Earthquakes

Earthquakes Earthquakes are natural vibrations of the ground caused by movement along gigantic fractures in Earth’s crust or by volcanic eruptions..

Cause of Earthquakes Most earthquakes occur when rocks fracture, or break, deep within the earth. Fractures form from stress, the forces per unit acting on a material, exceeds the strength of the rocks involved.

Stress There are 3 types of stress: Compression: is stress that decreases the volume of a material. Tension: is stress that pulls a material apart. Shear: is stress that causes a material to twist

Strain: this is the deformation of materials due to stress Ductile deformation: when stress exceeds the strain a material can withstand, ductile deformation occurs. Meaning the material becomes deformed or can no longer resume its former shape. Elastic strain: this allows a material to undergo strain and stress and return to its former shape. (think of a rubber band)

Faults A fault is defined as a fracture or system of fractures in Earth’s crust that occurs when stress is applied too quickly or stress is too great.

Normal Fault Reverse Fault Strike Slip Fault Normal Fault – horizontal tension Reverse Fault – horizontal compression Strike Slip Fault – horizontal shear.

The San Andreas Fault in California is a Strike Slip Fault

Seismic Waves Every earthquake generates 3 types of waves Primary waves (P-waves), Secondary waves (S-Waves) P and S waves move through the center of the Earth and are called Body Waves Surface waves

What do the waves do? P –waves: squeeze and pull rocks in the same direction along which the waves are traveling. (like a slinky)

S-Waves: cause rocks to move at right angles in relations to the direction of the waves.(like a rope)

Body waves give information about the Earth’s Interior

Surface waves - move in two directions as they pass through the rock. They move in an up and down motion similar to an ocean wave. This wave also causes rocks to move from side to side. Surface waves travel along the Earth’s surface. These waves cause the most damage because of their large amplitude.

Measuring Earthquakes Earthquakes are measured on machines called seismographs.

A seismogram is the record produced by a seismometer. Seismometer or Seismographs record earthquake events by detecting the vibrations

P waves arrive first, then S waves, then surface waves P waves travel the fastest and arrive first, then S waves, then surface waves

Anatomy of Earthquake The point within Earth where faulting begins is the focus The point directly above the focus on the surface is the epicenter

Most Earthquakes Occur Along Plate Boundaries

Most Volcanoes occur along plate boundaries too!

~80% of all earthquakes occur in the circum-Pacific belt

This area is know as the Ring of Fire

Measuring Earthquake Intensity Seismologists use the Richter Scale and the Modified Mercalli Scale to measure the intensity of Earthquakes

The Richter Scale was developed by Charles Richter in This scale is a base-10 logarithmic scale. Each number you go up on the scale, the magnitude measured on the seismograph goes up 10 times.

Mercalli Scale Invented by Giuseppe Mercalli in 1902, this scale uses the observations of the people who experienced the earthquake to estimate its intensity.