The Earth’s Anatomy and the Mechanics of a Temblor

Whether you live in an earthquake-prone region or not, you know that the Earth can move in mysterious ways. Preparing for an earthquake might make you curious about the actual mystery behind a quake. Familiarizing yourself with the essential facts behind earthquake science can not only quell your curiosity, it can help you determine how likely you are to face a quake in your future.


Earth’s Anatomy

To understand how earthquakes occur, you first have to familiarize yourself with the Earth’s anatomy. Our planet can be divided into concentric sections: Crust or lithosphere on the outside, below which is the mantle, followed by the outer core and the inner core, explains Kate Hutton, a seismologist at the California Institute of Technology in Pasadena, California. “The overall view is that there is heat escaping from the Earth’s interior, so part of it is undergoing convection (a ‘slow boil’ if you like),” Hutton says. “The outer core is actually liquid and the mantle is solid, on a short time scale, but flows on the time scale of geologic time. The crust is generally brittle, does not flow, but is riding around on top of the mantle below it.”the-earths-anatomy-5

The Earth’s crust contains a dozen or so large segments, along with many small ones called tectonic plates. Where the tectonic plates move past each other at their edges, the motion is generally bumpy, Hutton says. Because there is friction between the plates as they try to move past each other, strain builds up until a break occurs in the brittle crust, typically on the plate boundary. “When the break occurs, rock moves suddenly, sending out vibrations (think ripples on a pond when you drop a rock into it) to the surrounding area,” Hutton says. “These vibrations (‘seismic waves’) are what people feel as an earthquake.” A small break causes weak seismic waves, while a larger break causes more intense and more widespread seismic waves. The break, where the rocks are offset, is called a fault. Although we all live atop the Earth’s crust, people in specific areas of the planet are more likely to experience an earthquake. “The places that are most at risk are along the boundaries between major tectonic plates,” Hutton says. For instance, California, the Pacific Northwest, Alaska and Japan, among other geographic regions, are at high risk.


Quakes Beget More Quakes

If you’ve experienced an earthquake, you aren’t immune to another one in fact, the opposite could be true. “Most plate boundaries are splintered up from their past history of earthquakes, so there are more faults than just the actual plate boundary,” Hutton says. Southern California is an example of this, as are India, China and Tibet. “The Indian plate has been slamming into Asia at the geologic rate of about two inches per year, squeezing up the Himalayas in the process,” she says. “Most of the major earthquakes in China are due to ‘splinter’ faults caused or activated by this compression.” Even if you don’t live directly on a fault line, however, you may face a large quake, because the middle of a plate still withstands stress and strain. “There are large earthquakes less commonly at interior weak spots,” Hutton says. “The 1811-1812 series of earthquakes in Missouri and Kentucky is an example of this,” she adds.the-earths-anatomy-1

The most immune areas in North America are the prehistoric cores of the main continent, such as Wisconsin, Minnesota and parts of Canada. The largest earthquake since widespread use of the seismograph began around 1900 was the 1960 Chile earthquake, with a magnitude of 9.5. The next largest came 64 years later in Anchorage, Alaska, with a 9.2 intensity. The 2004 Sumatra earthquake was third, at 9.1, and two quakes tie for fourth: 2011 Japan and 1952 Kamchatka, Hutton says.

Future Unknown

Despite speculation to the contrary, the question of whether the biggest quakes are still yet to come is unclear. “The magnitudes of the largest earthquakes are generally determined by the size (surface area) of the largest faults, as well as the strength of the fault zones, neither of which should have changed by huge amounts over the Earth’s history,” Hutton says. “There was more heat escaping in the past, however, and plates may have been moving faster. Perhaps large earthquakes were occurring more often. We don’t really know, because no one was here to record it.”
While there’s a certain amount of mystery surrounding earthquakes, one thing is most definitely clear: They will continue to happen. So be prepared.

Know How Earthquakes Are Measured

We’ve all seen the squiggly lines of a seismograph, but do we really know the complex formula that scientists use to measure the strength of an earthquake ?the-earths-anatomy-4

This quick primer can give you an inside look at the science behind it. “earthquake magnitude is a measure of the intrinsic size of the earthquake, more or less how much rock moved how far when the break occurred,” says Kate Hutton. Another way to look at measuring an Earthquake is using “intensity,” a map which shows where the ground shaking was strong and where it was weak, she adds. “An earthquake should have a single magnitude, as well as a map of intensities based on damage, human perception and seismographic recordings,” she says. Damage from an earthquake is only partially determined by its size. “Distance away from the fault rupture is also important, as well as directionality of the seismic waves, soil conditions and building standards,” Hutton adds. A good building code and enforcement thereof can make a huge difference in whether an earthquake’s damage is measured in millions or billions of dollars.