By Jonny Lupsha, Wondrium Staff Writer
A single earthquake can kill hundreds of thousands of people. It can destroy even more buildings than the number of lives it takes. How can one earthquake be so costly—and fatal?
The death tolls of earthquakes are hard to believe. A 2004 earthquake and its resulting tsunami in the Indian Ocean claimed the lives of over 227,000 people; a quake in China in 1920 killed 273,400. The damages are equally as unbelievable, if not more so. The 2011 Tōhoku earthquake and tsunami cost Japan $360 billion, while 1995’s “Great Hanshin Earthquake” set the island nation back $200 billion.
Recent earthquakes in Turkey and Syria have claimed the lives of more than 40,000, with rescuers still doing search and rescue efforts looking for survivors. What makes earthquakes so devastating? In his video series The Nature of Earth: An Introduction to Geology, Dr. John J. Renton, Professor of Geology at West Virginia University, details the literally earth-shattering factors of earthquakes.
Shock Waves and Seismic Waves
“The energy that’s released whenever rocks break, they release shock waves,” Dr. Renton said. “A ‘shock wave’ is a general term; if it happens to be generated as a result of the movement of a fault—in other words, an earthquake—we call it a seismic wave. A seismic wave is simply a shock wave strictly associated with a fault movement.”
According to Dr. Renton, there are two very different kinds of shock waves, depending on their relationship to the “direction of propagation.” A direction of propagation is the direction in which the energy of a wave is moving. With a shear wave, as the wave moves away from you, it moves whichever material it’s moving through perpendicular to the direction of propagation.
“The idea is that as the shock wave goes through, what’s happening to the rock or whatever it’s going through is going out and in, and out and in like that, moving the stuff perpendicular—that’s the important part,” he said. “The compression wave is kind of totally different. In the compression wave, if you can imagine the shock wave moving away from you again, the material it’s moving through is moving the stuff back and forth in the [same] direction in which it’s going.”
To typify a compression wave, Dr. Renton mentioned a Slinky® held in two hands, its coils passing from one to the other. Shear waves can only pass through solids, while compression waves can pass through any substance.
Destructiveness of Earthquakes
In common discourse about earthquakes, the words “focus” and “epicenter” are often used interchangeably, but they’re very different—and vital to understanding why earthquakes cause so much damage. The focus is the point in the Earth’s crust at which the earthquake occurs. The epicenter is the point on the surface directly above where the earthquake occurs. The energy of the quake is released at the focus, miles beneath the surface, in what are called “body waves.”
“In terms of Earth movement—rock movement—the amount of energy that is being used to actually move the rocks as the body waves go through the body of the Earth—that’s why they’re called body waves—is very, very, very little,” Dr. Renton said. “The total amount of rock movement is on the order of, like, maybe a fraction of a millimeter.”
However, an earthquake has enormous energy potential. What’s the rest used for? Body waves travel through the Earth at nearly 24,000 miles per hour, reaching the epicenter on the surface first—and strongest. When they reach the surface, the body waves transfer all their energy to the surface and become surface waves.
“Surface waves are totally different from the body waves,” Dr. Renton said. “The main difference between them is the amplitude of the movement. Whereas the body waves move the rocks almost nothing, we’re talking now about fairly large movements of the rocks.
“So large, as a matter of fact […] that, in some cases, you can actually see the surface wave moving across the surface of the Earth.”
The Nature of Earth: An Introduction to Geology is now available to stream on Wondrium.