How Earthquakes Work

By Tom Harris and Patrick J. Kiger

Have you ever assured someone that your friend is reliable by saying that he or she "has both feet on the ground"? The fact that such a phrase exists shows how much comfort we take in the idea that the ground beneath our feet is unmoving, unchanging and dependable. Indeed, much of our civilization, from our houses and buildings to our energy, food and water sources, depends on unmoving earth.

In truth, however, our planet's seemingly stable surface is made up of enormous pieces of rock that are slowly but constantly moving. Those pieces continually collide with and rub against one another, and sometimes their edges abruptly crack or slip and suddenly release huge amounts of pent-up energy. These unsettling events are called earthquakes, and small ones happen across the planet every day, without people even noticing. But every so often, a big earthquake occurs, and when that happens, the pulses of energy it releases, called seismic waves, can wreak almost unfathomable destruction and kill and injure many thousands of people [source: Bolt].

That sort of cataclysm occurred on March 11, 2011, in Japan, when a massive quake, later estimated by Japanese Meteorological Agency to be 9.0 in magnitude, struck 81 miles (130 kilometers) east of the city of Sendai on the nation's northeastern coast. The forces of the quake, the fifth most powerful in the past century, set off a giant wave, called a tsunami, that engulfed villages, destroyed buildings and drowned and crushed people who lived there [source: Green]. The earthquake and tsunami also badly damaged a six-reactor nuclear power plant in Fukushima, 150 miles (241 kilometers) north of Tokyo, destroying the backup generators that powered its cooling systems and causing a dangerous release of radiation that forced people in the region to flee. In all, the quake claimed the lives of 20,896 people, according to the U.S. Geological Survey.

Though earthquakes have terrorized people since ancient times, it's only been in the past 100 years that scientists have come to understand what causes them, and to develop technology to detect their origin and measure their magnitude. In addition, engineers and architects have worked to make buildings more resistant to earthquake shocks. Someday, researchers hope to find a way to predict earthquakes in advance, and perhaps even control them.

In this article, we'll give you the latest scientific knowledge about earthquakes, and discuss how humans can cope with them. But first, here are some basic earthquake facts.

Earthquake Facts

Technically, an earthquake is a vibration that travels through the Earth's crust. Quakes can be caused by a variety of things, including meteor impacts and volcanic eruptions, and even sometimes man-made events like mine collapses and underground nuclear tests [source: Hamilton]. But most naturally occurring earthquakes are caused by movement of pieces of the Earth's surface, which are called tectonic plates. (We'll learn more about those plates on the next page.)

The U.S. Geological Survey estimates that, each year, there are as many as 1.3 million quakes with a magnitude greater than 2.0, the threshold at which humans can feel the vibrations [source: USGS]. The vast majority of them are very small, and many occur in remote areas far from people, so we don't usually even notice them. The earthquakes that capture our attention are the rare big ones that strike near heavily populated areas. Such earthquakes have caused a great deal of property damage over the years, and they've claimed many lives. Over the last decade alone, earthquakes and the tsunamisavalanches and landslides caused by them -- have killed 688,000 people around the world [source: Stoddard].

Perhaps the most lethal quake in history had a magnitude of 8.0 and struck China's Shanxi Province in 1556. According to historical accounts, city walls, temples, government buildings and houses all crumbled, and more than 830,000 people were killed. A scholar named Qin Keda, who survived the quake, later provided what may have been the first earthquake preparedness advice in history: "At the very beginning of the earthquake, people indoors should not go out immediately," he recommended. "Just crouch down and wait for chances. Even if the nest is collapsed, some eggs in it may still be kept intact" [source: Science Museums of China].

On the next page, we'll examine the powerful forces that cause this intense trembling and we'll discuss why earthquakes occur much more often in certain regions.

 

Earthquake Preparedness

Over the past 50 years, major advances have been made in earthquake preparedness -- particularly in the field of construction engineering. In 1973, the Uniform Building Code, an international set of standards for building construction, added specifications to fortify buildings against the force of seismic waves. This includes strengthening support material as well as designing buildings so they're flexible enough to absorb vibrations without falling or deteriorating. It's very important to design structures that can take this sort of punch, particularly in earthquake-prone areas.

But architects and engineers also are trying to develop innovations that would provide even greater protection against quakes. Greg Deierlein of Stanford University and Jerome Hajjar of Northeastern University, for example, have designed a structure equipped with structural "fuses" that, instead of toppling, deliberately collapse upon themselves and then reform after the quake subsides [source: Ward].

Additionally, scientists are developing "smart" building materials that are capable of coping with the tremendous forces generated by a quake. One idea is to include fiber-optic sensors that can sense when a structure is about to fail; the sensors would then send signals to tiny ceramic strips built into the walls and frame, which would change shape to absorb the energy [source: Stark]. (See How Smart Structures Will Work for more on how scientists are creating new ways to protect buildings from seismic activity.)

Another component of preparedness is educating the public. The United States Geological Survey (USGS) and other government agencies have produced several brochures explaining the processes involved in an earthquake and giving instructions on how to prepare your house for a possible earthquake, as well as what to do when a quake hits.

In the future, improvements in prediction and preparedness should further minimize the loss of life and property associated with earthquakes. But it will be a long time, if ever, before we'll be ready for every substantial earthquake that might occur. Just like severe weather and disease, earthquakes are an unavoidable force generated by the powerful natural processes that shape our planet. All we can do is increase our understanding of the phenomenon and develop better ways to deal with it. 

 

More Great Links

·         USGS Earthquake Hazards Program

·         California Institute of Technology: Earthquake Research Affiliates

 

Is it true that scientists are predicting a really big earthquake will sink western California?

 

This often comes up when people talk about earthquake activity along the Pacific coast of the United States. Seismologists have predicted that a massive scale (8.0 or higher on the Richter scale) earthquake will shake the region sometime within the next 30 years or so. This is the so-called "Big One" that makes many Californians understandably nervous and inspires a variety of apocalyptic disaster speculations.

But while the Big One would definitely wreak mass destruction, it would not sink part of California into the ocean, nor would it break the state off from the rest of the country. The idea comes from a misunderstanding of the seismic forces that cause earthquakes in the region.

Powerful earthquakes occur frequently along the west coast of the United States because the region is near a boundary between two tectonic plates. If you've read How Earthquakes Work, then you know that the earth's surface is made up of large, rigid plates that slowly drift over the mantle layer below. At the boundaries between plates, a number of things can happen. The Pacific plate and the North American plate simply grind against each other -- one creeps slowly northwest and one creeps southeast.

This boundary forms a fault line that extends under the ocean and on land along the west coast of the United States. The San Andreas fault in California is the piece that's on land. Smaller faults form in the crust material near the boundary line due to the forces of the plates pushing on each other.

Friction builds up along faults because the two sides are pushed very tightly together. If the force of friction exceeds the forces moving the earth, the two sides will become "locked," so they stop creeping. When this happens, tension builds up along the fault line until the force of movement is great enough to overcome the force of friction. Then the pieces of earth suddenly "snap" into place, releasing a large amount of energy that causes earthquakes in the earth's crust.

Many scientists estimate that there is enough tension built up along some locked California faults, that when they do finally slip, the earthquake will be extremely powerful. The Hayward Fault particularly concerns these scientists because it runs under heavily populated areas in and around Los Angeles.

The notion that part of California will break off was likely inspired by the San Andreas fault. After all, since the fault goes right through California, one part of the state is on the Pacific plate and one is on the North American plate. If those plates are moving in different directions, it make sense that the two pieces of California will move in different directions too.

And this is indeed the case. But, even in a massive shift along the fault, the plates travel an incredibly short distance -- a matter of feet in the most extreme shifts. The tension cannot build up to the point that one entire mass of land will shift many miles in relation to another one, so you will not see any sizable piece of land breaking away from another. Instead, the pieces of land will move away from each other very slowly, taking millions of years to make large scale changes. One end of California may slowly drift so that it is eventually under water, but this can hardly be construed as "sinking into the ocean."

Soft-story buildings, so called for having first stories much less rigid than the stories above, are particularly susceptible to earthquake damage because of large, unreinforced openings on their ground floors and in their typically wood-frame construction. These openings often accommodate parking spaces, large windows and expansive lobbies in residential and retail buildings. Without proper design, such structures are much less able to withstand the lateral forces; forces that push a structure side to side that earthquakes generate. Once the first floor folds, the upper floors pancake down on top of it, crushing anything underneath.

That's clearly a big problem in population-dense, earthquake-prone areas like San Francisco (and, theoretically, any high-density city, earthquake-heavy area with similar construction) have thousands of soft-story buildings in need of improvements. A study conducted by the California Institute of Technology found that, San Francisco faces a similar situation with its roughly 10,800 soft-story buildings.

 

Soft-Story Buildings

 

Some of the most susceptible structures to shaking damage are soft-story apartments and condominiums. A soft-story residential building is one that has open parking or commercial space on the first floor and housing on higher floors built prior to recent codes. In an earthquake, ground shaking causes such structures to sway and sometimes collapse. A soft-story collapse can have particularly disastrous consequences considering that they crush cars and kill people occupying the open areas.

ABAG modeling has shown that, in both a large earthquake on the Hayward or San Andreas faults, two-thirds of the uninhabitable housing units will likely be in soft-story residential buildings.

 [source: Association of Bay Area Governments]. Worse still, experts predict a major earthquake in the Bay Area over the next several decades. If such a quake occurs, 80 percent of San Francisco's soft-story buildings would collapse or be damaged beyond repair as a result [source: Selna].

And this is indeed the case. But, even in a massive shift along the fault, the plates travel an incredibly short distance -- a matter of feet in the most extreme shifts. The tension cannot build up to the point that one entire mass of land will shift many miles in relation to another one, so you will not see any sizable piece of land breaking away from another. Instead, the pieces of land will move away from each other very slowly, taking millions of years to make large scale changes. One end of California may slowly drift so that it is eventually under water, but this can hardly be construed as "sinking into the ocean."

What can be done to prevent such a disaster? That's where structural engineers like Dr. Scott Adan come in. Founder and principal of Adan Engineering, he specializes in a process known as soft-story seismic retrofitting.

"Lateral stiffness and strength are the key ingredients to make buildings 'earthquake safe.' In a soft-story building, the lateral components were either never considered or they are just not strong enough to resist the imposed earthquake forces," explains Adan.

Soft-story seismic retrofitting addresses those oversights, adding the structural components needed for buildings to remain standing after an earthquake hits. Read on to find out how the process works and how cities are encouraging building owners to make these improvements.