huangyhg

crazy seismic question
i am designing a one story, wood framed, "prototype" coffee shop (that is, it can be built anywhere in the country). it turns out that florida wind loads governed the lateral analysis, but i came across something curious when checking seismic loads.
unless i am reading the maps in ibc incorrectly, the "worst" seismic situation (based on the highest ss and s1) is not in california at all, but rather at the new madrid fault in missouri. is this correct?
daveatkins
i believe you are correct! surprising, isn't it?
i have never figured out how that works, how you can get a seismic hotspot in the middle of the tectonic plate.
there is a similar situation in the middle of australia but it is not as active and the population is minimal.
well, the new madrid area experienced a major, major earthquake in the 1800's, i think, but as far as i know, nothing since then. i've heard the earthquake in the 1800's changed the path of the mississippi river.
daveatkins
there are small ones in that area every few years, northeast arkansas experienced one a few years ago......4.0 magnitude if a i re
i am from missouri and it is high. it has to do with soil conditions (horrible - maybe a bit better than peat in some areas) and of course the fault. there is a very nasty fault line running thru that area. starting in about 1810 this area suffered the worst earthquakes maybe ever experienced in the world. some said they might have been as high as 10 on the richter scale!! the quakes and after shocks lasted 2 years!! tepees fell over!! people fell over!! log homes collapsed. the mississippi river ran backwards for 6 hours as it filled the newly formed reelfoot lake!! - from the depression. the quakes rang church bells in philadelphia - like 900 miles away!!
i haven't done any work in ca - but when you get to southern missouri (hell - even st. louis - at least we have decent soils) its gets nasty and some the public works departments have taken it to heart. unfortunately - most of missouri is not enforced. sure we are supposed to follow code - but no one enforces it except in the larger cities and counties.
and the next one is due any day ---- some people say??!!
we briefly studied that earthquake in my environmental loads class. there was nor ricter scale then, so it was all based on the mercalli intensity scale which was measured on historical data. there weren't many people around, and most of who were weren't literate, so the data was hard to come by. at any rate, they estimated that earth quake was the worst ever on the north american continent.
the mississippi is actually located in a failed rift in the middle of the north american plate.
yes, most of the quakes on the new madrid fault (near the bootheel of missouri, usa) are small. there were a series of powerful earthquakes, largest was about 6.5 on richter scale, during period 1811-1812, over a span of about 2 years. one was so powerful the ground shifted so much so that the mississippi was temporarily flowing north in sections of the river, finally the riverbed shifted to another spot. there were few people in that area at the time, so that far as we know, no one was killed. i am no seismic expert, so i can guess why it is considered such a hot spot--the earth's crust there is solid rock in many places, making the ground almost ideal for transmitting seismic shock waves; the waves propagated so well that church bells in boston, massachusetts allegedly rang when the shock waves reached boston. you may not have major plates coming together there, but it is obvious the crust is fractured there, as they are able to find and monitor action on the new madrid fault.
lots of links here
there aren't many earthquakes in that area and they are small, but there is a fairly consistent activity. i used to check the map linked to below regularly and was a little surprised to almost always find something on the map.
i went to a seismic seminar a few years ago and they spent a long time talking about that fault line. it is supposed to be due (in a geological time table sense) for a large earthquake that everyone on the east coast will feel. it does have something to do with the overall soil of the east coast and the way the waves will travel.
i am a student of the nw tennessee area....heck i am also a large landowner there as well....you are correct in saying this is the most seismically active area in the us. the area is overdue for an earthquake....the seismic coefficient (sa) for the area that i own property (dyersburg) is 2.2, as opposed to montgomery, al where i live that currently has an sa of 0.20. reelfoot lake was formed is the early 1800's as was alluded to before when the mississippi river ran backwards for 2 days filling this lake as it formed from the major earthquake of 1802 +/-.

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