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# What are some of the main characteristics of an earthquake?

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Earthquakes produce two types of potentially destructive waves that move through the earth from the point of the fault: primary, or pressure waves and secondary, or shear waves. Primary waves, also called P waves, exert a force of compression and travel through rock at speeds that can exceed 225 mph. Secondary waves, also called S waves, exert a shearing force and travel only half as fast as P waves, but are capable of causing much greater damage when they reach the surface.

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When secondary seismic waves reach the surface, they can have different characteristics and will move the ground in a direction perpendicular to their direction. Named after A.E.H. Love, the British mathematician who first described them, Love waves are S waves that cause horizontal shearing, which results in the ground moving from side to side. Rayleigh waves, named after John William Strutt, Lord Rayleigh, who first predicted them, move the ground in the manner of an ocean wave. These are S waves that cause the ground to roll over and under when they reach the surface.

An earthquake occurring under the sea floor can cause a potentially destructive ocean wave called a tsunami. A shift in the ocean floor causes a displacement of water and a transfer of energy that can produce a potentially forceful and highly destructive tidal wave when it reaches a shoreline. As a tsunami approaches the shallower depths near land, the horizontal force is transferred upward and can cause the wave to reach heights of 100 feet or more.

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## Related Questions

• A:

The Sumatran-Andaman earthquake on December 26, 2004, is the longest lasting earthquake ever recorded. The continuous shaking lasted from 500 to 600 seconds, or about 10 minutes.

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The United States Geological Survey keeps a record of all earthquakes around the globe. In addition to providing seismic data from survey sites, the USGS also offers an application that allows users to report earthquake tremors they have felt. The USGS records all geological activity, not just earthquakes.

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According to the Southern California Earthquake Center (SCEC), the difference between an earthquake, also known as the mainshock, and an aftershock is that an aftershock follows closely in the wake of a larger earthquake and in approximately the same area as that earthquake. Earthquakes are usually more powerful and longer lasting than aftershocks. An aftershock will not occur unless there is a mainshock that occurs first.

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The epicenter of an earthquake is the point on the surface of the Earth directly above the point in the crust where a seismic rupture occurs. This origin point within the crust is called the hypocenter or focus.