During the morning of April 18, 2008 (4:37 a.m. CDT) a magnitude 5.2 earthquake occurred in the Wabash Valley Seismic Zone. The shock was felt over a multistate area, with the quake's epicenter in far eastern Illinois. It is believed by many geologists that the Wabash system is related to and/or part of the New Madrid Seismic Zone (NMSZ).
The NMSZ lies within the central Mississippi River valley and runs from northeast Arkansas through southeast Missouri, western Tennessee, and western Kentucky to southern Illinois. Unlike many earthquake fault zones which are the result of plate tectonics at continental boundaries, the New Madrid zone is in the middle of the continent and associated with a failed continental rift. The characteristics of these mid–continent quakes are also different. The 1811–12 quakes were felt over a wider geographic area (10 times larger the 1906 San Francisco earthquake) and New Madrid quakes are focused 3 to 12 miles below the surface.
Since last month's shake–up, we have received several e–mails requesting information on the New Madrid quakes of 1811–12. Some of the media information about the magnitude, or strength, of this historic series of quakes has been contradictory. This has been especially true when the magnitude has been mentioned. A lot of the information put all magnitudes below 8.0, which is incorrect according to the U.S. Geological Survey (USGS).
I am not going to go into a detailed discussion of the New Madrid system and quakes. There is a huge volume of information available online. However, I am going to provide some links that I have found very useful in providing questioners with reliable information.
One site that discusses the structural features in a clear and thorough manner is the Wikipedia entry on the New Madrid Seismic Zone. Please remember that Wikipedia entries are created and edited by users, so if any information is questionable it should be checked against other more traditional sources. Nonetheless, this Wikipedia information is excellent and can be found here: en.wikipedia.org/wiki/New_Madrid_Seismic_Zone
Another useful webpage for reliable information on the historic 1811–12 series can be found on the USGS website here: earthquake.usgs.gov/regional/states/10_largest_us.php
A more in-depth discussion of those quakes is also available at the USGS site here: earthquake.usgs.gov/regional/states/events/1811-1812.php#december_16
The above page also provides links to photographs of the damage from these quakes. Interestingly, the photos were taken in 1904, almost a century AFTER the actual quakes. If you take time to explore the USGS site and links, you will find a wealth of information.
Our website also has a page with links to helpful sites. You can find that page here: www.scchealth.org/docs/ems/docs/prepare/earthq_links.html
The St. Louis University and University of Memphis sites are particularly interesting. We also have a list of a few of the books I have read on the New Madrid Fault. That list is here: www.scchealth.org/docs/ems/docs/prepare/earthq_add_reading.html
The second book link, "The New Madrid Earthquake (Bulletin 494)" is especially interesting if you want a very thorough and technical source of information. It is a reprint of a USGS bulletin from the turn of the early 20th century and has many photographs.
The "New Madrid Fault Finders Guide" is also an excellent book for those who live within driving distance of southeast Missouri (or even those who don't who want an interesting read with lots of photographs). It contains several tours of the area affected by the quake series and guides the user to features that still exist and can be seen.
Finally, I have had fun using Google Earth to examine the areas referred to in the above "Finders Guide." For those who are not familiar with Google Earth, it is free software from Google and very powerful. You can download it here: earth.google.com
Below is an image from Google Earth of sand blows on both sides of Interstate 55, north of New Madrid, Missouri. The light–colored, roughly circular areas show where sand erupted like a geyser to the surface. The sand blows still affect soil and vegetation color to this day, nearly 200 years after the series of earthquakes.
Sand blows are still in evidence in southeast Missouri, nearly two centuries after the New Madrid series of earthquakes. Image from Google Earth.