusually high energy content. Prior to the event, estimates about ground movement on the lakebed were generally accepted and a number of buildings were built on these estimates.

This year’s earthquake was of smaller magnitude than the one in 1985. So far, the indication is that fewer buildings have collapsed and although there is much loss of life, it probably will not compare to the 1985 toll (fortunately). But my guess is that the buildings that were affected in Mexico City in 2017 were similar in height and construction years to the ones that encountered the most trouble 32 years ago. Of course, each quake is different (distance from city, depth, magnitude, duration, and probably many other factors), and the resonance pattern might be quite different as well.

19 Responses to “Two Mexico earthquakes”

  1. Ymar Sakar Says:

    Time for some Soddom, Gomorrah, and Noah’s Ark, Divine Counsel.

    I know you entities up there want to slowly escalate up the hot pot temperature, but humans tend to be pretty dense on these matters.

  2. Mike K Says:

    The San Francisco 1989 earthquake had a similar effect on the Marina District which was built on fill. Many buildings sank beneath the liquified soil.

  3. Cornflour Says:

    I once heard that a seismic wave shaking Mexico City’s lake bed sediments was like someone shaking a rug to get out the dust. Of course, as noted by Mike K, soil liquefaction makes everything worse.

    The pre-Columbian city was built around lakes, and canals. Food and other common goods were moved around by water. If not for that, the city wouldn’t have grown so large. No wheeled vehicles were available. If goods can be moved only by loading them onto people’s backs, then the area covered by common trade in ordinary things is limited. The area of associated settlement size is consequently also limited, as is total population. Of course, smaller items that don’t spoil can be moved much longer distances.

    If the aquatic city hadn’t grown so large, then the Spanish wouldn’t have made it their capital. But the Spanish had horses, oxen, and wheels, so they filled in the lakes and canals and put up buildings on the fill. This greatly exacerbated the destructiveness of earthquakes. It goes without saying that the whole process was a plot by oppressive, patriarchal, white-supremacist, heterosexual, cis-gendered males. Without them, we’d never have Trump, which is always the most important point.

  4. Geoffrey Britain Says:

    I suspect there’s still a lot we don’t yet know about earthquakes.

    In the 30+ years I lived in SoCal I experienced one mild earthquake.

    My EX experienced the 1971 San Fernando earthquake. That earthquake knocked over much furniture in their house, including the bunkbed in her and her sister’s room which blocked the door. Her father had difficulty getting into the room, which as you can imagine was traumatic. She said plates and glasses were knocked out of kitchen cabinets and the house was a wreck.

    But… the house directly across the street had almost nothing disturbed…

    “the 1971 San Fernando earthquake occurred in the early morning of February 9 in the foothills of the San Gabriel Mountains in southern California. The unanticipated thrust earthquake had a moment magnitude between 6.5 and 6.7…

    “Damage was locally severe in the northern San Fernando Valley and surface faulting was extensive to the south of the epicenter in the mountains, as well as urban settings along city streets and neighborhoods. Uplift and other effects affected private homes and businesses.”

    A few years ago, I heard on the radio a gov. earthquake ‘investigator’ being interviewed about earthquakes and he said the people who most often survived collapsing buildings were invariably found next to something that prevented ceilings from collapsing fully upon them. Like someone happening to fall on the floor, against the back of a couch. Which caused a collapsed ceiling to rest at an angle providing an area of safety.

  5. F Says:

    My middle daughter and her husband and 4-year-old son live in Mexico City. She felt the quake, but said their was no damage in their neighborhood. She writes that she is very touched by the manner in which Mexicans from all walks of life are pitching in to help in the recovery effort, which she has also joined. She is also struck by the fact that unlike what happened here after the hurricane in Texas, she is seeing no reports of looting or vandalism. She is impressed with the fact that impoverished people are not stealing Adidas sneakers or breaking into half-destroyed houses to help themselves to valuables. I am too.

  6. parker Says:

    When the Madrid zone wakes up again, like 1811-12, the USA will be cut into 2 parts. The earth is a harsh mistress. We are “but a moments sunlight upon the grass”. Earthquakes, hurricanes, tornadoes, wild fires, droughts, floods, famine, and disase are par for the course. Only the strong survive.

  7. AesopFan Says:

    Sarah Hoyt’s blog carried an extensive series on the Madrid fault, analysis and explanations, etc.
    Very informative.
    The is the last segment:
    https://accordingtohoyt.com/2017/09/14/rock-and-roll-the-new-madrid-fault-system-part-vii-comparisons-by-stephanie-osborn/

    Ms. Osborn also had some great pictures of the eclipse:
    https://accordingtohoyt.com/2017/09/20/the-great-american-eclipse-from-the-centerline-by-stephanie-osborn/#comment-477839

    which led to this general link on eclipses:
    http://www.solareclipsetimer.com/

  8. Mark Drela Says:

    This may be picking nits, but the video is using incorrect terminology. The TN bridge was destroyed by an “aeroelastic instability”, also called “flutter”. This is not the same thing as “resonance”.

    Resonance is the amplification of already-present forcing vibration at a particular frequency, such as the shaking ground during an earthquake, or a very large platoon of soldiers marching in formation. There was no such forcing vibration present during the TN disaster, only a strong wind.

  9. Waidmann Says:

    This discussion reminds me of an explanation I heard many years ago for the walls of Jericho falling down when the Israelites simply walked around the city a bunch of times. The theory went that all the walking caused the vibrations, or perhaps “resonance” is more accurate, “and the walls came tumbling down”. I wonder if the ancient site of Jericho is on some sort of unstable ground.

    Waidmann

  10. om Says:

    Parker:

    Un, no.

  11. Ymar Sakar Says:

    Aesop, Doomsday stuff?

    Some people here don’t believe in Ragnarok or some kind of supernatural stuff. They prefer to rely on the Strong Horse Man in DC.

  12. Ymar Sakar Says:

    Om has gotten real subdued and quiet in his refutation of Parker there.

    He should go back to his hate centered life philosophy back when he was taunting Trum voters and supporters. That hate, while not being the True Hate that is powerful enough to kill the hosts of evil, is sufficiently corrosive that the soul fuel won’t last for very long.

    It’ll be a contest and sight to see, which one wins out. Human emotion or the human spirit. I’m also testing to see if OM is true to his word, whether he’ll keep scrolling past. Human words and promises ain’t what it used to be.

  13. Ymar Sakar Says:

    http://www.foxnews.com/science/2017/08/28/nasa-wants-to-prevent-yellowstone-super-volcano-from-destroying-us.html

    NASA is going to save us, haha.

    Now that’s funny.

    If divine punishment doesn’t spew the wicked people of this continent from its face, then the human handmaidens of evil just might “engineer” the collapse at their own timing.

  14. om Says:

    “Scroll by,” take a hike.

    The New Madrid fault isn’t going to split the North American craton. Nor will the next big one on the San Andreas cause California to fall into the sea. Or maybe you (not Parker) believe that Guam is going to “tip over?”

    So get a grip “Scroll By.”

  15. Ymar Sakar Says:

    Aw, om went back on his word, again, to scroll past. That’s all humans amount to in the end. Talk the talk, but can’t walk the walk.

    People don’t even have the integrity to obey their own made up rules, let alone the nation’s rules. Not that I advocate obeying the rules of human laws.

  16. om Says:

    Scroll By:

    Keep making it up as you go along, it’s just the way you roll. Most just roll their eyes and scroll by I guess. Continue to yammer and rant, you have my permission.

  17. Ymar Sakar Says:

    Don’t forget that it was Om that said he would scroll by Ymar’s comments, as they were unworthy, tl;dr, or some such tantrum.

    It wasn’t a neat nickname you had come up to taunt; it’s like somebody forgot about the whole matter.

    I’m merely holding your feet to the fire on the premise of your own words. Not that they actually mean anything, as proven recently.

    Why would anyone need your permission to write anything here… but do you need your own permission to adhere to your own promises and statements, maybe not.

  18. Ymar Sakar Says:

    Om, you should follow and obey your own rules, even if you taunt and belittle everybody else here that stood in your political religious way.

    Make it easier on us, and just scroll past the comments you don’t like. It would save everybody a little bit of their soul.

  19. neo-neocon Says:

    Ymarsakar; om:

    Please desist from the personal sniping.

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Previously a lifelong Democrat, born in New York and living in New England, surrounded by liberals on all sides, I've found myself slowly but surely leaving the fold and becoming that dread thing: a neocon.
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