Earthquakes!

Imagine that you are watching the World Series on TV when all of a sudden you see people looking around as if they don’t know what is going on. People start to run out on the field and the game commentators announce that it appears an earthquake is happening. Cameras shake and the TV screen shows the shaking in the stadium as people rush to escape injuries if the concrete structures fall.

This really happened during the World Series in 1989 in San Francisco. Do you know why earthquakes happen? Read on to find out more.

Learning Objectives

To understand why volcanoes are a major natural disaster, you should be able to:

Describe why and where earthquakes occur.
Compare and contrast three types of seismic waves.
Explain how seismologists study earthquake seismic waves.
Describe how earthquakes cause health risks and injuries.

Vocabulary

earthquake—a sudden, violent shaking of the ground caused by movements within the earth’s crust or by volcanic action.

epicenter—a location directly above the origin of an earthquake.

Moment Magnitude Scale—a scale developed to measure the strength of an earthquake that provides an estimate that is more accurate over differing strengths and locations of earthquakes.

seismic waves—the form in which most energy is released from an earthquake.

seismologist—a scientist who studies earthquakes.

seismograph—a machine that records the different waves released during an earthquake.

transform fault boundary— where two crustal plates scrape against each other.

Why and Where Earthquakes Happen

An earthquake is a sudden, violent shaking of the ground caused by movements within the earth’s crust or by volcanic action.

Earthquakes occur at transform fault boundaries—where two crustal plates scrape against each other.

Remember: There are three ways crustal plates move:

Toward each other at convergent plate boundaries
Away from each other at divergent plate boundaries
Scraping past each other at transform plate boundaries

Image: USGS

As the crustal plates on opposite sides of the transform fault move, they build up stress. When the stress is too great for the rocks to withstand, they fracture and release seismic energy in waves. Seismic waves can travel along or near the surface (L waves) or through the earth’s interior (P and S waves).

Seismologists (scientists who study seismic waves and earthquakes) use the L, P and S waves to measure the strength of an earthquake. A seismometer is a machine that records the different waves released during an earthquake. Seismologists can study the record of waves and estimate both the strength and the location of the epicenter.

An epicenter is the location directly above the origin of the quake.

The Richter scale, a numerical scale used in measuring the magnitude of an earthquake, has mostly been replaced by the Moment Magnitude scale. The Richter scale was accurate only for certain quake frequency and distance ranges from the earthquake. The Moment Magnitude scale was developed to provide the most reliable estimate of earthquake size regardless of frequency or location.

There are many seismic stations all

over the world. When an earthquake

occurs, multiple stations record the

seismic waves and seismologists can

measure the magnitude of the quake

and calculate its exact epicenter.

About 90% of the world’s

earthquakes occur in a zone around

the Pacific Ocean called the

“Ring of Fire”. Many tectonic plates meet in this huge zone, including the Eurasian, North American, Juan de Fuca, Cocos, Antarctic, Indian, Australian, Philippine and many smaller plates. They all encircle the large Pacific Plate.

Did you know?
It is estimated that 500,000 earthquakes occur in the world each year. Southern California has about 10,000 earthquakes per year, but most are too small to be felt.

Earthquake Damage

Earthquakes are natural disasters because of the great damage they can cause and because of the many injuries and loss of life that can occur. Earthquakes can cause buildings to collapse, highways and interstates to crumble, and landslides to bury houses and roads. Broken gas lines trigger dangerous fires and downed power lines trigger power outages and electrical injuries.

In 1960, a 9.6 magnitude earthquake, the largest ever recorded, occurred along the coast of Chile in South America. Great stress had built up along two plates and the rocks fractured, producing ground movements over an 800 km distance.

Interestingly, China has had many deadly earthquakes. Some destroy entire towns. China’s earthquakes are not due to transform fault activity. They result from the massive collision of India with Asia that still continues. China’s population is large and many regions are tightly packed with people. Natural disasters (earthquakes, floods, famines) in China are almost always worse than anywhere else because China’s population is huge.

Look at the map below. If you have read the lesson on plate tectonics, you have seen this map. Everywhere you see the arrows at a plate boundary sliding past each other can be a location for possible earthquakes. Do you live near one of these locations?

Check Your Understanding:

1. What is an earthquake?

2. Describe why and where earthquakes occur.

3. Compare and contrast three types of seismic waves.

4. Explain how seismologist study and measure earthquake seismic waves.

5. Describe how earthquakes cause health risks and injuries.

3 diagrams showing plate movements away from each other, toward each other and sliding past each other

map of Pacific Ocean showing the location of the Ring of Fire

world map showing the location of major crustal plates in different colors