The Seven Types of Earthquakes: What They Are and How They Can Affect Us

A leader in the plate tectonic revolution, Sykes also spearheaded studies of earthquake hazards and prediction for five decades. His unvarnished reflections and insights will captivate anyone intrigued by the irregular rhythms and spasms of the earth or curious about the scientists who strive to decipher them.

~Rob Wesson, former chief, USGS Office of Earthquakes, Volcanoes, and Engineering, and author of Darwin’s First Theory: Exploring Darwin’s Quest for a Theory of Earth

On Tuesday, six earthquakes of 3.5 or greater hit California–a little more than a week after a 7.1 magnitude earthquake hit Southern California—the strongest to hit the area in nearly 20 years. In light of the earth-shaking events of the past few days, this week, we’re featuring Plate Tectonics and Great Earthquakes: 50 Years of Earth-Shaking Events, by Lynn R. Syke. Sykes played a crucial role in the birth of plate tectonics, conducting revelatory research on earthquakes. In this new book, Sykes delivers an invaluable insider’s perspective on the theory’s development and its implications. He also describes sizes of Earthquakes and their magnitudes. In today’s post, Mathilde Grand d’Esnon MSc candidate in Management at EMLyon Business School discusses the seven different categories of earthquakes, and their effects. 

Remember to enter our drawing for your chance to win a copy of Plate Tectonics and Great Earthquakes: 50 Years of Earth-Shaking Events.

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Not all earthquakes or the same, nor do they affect us in the same way. Some are so small that they’re only noticeable under specific circumstances, and others are so strong that they tumble buildings. News headlines often highlight the earthquake’s magnitude, but what do these numbers mean, and what are the effects of these tremors? 

The Difference Between  Magnitude and Intensity

According to United States Geological Survey, magnitude refers to the energy released by the earthquakes and is based on measurement of the maximum motion recorded by a seismograph. Intensity describes what is felt during the earthquake.

Microearthquakes

There are seven sizes of earthquakes, the smallest of which are known as microearthquakes. These earthquakes have a magnitude of less than 3.0 and are almost impossible to feel. On June 1, a 2.0 magnitude earthquake was recorded 37 miles off the coast of Brigantine in Atlantic County, NJ, and there were no reports of damage or injures.

Small Shock Earthquakes

Earthquakes with magnitudes between 3.0 and 5.0 are known as small shocks. According to a recent three-year data sample, an average of 234 small shock earthquakes occur per year in California and Nevada. The smallest of these are noticeable, but many people don’t always recognize them as earthquakes. A 4.0 to 4.9 magnitude earthquake, however, might wake you up and cause dishes to break and objects to overturn. A 4.0 magnitude earthquake was recorded about 20 miles northeast of Cleveland on June 10. 

Moderate-Size Earthquakes

Moderate-size earthquakes can be frightening. With a 5.0 to 7.0 magnitude, they are felt by all and can cause moderate damage in well-built ordinary structures to considerable damage in poorly built structures. In June, two earthquakes, magnitudes of 6.0 and 5.3 shook Changning County of Yibin City in China. They caused the death of eleven people, and 122 were injured. 

Large Earthquakes

Large earthquakes have a magnitude of 7.0 and greater. But within that category, Lynn R. Sykes defines three more specific descriptions of earthquake sizes.

  • Major earthquakes (magnitude 7.0 to 7.75)
    On November 30, 2018 a 7.0 magnitude earthquake hit north of Anchorage Alaska. While no casualties were recorded, several building and highways were damaged.

  • Great shocks (magnitude 7.75 to 8.5)
    In 1906 an estimated  7.9 magnitude earthquake struck San Francisco and caused nearly 700 deaths.

  • Giant earthquakes (magnitude 8.5 or greater)
    On March 11, 2011 a 9.1 magnitude earthquake took place 231 miles northeast of Tokyo and caused a tsunami with 30 foot waves.

Earthquakes can vary vastly in magnitude and intensity. And the after effects can be devastating. But what causes them to be drastically different? Check back tomorrow for an excerpt from chapter 1 of Plate Tectonics and Great Earthquakes: 50 Years of Earth-Shaking Events, in which Sykes discusses the main elements of plate tectonics, the types of faults, and the mechanisms of earthquakes.    

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