The Shaky Science Behind Earthquakes
Earthquakes can rock your world – literally. At least a million earthquakes occur worldwide every year. Most of them are so small only very sensitive instruments can detect them. About 12,000 to 14,000 earthquakes each year are big enough for people to feel.
Even most of those don’t cause real damage. But the biggest earthquakes are incredibly powerful. They can cause entire buildings to shake into pieces, trigger tsunamis and landslides, and shift the land surface.
From minor to mega: the moment magnitude scale
Clearly, earthquakes come in all sizes. To describe their intensity, seismologists—scientists who study earthquakes—use a special scale called the moment magnitude scale. The scale goes from Magnitude 1 (M1) to Magnitude 10 (M10). The scale is logarithmic. That means that each number represents an increase of 10 in intensity. On this scale, an M5 earthquake is 10 times as strong as an M4 earthquake. An M6 earthquake is 10 times stronger than an M5 and 100 times stronger than an M4.
Each year, about big 16 earthquakes over Magnitude 7 shake the planet. Earthquakes over M8 are called “Great earthquakes.” On average, these occur only about once every 1.5 years. The strongest earthquake ever recorded was a Magnitude 9.5 that occurred in Chile in May,1960.
Great earthquakes can rattle the entire planet like a bell ringing. That happened in 2011, with a huge earthquake near Fukushima, Japan. The force of the quake moved Japan’s main island eight feet closer to the US. The movement was so big it changed the weight distribution on the planet. This caused the Earth spin faster on its axis, making each Earth day 1.8 microseconds shorter.
Plate tectonics and earthquakes
Naturally, most earthquakes aren’t just random. They typically occur where pieces of Earth’s crust—the planet’s thin, brittle outer layer—shift along fractures called faults.
The crust, along with the very top of the upper mantle just below, is broken into about twelve or so pieces called tectonic plates. Those plates slowly move, shifting in relation to each other. Typically, plate motion isn’t smooth. It catches and builds up tension. Then, it suddenly slips. That slip causes earthquakes.
Two tectonic plates sliding past each other creates a strike-slip fault. The San Andreas fault in California is an example of a strike-slip fault.
Head on collisions between plates also cause earthquakes. That’s happening in India and Nepal, where two plates slowly smash into each other. The collision has pushed pieces of the crust up like a rug wrinkling, creating the towering Himalaya mountains and triggering many earthquakes in the region.
Other plate collisions form thrust faults, where two pieces of crust overlap like tiles being pushed together. The biggest thrusts are subduction zones. In subduction zones, oceanic plates dive under lighter continental plates.
Subduction zones are responsible for some of the largest quakes on Earth: megathrust quakes. The M9.5 earthquake in Chile, was a megathrust earthquake.
When two parts of the crust pull apart, a rift or trench, such as the Great African Rift, forms between the parts. The faults on either side of such a rift are called normal faults. Earthquakes can occur as the sides get pulled further apart.
It’s no surprise that nearly all of Earth’s very biggest earthquakes occur along active plate-boundary faults. In fact, a worldwide map showing where earthquakes are most common also outlines the edges of many of Earth’s tectonic plates. These types of earthquakes originate very deep in the crust, from 16 to as much as 400 miles below the surface.
Plate motion also causes faults and earthquakes in the middle of plates. Imagine a car crash. The whole car can crack and bend, not just the bumper that got hit. Like the car, the crust may stretch, slide and shift in response to plate motion.
Learning from Earthquakes
When an earthquake lets loose, it sends ripples of energy through Earth called seismic waves. There are two main types of seismic waves that travel from the site of an earthquake: P waves (for “pressure”) and “S” waves (for “secondary” or “surface”).
P waves are faster and therefore arrive first. They are compression waves, moving the ground back and forth sort of like a Slinky. S waves are slower, but can cause much more destruction. They cause the ground to ripple up and down, like shaking a rug. The closer you are to an earthquake, the closer together the P and S waves arrive.
To study seismic waves, seismologists use sensitive instruments called seismometers. By analyzing the way these waves travel through and across Earth, how fast they arrive, and how close together, seismologists learn a great deal. They can determine the earthquake’s magnitude, and pinpoint the epicenter, or exact location, of the earthquake on Earth’s surface. They can locate the place deep underground where the fault that caused the earthquake slipped, the hypocenter. They also use seismic waves to explore Earth’s interior layers.
Directly or indirectly, tectonic plate motion causes most of Earth’s earthquakes. But human activity can also trigger small, shallow earthquakes. For example, water filling a reservoir or injecting water into underground rocks can cause small earthquakes. Big explosions also register on seismometers. Seismologists use waves generated from these human activities the same way they study waves from natural earthquakes.
Sometimes, seismic waves come from unexpected sources. On January 8, 2011 Seattle Seahawks football player Marshawn Lynch, nicknamed Beast Mode, made an incredible touchdown run. The crowd of over 60,000 spectators went wild. Seismometers nearby registered their cheering and stomping as the same intensity as an M2 earthquake! The crowd-created “earthquake” was called Beast Quake in honor of Lynch.
Studying Beast Quake and other shaky events has helped seismologists develop an earthquake early warning system. This enables them to detect an earthquake just seconds ahead, and send out an alert by text and email. Seconds may not sound like much time. But it’s enough to give people time to duck into doorways or under desks for safety.
Written by Laura McCamy
Edited by Beth Geiger, MS Geology
Illustrated by Renee Barthelemy