# What is an Earthquake?

Introduction
What is an earthquake is a question often asked by people. Earthquakes are seismic waves inside the Earth that follow the release of energy that has built up inside the rocks. Rocks fracturing, volcanoes erupting, and man made explosions trigger earthquakes.

Parking car garage collapsed during
an earthquake
D Carver, USGS

Focus of an earthquake
The point where the rocks rupture is the focus of an earthquake. The geographic location on the surface of the Earth directly above the focus is the epicenter of an earthquake.

P waves
All earthquakes produce P waves and S waves that travel through the Earth. P waves are compression waves and arrive first at reporting stations. The waves cause the rocks to compress and expand like movement through a slinky as the waves travel through the Earth.

S waves
S waves are secondary waves because they arrive at reporting stations after the P waves. S waves only travel through the solid Earth and stop when they reach the outer core of the Earth which is liquid.

Plate boundary earthquakes
Ninety percent of the earthquakes on Earth are produced at plate boundaries. A convergent boundary is a place where two plates are coming together creating a subduction zone. Great earthquakes that produce tsunamis occur when rocks fracture in these subduction zones.

Transform boundary earthquakes
A transform boundary like the San Andreas Fault produces thousands of earthquakes each year. The mid ocean ridge forms where two plates are separating. The boundary produces small earthquakes as molten rock moves from the mantle to the surface of the ocean floor.

What is an earthquake scale
There are two major scales that describe the size of earthquakes. The Richter scale is used generally for reporting earthquakes. It was designed in 1934 and when news reports describe an earthquake as a 6.2 magnitude quake they are using the Richter scale.

Moment magnitude scale
Great earthquakes that occur along subduction zones use the moment magnitude scale because it includes the size of the earthquake as well as the size of the seismic waves. The largest earthquake ever recorded was the 1960 Chile earthquake that registered 9.5 on the moment magnitude scale.

Convergent Boundary There are two types of boundaries where plates converge. They produce great mountain ranges and large earthquakes.

Transform Boundary  These boundaries are found all around our ocean floors where oceanic plates are separating.

Pacific Plate The Pacific Plate is the largest plate on Earth. Great earthquakes and towering volcanoes surround the plate along the Pacific Ring of Fire.

Ocean Trenches Find out how plates diving beneath other crustal plates create trenches on the ocean floor that are deepest points on Earth.

S Waves Find out about S waves and how they are produced and travel through the Earth.

What is an Earthquake Find out what causes the Earth to shake, rattle and roll during an earthquake.

Plate Tectonics Find out lots of fascinating facts and interesting trivia on plate tectonics.

Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more.

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