Chapter 8
Chapter 8: A Violent Pulse: Earthquakes
Feature Articles
The Rest of the Story: What’s Happening at the Atomic Scale During Faulting?
by Stephen Marshak
To fully understand the process of faulting, we need to examine the behavior of atoms during the development of a fault. Recall that chemical bonds hold the atoms making up the minerals in rock together. We can picture these bonds as tiny springs. A chemical bond, like a spring, has a specific length when not subjected to a force. The application of a small stress to a rock causes the bonds to begin to stretch—the same process happens when you pull on the ends of a spring—creating an elastic strain; if you remove the stress, the bonds return to their normal length. But if instead you increase the magnitude of the stress, the bonds stretch further until eventually they break. If many bonds break along a surface in a mineral, a crack develops, across which the two halves of the mineral no longer connect. If the stress becomes larger still, the crack grows, and eventually more cracks form and grow until they all link together to form a single large fracture. When this happens, the rock as a whole divides into two, and one piece can slip past the other.
The Rest of the Story: The Mystery of the New Madrid Seismic Zone
by Elizabeth Lane Mason
Most earthquakes occur at plate boundaries, but in 1811 and 1812, three magnitude 8 earthquakes struck the interior of the North American plate near New Madrid, Missouri. Since then, geologists have struggled unsuccessfully to solve the mystery of the New Madrid seismic zone. However, new research has identified a likely culprit: the melting of an enormous glacial ice sheet at the end of the last ice age.
Geologists have long been aware that the anomalous seismic zone located at the corner between Missouri, Arkansas, Tennessee, and Kentucky lies directly above a failed rift zone. Near the end of the Precambrian, upwelling magma beneath the crust threatened to break apart the North American plate. Before the rift occurred, the upwelling stopped. But the process had introduced a weakness in the plate. The existence of this weakness alone doesn’t cause earthquakes, however.
During the last ice age, much of the North American plate was covered by a thick sheet of ice that reached as far south as Illinois. Though the ice didn’t extend all the way to New Madrid, the weight of the sheet caused the earth to flex in that area. When the ice melted between 8,000 and 19,000 years ago, the removal of all that weight allowed the crust to rise to its original position. This rebound may be what is triggering the earthquakes in the failed rift zone.
The crustal rebound process is extremely slow. In fact, North America won’t completely bounce back for another 10,000 years. This means that seismicity in the New Madrid area will also continue well into the future.
Evidence of liquefaction preserved in the sediments of the New Madrid seismic zone shows that major earthquakes of magnitude 7 or higher have occurred every 200 to 900 years for the past 1,200 years. If the trend continues, this heavily populated area of the Midwest may be due for another big one any day now. Because the interior of the plate consists of old, rigid crust, seismic waves will travel far and a major earthquake there would affect a relatively large area. In 1812, the region was sparsely populated, but today it encompasses several large cities including St. Louis and Memphis. Compounding the danger, very few structures in the area were built to withstand an earthquake.
Scientists hope that if the deglaciation theory is correct, models of glacial melting and crustal rebound will help them more accurately predict if large earthquakes will strike the New Madrid seismic zone in the near future.
REFERENCES
Grollimund, B. and M.D. Zoback. 2001. Did deglaciation trigger intraplate seismicity in the New Madrid seismic zone? Geology 29, no. 2: 175-178.