Add-on:New Madrid earthquake intensities

This add-on illustrates the Modified Mercalli earthquake intensities as estimated by Dr. Susan Hough, a seismologist for the USGS, for the first of the series of 1811-1812 earthquakes.

The New Madrid Seismic Zone in the central United States is a failed rift zone (just like in east Africa, which is non-failed rift zone). The fault system there causes severe earthquakes roughly every 400-700 years, the most recent being in 1811 and 1812. These were actually three earthquake, in December, January, and February of those years. Evidence in the archeological and geologic record suggest that a sequence of several severe earthquakes is a common mode of rupture for this fault system.

The Modified Mercalli scale is a system of measuring the intensity of ground shaking in a particular location based on eyewitness observations. This system is fairly qualitative, which doesn't make it useful for design, but can be used to estimate the size of earthquakes that occured before we had instruments to measure them precisely.


 * I. Instrumental - Not felt except by a very few under especially favorable conditions.
 * II. Feeble - Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
 * III. Slight - Felt quite noticeably by persons indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
 * IV. Moderate - 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. Dishes and windows rattle.
 * V. Rather Strong - Felt by nearly everyone; many awakened. Some dishes and windows broken. Unstable objects overturned. Clocks may stop.
 * VI. Strong - Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.
 * VII. Very Strong - Difficult to stand; furniture broken; damage negligible in building 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. Noticed by persons driving motor cars.
 * VIII. Destructive - Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.
 * IX. Ruinous - General panic; 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. Disastrous - Some well built wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.
 * XI. Very Disastrous - Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
 * XII. Catastrophic - Total damage - Almost everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

One immediate observation about this earthquake in the central US is that it was felt as far as 1000 miles away in New England. Earthquakes that occur in California don't affect this large of an area because the bedrock on the west coast is lower quality than in the central US. Earthquake waves can travel further through better bedrock.

Another interesting thing to observe in this data is the dependency on local geology in the intensity observed. In the screenshot below, you can see that in many locations in southern Illinois, Indiana, Kentucky, and Tennesee, higher levels of shaking were felt near rivers, even for sites that were a greater distance from the epicenter of the earthquake. This is because the younger, softer soils in river valleys tend to amplify the shaking, just like shaking a block of soft jello. Sites that were on higher ground, and thus on stiffer, older soils didn't experience shaking that was as severe.


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