# Earthquakes Chapter 19.

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Earthquakes Chapter 19

What is an earthquake? Natural vibration of the ground
Caused by movement along fractures or volcanic eruptions

Most earthquakes occur when…
Rocks fracture or break deep within the Earth. Fractures form when STRESS, the forces per unit area acting on a material, exceeds the strength of the rocks involved.

3 Kinds of stress Compression: Pushes together Tension: Pulls apart
Shear: Twists

Faults Many rocks that make up the crust fail when stress is applied to quickly. The result is a system of fractures called FAULTS.

Three Types of Faults Three types of faults
Form depending on type of plate motion and complex reaction of earth’s lithospheric blocks Strike-slip Normal Thrust

Seismic Waves Vibrations of the ground during an earthquake are called Seismic Waves (there are 3 types).

1. Primary Waves (P Waves)
Squeeze and pull rocks in the same direction along the wave direction. The first wave to arrive at an earthquake

Secondary Waves (S Waves)
Cause rocks to move at right angles in relation to the direction of the wave.

Surface Waves Move in 2 directions as they pass through rock.
Produces motion in the upper crust Motion can be up and down Motion can be around Motion can be back and forth Travel more slowly than S and P waves More destructive

Focus –. point inside the Earth where an
Focus – point inside the Earth where an earthquake begins Epicenter – point on Earth’s surface above focus

Seismology: the study of earthquakes

How Seismographs (seismometers) Work
the pendulum remains fixed as the ground moves beneath it

Typical Seismogram

How do scientists calculate how far a location is from the epicenter of an earthquake?
Scientists calculate the difference between arrival times of the P waves and S waves The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

How are Earthquakes Measured
How are Earthquakes Measured? Richter Scale: The size of the largest seismic wave in magnitude.

How are Earthquakes Measured? Mercalli Intensity Scale

Most earthquakes…. Are associated with tectonic plate boundaries
Most earthquakes…. Are associated with tectonic plate boundaries. Almost 80% of all earthquakes occur in the Circum-Pacific Belt!

Destruction to buildings
“Pancaking” Where structural buildings fall and debris resembles a stack of pancakes.

Tsunamis

Formation of a tsunami

Tsunami Warning System