What is an earthquake? An earthquake is a sudden, rapid shaking of the Earth caused by the breaking and shifting of rock beneath the Earth’s surface. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth’s surface move slowly over, under and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. (This should be review) When the energy grows strong enough, the plates break free causing the ground to shake.

Where do earthquakes appear? Most earthquakes take place near the edges of tectonic plates. Earthquakes can also occur at faults, which are cracks in the Earth’s crust.

What causes earthquakes? Elastic deformation leads to earthquakes. Rocks deform in response to stress in the Earth’s crust. When the rock can no longer bend under stress it will break Elastic rebound is the return of the deformed rock to its original shape. During this process energy is released as seismic waves and these waves result in earthquakes.

What Is Seismology? Seismology is the study of earthquakes and seismic waves that move through and around the earth. A seismologist is a scientist who studies earthquakes and seismic waves.

How do earthquake waves travel? Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs. There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the Earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves.

Seismic waves traveling through the Earth

BODY WAVES Traveling through the interior of the earth, body waves arrive before the surface waves emitted by an earthquake. These waves are of a higher frequency than surface waves.

P-waves Aka pressure waves or primary waves They are the first waves to be detected. They can travel through solids, liquids and gases. P wave animation

S-waves Aka shear waves or secondary waves They are the second waves to be detected. They cannot travel through liquids. S wave animations

SURFACE WAVES Traveling only through the crust, surface waves are of a lower frequency than body waves, and are easily distinguished on a seismogram as a result. Though they arrive after body waves, it is surface waves that are almost entirely responsible for the damage and destruction associated with earthquakes This damage and the strength of the surface waves are reduced in deeper earthquakes. Surface wave animations

Locating Earthquakes Seismographs-instruments located at or near the surface of the Earth that record seismic waves Seismogram-a tracing of earthquake motion created by a seismograph S-P time method- allows one to locate the epicenter of an earthquake; a method of comparing numerous seismograms by lining up data for p and s waves on a distance-time graph How a seismograph works

Locating Earthquakes Epicenter-the point on the Earth’s surface directly above an earthquake’s starting point Focus-the point inside the Earth where an earthquake begins

How strong was the earthquake? The Richter Magnitude Scale Created in 1930 by Charles Richter Measures the strength of earthquakes (aka magnitude) It is a logarithmic scale-each time the magnitude increases by one unit, the measured ground motion becomes 10x larger