Transcript for Hidden Fury, segment 07 of 11
The earthquakes trace out a zigzag pattern when plotted on the map, but their epicenters are actually located two to ten miles below the surface along deep faults in the Earth's crust.
These cubes represent earthquakes along deeply buried faults. Swinging below New Madrid, seismicity picks up, then dies out farther north. To learn what causes these quakes, we must journey back in time half a billion years.
At that time the continent began to slowly pull apart. Molten magma swelled up from below, causing the crust to bulge and crack open at the surface. Extreme heat and pressure continued splitting the rigid crust, allowing magma to shoot up through fissures and erupt on the surface, forming volcanoes. The Real Foot Rift was tearing the continent in half.
Now, this is a fairly common geologic process worldwide, and when it happens on a large global scale, it can form new oceans. A new ocean is opening up right now in the Red Sea area, for instance so - but many rifts are not successful. They fail. They never make it to the stage of actually splitting a continent and creating new oceanic crusts, and Real Foot Rift was one of those.
Over the next three hundred million years the volcanoes in the doomed rift went extinct. The thin, fractured crust cooled back down, collapsing under its own weight to form a long, wide valley. Ancient oceans then flooded much of North America for millions of years. They left behind thousands of feet of sediment which collected in the sunken Real Foot Rift. Through the eons the rift made several weak attempts to pull apart again. Magma started up old fractures but never reached the surface.
Sixty million years ago rifting stopped. The crust sank down once again, and natural erosion filled the rift with another layer of sediments. The Mississippi River began flowing down the rift valley after the last Ice Age, about ten thousand years ago, but down below, the shattered, rifted rocks remain.
You've got a much weaker overall crust than you would out on the rest of the criton, and that's where we find large earthquakes in these stable continental areas so we feel that it's the, it's the ancient faults of the Real Foot Rift system that are generating the earthquakes today.
The ancient faults are kept alive by subtle tectonic forces. Stress in the Earth's crust is squeezing the rift from the east and west. When stress is great enough, the old faults, which are at an angle to the pressure, slip suddenly, causing earthquakes. The southern end of Real Foot Rift near Memphis is currently under the most stress. With the tendency for big earthquakes to strike at the ends of faults, this is a good candidate for the zone's next big quake. The question is, when?
I would not be surprised to witness a greater than magnitude six earthquake in my lifetime out here. I'm comfortable that we know roughly where, in the broad sense, somewhere in the current very active seismic zones, there will be a magnitude six or greater earthquake certainly in my lifetime, but we still have a lot more to learn. We don't know when the magnitude eights will occur, probably not in my lifetime, but possibly. We just don't know.