Transcript for Hidden Fury, segment 08 of 11


{{{MUSIC}}}

With few defenses against earthquakes, even a magnitude six to seven earthquake could inflict heavy losses in this region. Damage will be concentrated where geologic conditions are most hazardous. A fascinating phenomenon called liquefaction will be widespread along rivers and streams.

It's a phenomenon where earthquake waves start shaking the sand. Soft, wet sand will start compacting together like when you take a box of cornflakes and shake it and the cornflakes in the box settle. Well, that happens with the sand, and as the sand starts to settle, the water that's between the grains of sand tries to get out from between the grains, and if there's something solid on top of the sand like a layer of clay, the pressure breaks the surface layer of clay, and sand will come erupting to the surface in sort of a liquefied form so we call it liquefaction, and what this often causes is, first of all, the ground to break up. The ground also loses all capacity to hold up objects such as buildings so buildings will settle, sink, or tilt during liquefaction, and in many earthquakes there are things called sand boils which form. The upper layer of clay will crack, and the sand will come erupting to the surface in sort of a volcano and form a circular patch of sand maybe twenty, forty feet across with a crater in the middle. That's very typical. We see it in almost any earthquake over magnitude six anywhere in the world.

Well, what I'm going to do is walk on this pristine sand that can bear my weight, and once I start bouncing up and down, you'll see that, as the sand starts to compact and the water comes out of its pores, the sand liquefies, losing all ability to bear my weight, and if we watch afterwards, we will see that water erupting to the surface in miniature sand boils.

{{{MUSIC}}}

This is a small sand-filled fissure caused by liquefaction during the eighteen eleven and twelve earthquakes. From the air, huge light-colored patches of liquefied sand from those quakes are plainly visible in farmers' fields today. Liquefaction during the eighteen eleven and twelve quakes devastated a fourteen hundred square mile area along the Mississippi River. Today this same area is covered with thousands of farms and many communities. Highway fifty-five traverses the entire liquefaction zone. A modern, well-built interstate, it is defenseless against a liquidlike foundation. The earth levees along the river are also vulnerable, especially during high flood stage. The river's natural banks, now lined with petroleum storage tanks, grain-loading facilities, a regional power plant, and long bridge approaches are especially vulnerable.

Failure of these kind of riverbanks normally occurs by a process that we call lateral spreading, which occurs when liquefied sand and water layers beneath the surface of the ground are shaken repeatedly over a long period of time during a major earthquake, but the eyewitness accounts from the eighteen eleven and twelve earthquakes indicate that even those low riverbanks experienced massive failure.

Transportation may be disrupted for years by liquefaction alone, but other earthquake effects will add to the problem.

{{{MUSIC}}}

In a future major earthquake there's very little doubt that there will be major landsliding from bluffs such as this as well as bluffs that are farther away from the river. Large amounts of this land will simply flow towards the river, and trees and farmland and the ground itself will simply be precipitated into the river. When we think ahead to future possible earthquakes in the midcontinent, navigation is probably one of the issues that we ought to deal with most seriously because obviously a large amount of barge traffic goes up and down the Mississippi River, and after a major earthquake, it is likely that the river would be unnavigable for quite some period of time, maybe weeks or even months.