Week 5 - Converging Plates

The New Madrid Seismic Zone

A long week of work, chores, and errands prompted the family and I to take a trip to Southern Missouri to learn a little more about our state. I remember hearing about a large-scale fault near the boot of Missouri when I was grade school, but haven't given it much thought since. The faults lie under one to two hundred feet of alluvium, which makes it difficult to identify the fault line, as the soft soil covers them up. Even still, there is plenty of great camping, fishing, and hiking down south to make the trip fun and exciting. Now, we had a reason to turn this trip into something that was somewhat educational for the girls.

Earthquake intensities

The New Madrid Seismic Zone formed about 750 million years ago with a sub-surface rift formed during the break up of the supercontinent called Rodinia. It obviously did not split the Americas in half, but left what is called an aulacogen. An aulacogen forms when a plate undergoes a three-way break. The third failed arm becomes a filled graben system, and the rocks that make up this system are very brittle compared to the rocks before the break. In this case, the large aulacogen formed much of what would become the Mississippi River. What is interesting to me is that these types of faults are pretty uncommon, as they don't necessarily form near plate boundaries. Because of that, Missouri observes very frequent earthquakes, but most of them are incredibly weak.

Geologic Model

A compressive east/west force stresses the rift found under the New Madrid Seismic Zone, where the syncline is almost centered over the boot of Missouri. The rock under this feature is much more ductile, which enhances the affects of compression in the region. Because of the awesome forces that are at play here, a catastrophic failure of the fault would be inevitable if the fault reactivated.

Significant earthquakes occurred during 1811-1812, when sand blows erupted due to exceptional ground pressure on underground water. The estimated magnitude of these earthquakes were about 7.5. In modern history, we have seen upwards of 5.2 in magnitude with annual earthquakes just reaching a strength which is perceptible.

Sand Blows