Rock and Roll: The New Madrid Fault System Part III: The Modified Mercalli Intensity Scale By Stephanie Osborn

Rock and Roll: The New Madrid Fault System Part III: The Modified Mercalli Intensity Scale By Stephanie Osborn


Rock and Roll: The New Madrid Fault System Part III: The Modified Mercalli Intensity Scale

Excerpted from Rock and Roll: The New Madrid Fault System, ©2017

By Stephanie Osborn

Images in this article are public domain, unless otherwise specified.


This whole collection of writings started off with an email exchange, months ago. Our illustrious hostess was part of the discussion, and expressed interest in my converting the info to one or more blog articles. Then, when the LibertyCon programmers heard about it, they asked me to give a presentation on same, which I did.

The presentation was a full house, and at the end, there was a request for me to convert it to blogs and/or an ebook. I asked how many would like to see an ebook of the material; virtually every hand in the lecture hall went up.

A little over a month later, with additional research under my belt and factored into the manuscript, the book has gone live. And as promised, I am providing Sarah a series of blog articles on the subject. This series of blog articles is only a small fraction of the material contained in the ebook; it may be considered in the nature of a series of informative abstracts of the information contained therein. For additional information, may I recommend that you check out Rock and Roll: The New Madrid Fault System.


Part III: The Modified Mercalli Intensity Scale

The intensity of a quake is defined by its effects on Earth’s surface. The Richter scale, long the definitive measure of an earthquake’s strength, is a measure of quake energy, and even that has been recently superseded by the moment magnitude scale (as in recent years, the Richter scale was found to be inaccurate for the most powerful quakes), but before the advent of seismographic data, geologists developed various means of estimating quake intensity based upon the damage produced. The one that eventually survived is the Mercalli Intensity scale. It was originally developed by Italian vulcanologist Giuseppe Mercalli in 1806, and later modified to accommodate new data.

According to the U.S. Geological Survey, “The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally — total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

observed effects

[Modern quake damage to chimney; Mercalli VII]


“The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place.

that place

[Modern home with crawlspace, shifted off foundations with partial collapse;

Mercalli VIII-IX]

“The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above.”

The Modified Mercalli Intensity Scale

Intensity Shaking Description/Damage
I Not felt Not felt except by a very few under especially favorable conditions.
II Weak Felt only by a few persons at rest, especially on upper floors of buildings.
III Weak Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
IV Light Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V Moderate Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI Strong Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII Very strong Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
VIII Severe Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX Violent Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X Extreme Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

san francisco

[San Francisco, 1906; Richter mag. 8, Mercalli XI. ]


The USGS adds, “Another measure of the relative strength of an earthquake is the size of the area over which the shaking is noticed. This measure has been particularly useful in estimating the relative severity of historic shocks that were not recorded by seismographs or did not occur in populated areas. The extent of the associated felt areas indicates that some comparatively large earthquakes have occurred in the past in places not considered by the general public to be regions of major earthquake activity. For example, the three [principal] shocks in 1811 and 1812 near New Madrid, MO, were each felt over the entire eastern United States.”



To obtain a copy of Rock and Roll: The New Madrid Fault System by Stephanie Osborn, go to:

116 thoughts on “Rock and Roll: The New Madrid Fault System Part III: The Modified Mercalli Intensity Scale By Stephanie Osborn

    1. Yeah, well, I’m on the VII side of that border.

      If another quake of that intensity hits in the same place, I’d say most of downtown Cincinnati would be a death trap, since I’m pretty sure no one builds with earthquakes in mind around here.

      1. …also realize that, if the next “Big One” on the New Madrid is NOT epicentered where that map shows, but is farther north or south, the intensity contours will shift accordingly.

        1. I assume that a lack of reporting observers to the west of the epicenter accounts for lack of contours that way. Or, Minnesota won’t be spared in the next one, either. Right?

      2. I actually got it a long time ago, but I’m still enjoying this series.

        What a lot of people don’t think about are the bridges here. I don’t know if any of the Interstate Highway bridges are capable holding together. And that’s a major shipping corridor. The disruption to distribution of all kinds of things all along the mideast and quite a bit of the eastern states would be royally screwed.

        1. Oh, it gets worse than that. The various highway departments, etc. are unsure if ANY of the bridges across the central Mississippi River will remain standing. That’s a major disconnect in ground shipping and transportation between eastern US and western US.

          1. one in the right . . . er . . . wrong place and crossing will be limited to some bridges in Minnesota/Wisconsin, maybe Dubuque and Prairie Du Chien, and the New Orleans area. That’d be a slight issue, very slight . . . like the ice age was slightly cooler slight. Well if the mud in Baton Rouge isolates it instead of liquefying, US61 might be the only hwy with two routes over the river.

            1. Well, I don’t think the shaking in Baton Rouge would be such as to liquify. Louisiana noticed it but not sufficiently to sustain much damage. It’s a ways downstream.

              But yes, the FEMA worry is that you’d have to go to either the extreme southern Mississippi, or the extreme northern, in order to find an intact bridge.

              1. yeah, the tales I recall from living in NOLA and Kenner were of the river going the wrong way afterwards, and nothing about the shakes. But their houses withstand a good shaking from the winds so not much was gonna damage them that far off.
                I actually worked in a few houses that were around for that quake. big two story places with 16foot high ceilings.

              2. This. I was going through my mind chimneys from prior to 1811 as a sort of reference, then realized that there just weren’t enough. The oldest I knew off locally was from 1816 – 1818. The country just wasn’t heavily settled at that point.

          2. A lot of folks aren’t aware, but an enormous amount of goods and raw materials still travel by barge up and down the Mississippi and major feeder rivers. Drop a couple of bridges and that traffic stops cold as well.

            1. There is also the matter of bridges across said tributaries, to include major rivers such as the Ohio, the Tennessee, the Cumberland, and the northern end of the Tenn-Tom.

            2. That’s true, but I would imagine that it wouldn’t take TOO much effort (comparatively speaking – fixing other things taking much more work) to make safe passages through downed bridge debris.

              1. If that were all that had to be dealt with, you’d be right. But do you remember the stories of Mark Twain, and all of the “snags” in the river? Those “snags” were dead trees, half-buried logs, and the like that had fallen into the river along with the various landslides from the banks in past quakes.

                Another quake big enough to take the bridges down is also going to add to the “snags” in the river.

                1. Well, I guess part of that depends on whether the dams survive, because the Ohio, at least in the general portion where I am, is about 15 feet higher than it was in Twain’s day (my dad said that one really dry summer, the river got low enough that you could wade across – no way it would come close now), so if the level remains the same, it would be more forgiving of such snags being dropped into the river.

                    1. Yeah, I know. I realized I’d typed the homophone about the time I hit the return key. Then I stared at it long enough to confuse myself, then I posted the correction.

                      I hate it when that happens.

                    2. “Levies are what the government says it employs to raise the money to pay for the damn levees.”

                      Even a cursory inspection of the books requires that correction.

                    3. Can you spell Atchafalaya? I live in Louisiana – so I can. It would probably take much less of an earthquake on the New Madrid complex than most think to cause an *amazing* amount of trouble for Louisiana. Old Man River does *not* like how we’ve locked him down, and desperately wants to do a different way. And once he does, there’s a 18.2 mile Atchafalaya Causeway that’s going to be very very unhappy, blocking I-10. (and it’s Levees, you were right the second time.)

                    4. I can spell it too. 🙂

                      No, no. I get that you think it would be horrible, but look at where it is on the Mercalli map for the 1811-12 quakes. It’s in the II-III range. That means, at the maximum, “Felt quite noticeably by people indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.” And unless the bridge is awfully poorly made, that’s not going to affect it, and that shouldn’t generate any significant liquifaction.

                      Will it be felt? Probably. Will it cause damage? Unlikely. Will people even know what it is? Chances are, they will not.

                      Now if the thing starts off at the extreme south end of the system, it might be a bit stronger for those in Louisiana. But 1) the main axis of the graben actually tends to direct the energies AWAY from Louisiana, along a SW/NE axis, 2) the most likely site of epicenters of substantial quakes is near the focus of the grabens, and that is up in the area between New Madrid, MO and Reelfoot Lake.

                      Now, all that said, the levees all along the Mississippi are gonna be a problem. But the worse problem is likely to be well upstream of Louisiana.

                      In the next few installments, you’ll get to see some of the eyewitness accounts, and you’ll realize that in the main, New Orleans, for instance, didn’t really know anything much had gone on, though they felt a bit of shaking. (The Mobile area felt it too, but again, was not damaged.)

            3. Yup.

              I recalled that in this decade there had been a serious drought which effected the Mississippi. This had a serious effect on movement of trade goods in the country. Looked it up on-line. the following excerpts are from two article, among many:

              From an online Business article in the Saint Louis Post-Dispatch from that summer, dated July 17, 2012:

              Operators up and down the Mississippi, the main artery of the nation’s 12,000-mile inland waterway system — and a conduit for millions of dollars in goods to ports around the world — are tying up their fleets. Others are loading less tonnage on more boats, managing to ply the water with lower weight.

              And from the Washington Post, January 6, 2013:

              On a stretch of the Mississippi River, the U.S. Coast Guard has been reduced to playing traffic cop.

              For eight hours a day, shipping is allowed to move one way in the 180 miles of river between St. Louis and Cairo, Ill., depending on the hour. For the other 16 hours, boats go nowhere, because the river is closed to traffic.

              If drought limiting shipping was a problem, what would happen if the river was closed altogether?

            4. A solution is pontoon bridges, even some cobbled together on barges, but DOTs seem reluctant to do such. It would probably be heavily restricted. I guess it falls under the category of how bad do they want it.

              1. The problem with pontoon bridges is that a line of floating barges overtopped by a solid roadway is a pretty effective barge barrier too. The bridge has to be either high enough to let barges under, or easily movable enough that it doesn’t take a couple of hours to open and close it.

                  1. An awful lot of traffic passes over the Mississippi. Attempting to ferry that traffic would be a nightmare.

                    What I could quickly find …

                    I have read that I-40 is one of the busiest interstates in the nation. It crosses the Mississippi at Memphis.

                    In Memphis in September of 2007 the Hernando de Soto bridge which carries I-40 across the Mississippi at Memphis was closed for examination due to structural settling. It was found that only west bound side needed repairs, and the east bound side was reopened. According to local news reports at that time some 35,600 vehicles traveled the bridge westbound daily from Memphis to Arkansas. The east bound traffic was diverted to I-55 and the Memphis-Arkansas Memorial bridge — which already carried similar traffic loads on a normal day. I doubt the numbers have decreased.

                    Now admittedly, if there were a major quake on the New Madrid line there would be a decrease in traffic.

                    1. Perhaps we could use pneumatic cannon to launch vehicles across the river?

                      I acknowledge some minor details would want to be worked out, such as a reliable means of “capturing” them on the receiving side, but those seem trivial when you consider the ease of quickly reestablishing cross river traffic.

                    2. Note that I said, “alleviate”, not “solve” the problem.

                      The problem in Cincinnati would be far worse. As I pointed out above, the main interstate bridge, which carries I-71/I-75 across the Ohio river, carries over 35,000 TRUCKS per day. The total traffic number is 185,000 vehicles per day (which is actually down from 2005, when it carried 196,000/day).

  1. [T]he Modified Mercalli (MM) Intensity Scale … does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

    There’s your hard science, right there.

    1. Yes, but it is well-founded on observed effects, and when we are talking about areas where there are NO seismographs, or in time frames where there are observations, but before the seismograph was invented, it’s really all we have to work with. And it has been cross-correlated for consistency. These days, one can actually roughly translate the central Mercalli intensity into a Richter magnitude. Don’t make the mistake of thinking that, because it’s “arbitrary,” it doesn’t have a close correlation to the real world.

      1. To perhaps stretch things a bit, Mercalli is sort of like Beaufort (wind speed)? Observed effect(s) that can be translated/correlated to more standard or familiar measures?

        1. Actually that’s a pretty good analogy. You have the Beaufort scale for the wind speeds and their effects, but it doesn’t tell you about the strength of the tropical system, only that at this point you have a wind speed in this range, generating this level of effect. The Saffir-Simpson scale measures the central sustained winds of the cyclone and gives you the intensity of the hurricane.

  2. Once again, thank you Stephanie Osborn for your guest post series. I am enjoying it very much.

    I recall reading articles after the 1989 Loma Prieta earthquake which explained that the level of destruction could be effected by the land on which the building were built. This was the reason given for why those buildings in the area developed on land fill in the San Francisco area received more damage than buildings in nearby areas.

    Question: How does the Mercalli system allow for such circumstances when analyzing the effect of an earthquake?

    1. The Mercalli system does not, as such. That’s why, if you are trying to translate it to Richter magnitudes, you go for the Mercalli intensity at the epicenter, and then try to take things like that into account.

      Remember, the Mercalli Intensity scale is simply a means of estimating the degree of shaking at any given point, NOT a means of determining overall quake energies, like the Richter scale.

    2. and I’ll point out if there is another major quake of any intensity in San Francisco, the neighborhood that is likely to be most damaged is the new million+ dollar condos that are built in a landfill.

        1. Our previous house was 50 miles nearer to the Loma Prieta epicenter than SF, our current house a few miles closer. Neither took appreciable damage (in our previous place, we had a couple of bookshelves fall over).

          But both are several miles off the known active fault lines, built on solid ground with a fair amount of underlying rock, and bolted to their foundations. The most damaged neighborhoods in the Loma Prieta quake were on or close to the fault line, and built on landfill. There’s a big difference between building where the land is solid and moves as a unit, and where it acts like a thick pudding when shaken.

    3. Our house was 11 miles from the epicenter of the Loma Prieta ‘quake, in fact you can see Loma Prieta peak from our back yard. The Marina district (an part of San Francisco built primarily on fill over what used to be part of SF bay) is about 55 miles away from the epicenter.

      While the event was quite sporty around our place, there was very little damage beyond things being tossed off shelves in local groceries. The Marina district got hammered.

  3. I really don’t like the Mercalli System because it does seem very subjective and dependent on a lot of factors very hard to analyze beforehand.

    1. It actually is NOT subjective, but quite well defined. Geologists using the scale have sets of checklists, as well as images of typical-for-that-intensity-level damage, to go by. In areas without much in the way of seismographic systems, this can be the first estimate of intensity, and even with worldwide seismic systems giving estimates, it is sometimes the means of refining those estimates.

      Remember, anything approaching a modern seismograph didn’t really get developed until around 1890-ish, over 80 years after the historic quake series we’re about to discuss in subsequent installments.
      And the seismograph system for the area didn’t get set up until the 1970s, over a century and a half later.

      Look at it like this: when you step on a balance scale at the doctor’s office to be weighed, your actual weight, the gravitational force produced by Earth pulling on your mass, is NOT what’s being measured; your weight is being compared to a set of calibrated weights (masses) on the scale. The better calibrated those weights are, the more accurately your own weight will be known. But you wouldn’t say that your weight is subjective.

      So too is the Mercalli index calibrated. No, it doesn’t measure the earthquake’s energy directly, but it still gives a reasonable indication of the size of the quake. It isn’t a direct measure, no, and trying to convert to Richter or moment magnitude intensities is very difficult and requires knowing a lot of factors. Sometimes, due to too many of those factors, it isn’t possible. But a collapsed house is still a collapsed house. And you don’t want to be in it when it collapses.

  4. When I was in California, I experienced 3 quakes. One was at home; sounded like someone hit my house with a bat, twice. Another was on the freeway, and I didn’t feel it. The last one was when I was standing in a building, and felt dizzy for a few seconds, and was told about the quake about a half hour later, explaining the dizziness.

    1. Yes, the “baseball bat” would have been the initial shock wave, the so-called P wave. You didn’t feel it on the freeway not because the freeway wasn’t moving, but most likely because your car’s shocks compensated for the movement. And in the building, it sounds like you felt elliptically-polarized Rayleigh waves, which can be very hard on tall vertical structures. A large-enough-amplitude elliptically-polarized wave can cause collapse in such structures, due to the fact that, as the wave propagates up into the structure, the base of the building is moving in a different direction from the middle of the building, which is moving in a different direction from the top of the building.

      1. I grew up outside Clarksville TN which is rock-chunking distance of Reelfoot Lake and the northeasternmost graben. We used to regularly feel the quakes when I was growing up. It wasn’t unusual for us not to consciously feel the quakes, only to see effects, such as standing waves in glasses of water, milk, tea, or the like; or the dining room chandelier, or kitchen table hanging lamp, swaying back and forth.

        I recall one good-sized temblor when I was a kid; I remember it clearly because it was Saturday morning and I was watching cartoons. The house shook so badly it nearly threw me out of the armchair to the floor. All of the pots, pans, and dishes in the cabinets behind me made a cacophony of rattling, and the quake itself sounded to my young ears like giants rolling boulders down the roof. Except the sound came UP, not DOWN, if you get me. Oh, and it plainly came in from behind me, which was roughly north, so I could even approximately determine the direction of approach.

        My parents were outside working on something, and they said they saw the house move, swaying, with spalled mortar and dust (brick home) falling from the walls. They made me come outside immediately.

        We took the New Madrid fault system VERY seriously when I was growing up.

        1. We observed similar auditory effects in the aftershocks post Loma Prieta. Sounds similar to an approaching train (local tracks were about a half mile west of us) became apparent before the ground noticeably started moving.

    2. The most severe earthquake movement* I have ever experienced was from the one on the Eastern Seaboard a few years ago, that damaged the Washington Monument. I was in Cincinnati. No one I talked to who was on ground level felt anything, but on the 20th floor of the building I work in, we experienced some disturbing levels of movement (for people not used to earthquakes), [putting it at a Mercalli intensity of about II). It went on for about 30 seconds.

      * The most intense building shaking I have experienced came from a jet airplane with its engine exhaust pointed directly at my house, when I lived right next to the airport.

      1. Here in the NC Piedmont (about 30 – 40 miles NW of the state’s center, aka the NC State Zoo … not to be confused with the state legislature, which meets about 75 miles East of here) the quake’s effects were notable on the second story where the first reaction was “has the Washer once again gone walkabout during the rinse cycle?”

      2. I was living in West Virginia when that one happened. I sitting in my (ground floor) apartment, working at my computer. I thought I felt a dizzy spell, then something rattled, and I thought, “no, I’m not dizzy…it was everything else that moved. Hey, I bet that was an EARTHQUAKE!”. First one I ever recall experiencing. I’d put it about III.

    3. I’ve lived through a significant and a serious quake. I don’t know the numbers because it was in Portugal and for first one (some houses fell) I was 3. For the second, it hit while I was on the stairs and sent me flying to the bottom…

      1. Any idea how well-built the houses that collapsed were?
        And I hope you weren’t hurt when you fell down the stairs. I’ve done that before, when I was younger, and came away rather bruised, but nothing more serious. Any other info on structural damage in that second quake?

        1. The second quake didn’t cause much damage. Part of the reason I fell was I was wearing socks and doing my usual rush down the varnished stairs. Nothing bad, I just bruised my butt, which is what I landed on. As in, had to stand for classes for a week or so.
          First one — most village houses were well-built. As in massive blocks of stone, fitted, and massive beams holding roof down. So it was pretty significant for a couple to fall down.
          I no longer know if fissures opened on the street or people THOUGHT fissures might open on the street. I was three and slept through my brother grabbing me and rushing out of the house, and through the entire village congregating on the street in the middle of the night, so anything I know is hearsay.

    4. felt 3 when I lived in Texas. one at work, I was on a second story floor in a room filled with metal industrial shelving loaded with 500ml bottles (product QC samples). How none fell over is pure luck, they did a heck of a dance. co-worker on different shift had items fall off the mantle at home, and a window crack.
      One of them when I was at hone was actually higher than the one at work, but further away. The neighbors were out in the horse barn, and couldn’t figure why the horses were acting up.
      The other at home was only slightly noticeable to me.

      1. Was in Colorado once, up in the Front Range west of CoSpr, when they had a quake. A friend of mine and I were trying to bring up some equines to go on a trail ride, only they were twelve kinds of going nuts and doing weird crap. My friend, the more experienced horsewoman, decided that yeah, we could push through and go, but it wouldn’t be a pleasant ride with our mounts acting out like idiots, so we chose to abort the ride.

        We later found out there had been an earthquake epicentered only a couple of miles away from that ranch.

        Animals are definitely more sensitive to the effects than we are.

        1. I had two small cats in the ’80s, and while they never acted any odder before an earthquake (they were definitely Odd), they were interesting during and after.

          For a mild shaker (Richter 5, maybe IV on the MM scale), they’d dig claws into carpet/pants/people legs and hang on, then would run for shelter.

          For the Loma Prieta (strong VII at home), they ignored the roast beast that moved from a slow cooker on the counter to a mess of the floor and spent the next few days on the roof. Those cats passing up free food was unusual, but flying beef was a bit much.

    1. Yes. The only real reason we don’t have an isoseismal map for the west is because there were so few people in the area to provide reports, and so few “modern” structures against which to compare the damage. There were a handful of mountain men and explorers, a VERY few odd white settlers, and fair numbers of Native Americans, mostly in tepees, the majority of whom didn’t really “get” what was going on, judging by the reactions of those who lived in the near vicinity of the epicenters. And only the settlers might have anything resembling a permanent structure against which to compare damage.

        1. Oh no, we have enough anecdotal reports to have some idea about that, especially given the knowledge of the geology. Don’t worry about the New Madrid setting off the San Andreas, because it’s going to attenuate in the fault-block mountains before it gets there.

          1. Damn, there goes Arizona’s next chance at ocean front property!

            *shakes fist at Gadsen for getting cheap at the last minute*

            1. No, that will never happen. The San Andreas is a strike-slip fault. Remember Part I of the series? A strike-slip fault moves horizontally, not up and down. SW California and Baja is gradually moving north as the main body of California moves south. Eventually Los Angeles will be a suburb of San Fran. (Don’t hold your breath waiting.)

              1. i forget the numbers but i figured at some point a long time from now, Juneau will be a suburb of L.A.

  5. Just a note on photos as above of the 1906 SF quake aftermath: While it’s true the SF press played up the fire and downplayed earthquake damage in order to not undermine the city’s reputation in newspapers back east, there was in fact a honking big fire after the quake, and those fighting the fire did a fair amount of dynamiting of buildings to try and contain things. Combine that with the widespread use of unreinforced brick construction and you get possibly an overlarge amount of destruction vs. modern construction locales using the same scale.

    To get an idea of how much destruction there was, one of the stories in this area is that the “used bricks” used in the 1950s-1960s housing boom to build chimneys and such in the San Jose area were the tail end of the bricks salvaged from the city of SF after 1906.

    1. All true, based on my studies.

      But there were some (primitive by modern standards) seismographic instruments in the area, so there’s not so much need of using the Mercalli to determine intensity.

      1. It’s been way too many years, but Lick Observatory near San Jose, CA has a seismograph running. I can’t remember which earthquake was on the displayed graph; might have been the Loma Prieta. I lived in San Jose for that event, and without the stainless liner, my chimney would have collapsed like most of my neighbors’ did. (It was shot, and had to be rebuilt when we sold out in the early 2000s.) That neighborhood was built out in the 1930s. 5-10 miles closer to the epicenter and it got really lively.

        As I recall, the damage in SF was partly due to reflections of the subsurface waves from underlying rock. There were some landfill/alluvial soil areas closer to the epicenter that fared much better.

        Our area in Oregon had a big quake in the mid 90s, but the fault system is pretty poorly mapped around here. We secured our furniture as best as we can, but wildland fire is a more immediate concern…

        1. I have an undergrad minor in geology, and a graduate subspecialty in same. The only reason I don’t have a bachelors in it is because I needed to get my behind on into grad school and get with that; if memory serves, I had somewhat more than enough credits for the minor anyway. But I studied seismology, among other things, and I proved fairly good at it (partly because I’d already had a full year of optics in the physics dept).

          Now, whenever there’s a media interface, waves will tend to both reflect and refract. And when they do, they tend to develop multiple polarizations. (This is why polarized sunglasses work so well for reducing glare.) The trick is that there are slightly more ways to polarize quake waves than there are light waves (because light can’t generate acoustic-style compression waves). And another trick is that whenever there is an interface — due to, say, a different rock stratum — you get that exact thing happening: reflection/refraction/polarization. So EVERY TIME the quake wave hits a new rock layer, you get this. Which is how seismologists can get cross-sections of stuff underground and tell you what layers are where. Now, the conversion that takes the raw data to the cross-section is very complex, and learning to read the resulting cross-section isn’t easy, but I got fairly decent at the latter, at least. (The former is generally done using computer algorithms. Fourier transforms figure heavily in most instances, along with other types of calculations.)

          More, certain types of wave do not transmit through fluid. So we can tell you there’s a water pocket down there, or we can say there’s hot magma in that chamber, or there’s NOT hot magma in that chamber.

          It’s really fascinating. Or at least I find it so. I visited Mt. St. Helens a few years back, and the Johnston Observatory (volcanic observatory, on the ridge to the north of the volcano, where the blast went) has several drums, recording the movements detected by seismometers on the slopes of the volcano, 4.3 miles away and radioed back to the recording drums. I stood there watching, with one eye on the drums and another out the window at the volcano — because I’d never seen a phreatic eruption on seismograph before, so I had to verify by watching for the steam/ash plume. So pretty soon I was watching and calling off the type of event. “Rock fall…rock fall…quake…phreatic vent…rock fall…rock fall…” and so forth. Mind, I wasn’t yelling or anything. I wasn’t even speaking in a normal tone of voice. I was talking to myself, mostly under my breath; I didn’t want to disturb anybody.

          What I didn’t know was that one of the park rangers was standing behind me, watching and listening. He came up and said, “So you really know how to read these things, huh?” I explained that, while I wasn’t an expert, yes, I had some knowledge, and this was my educational background and that was what I did for a living, etc. Then we moved over to the diorama/map of the volcano and environs and discussed how everything “went.” (I clearly remember the eruption. I’m older than dirt. Some dirt, anyway.) His response was, “Wow. Then I’m gonna send the tourists with questions to you!” Turned out HIS educational background was forestry, and he’d had to learn all kinds of stuff when he got assigned to the volcanic observatory…

          And yes, he did start sending the other tourists over to me… 😀

  6. Best examples of Level’s IX and X I have seen in person was the aftermath of the Schezuan 2008 earthquake. Visited the destroyed city in 2011 and looking at buildings that are literal shells where the front and interior collapsed with floors hanging like laundry on a clothesline. Or fields of rubble that used to be factories and buildings. Looking to the surrounding mountain sides and seeing places where the mountain gave away and fell. It was simply awe inspiring.

  7. One of the Army Corps dams on the Republican-Big Blue-Kansas river system sits atop a fault in KS. The fault has a history of moderately strong quakes (estimated R 6.8). The hydrology and geology folks familiar with the situation have a unofficial betting pool about if the dam will liquify in a quake or if the reservoir will silt up before the next temblor. Last I heard the reservoir silting up was favored 3:1.

    1. The Humboldt Fault, perchance?

      FWIW the Humboldt is part of the Nemaha Ridge, which is a section of the ancient Keweenawan Rift, also referred to as the Midcontinent Rift System. This is an older system than the Reelfoot Rift, and came much closer to tearing apart the craton — it almost wound up ocean. The only thing that stopped it was the development of the Grenville Orogeny, a mountain-building collision of continental plates on the now-eastern side of the plate, which forced the rift closed.

      1. Can’t be. Humboldt swore he was in Germany—or was it Latin America?—and had nothing to do with it. So it’s most certainly not his fault. 😉

        Yes, that’s the one.

        1. Some of the people I know in western Kentucky joke about having a pool for how far into Illinois across the Ohio bits of Paducah will reach, if one or both dams at Land Between the Lakes fail due to a New Madrid quake.

        2. I remember learning in my AP American History class that this was something that government just *had* to do, because it wasn’t economically viable for all those power companies to figure out how to produced energy for farmers and then put out electric wires to all those sparsely populated farms.

          I accepted the claim “only government can do it!” for years, but just a couple of years ago, I was thinking about this, and it occurred to me: “You know, I think the conventional wisdom is wrong on this! Yes, perhaps conventional power plants would never have sent wires to all those farming communities. But perhaps that wasn’t the most efficient way to get electricity to farmers. Perhaps small coal-powered plants would have been the way to go. Or perhaps generators for each farm. Or perhaps even something like some sort of ethanol or biofuel brewed on-farm or in the nearest town. Or maybe even something else entirely…. But because government stuck their nose in the matter, we’ll never know for certain!”

          1. AP History — gee, can’t imagine any reason for them to endorse and justify intrusive government.

            Perhaps somebody somewhere has written a seriously entertaining history of the Edison/Tesla battle over whether AC or DC would rule the nation’s electricity supply, but I don’t recall ever seeing it. There may have been a History Channel (possibly PBS) miniseries about it, but if so that is the sum of my recollection of it.

          2. Meh. That was an excuse. TVA came about during the Great Depresssion when so many people were out of work and FDR created his New Deal. Which should tell you a lot about the politics. But since it was in the South with those good ol’ practical redneck engineers like Uncle Timmy eventually put in charge of running it, it actually worked.

          3. You’re overlooking a couple of things.
            1. Economies of scale. When producing a mass volume good (and energy is such), combining all the fixed costs in one package is cheaper. Those fixed costs include the extra skilled labor to keep each individual (home or small town) generator functional.
            2. Technology: your individual home installation literally wasn’t possible when TVA was being built. Even if there was such a thing, it certainly wasn’t widespread especially n rural America. My family in rural AR was still using a mule drawn wagon in 1936, and we have photos to prove it.
            3. Legal: Once you get above the individual house, you get into issues of power line right-of-way, permission to build, acquiring land, etc.. These things require some sort of government intermediary, otherwise one stubborn farmer is either allowed to derail the whole project, or he and his family get to flee / die from a mob of neighbors.

            Government is both necessary and evil.

  8. That number seven line passes right through my hometown as it goes from Illinois into Indiana. ‘Course, I am presently on the south eastern coast of Virginia, so more worried about Irma than Prince Roger’s father. And will, in about 8 months time, be back on the Pacific Rim for all the fun and games over there.

      1. I have a bunch more in that album that I took there. Very scary walking through the site. Currently it’s a heritage site to show the power of large earthquakes.

  9. The only earthquake I’ve actually felt:

    This was in Decatur IL, while spending the weekend at an amateur bowling tournament with some friends. I had left the lounge at the Ambassador Hotel and was in bed watching television when I felt the shaking. My estimate is Mercalli III intensity at that location, though it could have been intensity II since I was on an upper floor and resting in bed. Enough people felt it regionally that it made the local papers.

    I have family who experienced the 1989 Loma Prieta quake In San Francisco. Their home was on solid ground, and experienced minimal damage though stuff bounced around and fell off shelves.

  10. I see the San Francisco quake is assigned a XI, but the table only goes to X. Any suggestions about where I can go to find out about higher numbers, and about how structural engineering contributes to the evaluation?

  11. Weirdly, I can’t find an Earthquake online for 1965, Portugal. I find one for 1969, which is entirely possible, as I tend to agglutinate ages in memory. If it was that one, as seems likely, it was a 7.8.

  12. Off topic:


    At 09:10UTC/04:10amCDT this morning, sunspot group 2673 produced an X2.2 solar flare (X class is the strongest currently extant), with possible associated CME, which would be Earth-directed if produced. (Still looking to see if it produced one.)

    At 12:02UTC/7:02amCDT the same spot group produced an X9.3 flare, with confirmed Earth-directed CME. The high-energy photons have already ionized the upper atmosphere and have caused radio blackouts on the Sunward side (day side) of Earth.

    The Carrington event, while producing much stronger flares (est. X100), was essentially a double-barreled event, with the first CME clearing the interplanetary medium in front of it, allowing the second CME to come in much faster and more energetically.

    The first X class flare did NOT produce an Earth-directed CME., But if it had, then a similar event could have happened here. In any case, expect major geomagnetic storming sometime in the next few days. A CME will affect the entire planet, interacting with the magnetic field mostly, but to a lesser extent the atmosphere as well. There is no “average speed” for a CME; their velocities depend on numerous factors. Expect the “impact” later this week (2-3 days), with aurorae down to at least mid-latitudes. I’m in northern Alabama and I’ll be going outside at night to look.

    DO NOT PANIC. Recommend being prepared to throw your laptop in the microwave, and unplug the microwave. Some high-latitude power blackouts or brownouts could occur.

    Here is a video of the X9.3 flare, taken in several different wavelengths, by the Solar Dynamics Observatory space-based probe.

    (To our esteemed hostess, I also will post this as a comment to today’s blog, in order to ensure folks see it.)

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