New York State High School Regents Exam Prep
Earth Science

What are earthquakes?

An earthquake is any shaking or vibration of the earth's crust. They occur where stress builds at a zone of weakness or at a break in the earths crust called a fault . Seismic waves are generated in all directions from the point on the fault that moved. This point is called the focus. The point directly above the focus on the earth's surface is called the epicenter. This is what you see plotted on a map to represent the location of the earthquake.

Where do earthquakes occur?

Earthquakes and volcanoes do not occur randomly on the surface of the earth. They are found on or near specific areas called plate boundaries. The dots on the diagram below represent the location of earthquakes and volcanoes.

Now look at page 5 in your reference tables. Notice any similarities? I hope so!! The pattern of dots matches with many of the plate boundaries shown on the map. One of the more famous plate boundaries is called the "Ring of Fire". This is a ring of volcanoes and earthquake activity that surrounds the Pacific Ocean.

How are earthquakes measured?

Earthquakes are measured using instruments called seismographs. These instruments measure the vibrations traveling through the earth's surface.

Scientists can use the information from the seismograph to determine the magnitude (strength) of the earthquake. Scientists measure the amplitude or the amount of shaking and compare it to the Richter Magnitude Scale. The Richter Scale is a logarithmic scale and is related to the energy released by the earthquake. An increase of one unit means a ten-fold increase in amplitude (shaking). Therefore, a magnitude 4 earthquake has an amplitude that is ten times greater than a magnitude 3 earthquake.

The other scale used to measure the intensity of an earthquake is the Mercalli Scale. This scale is based on the observed amount of destruction and information from the people who experienced the earthquake. This scale does not measure or take into account the energy released by the earthquake.

For more information and resources about earthquakes click on Topic 3: The Dynamic Crust. Also see Topic B: Earthquakes and Earth's Interior towards the end of the subject outline.

New York State High School Regents Exam Prep Center: Earth Science