Earthquakes.  Natural vibrations of the ground caused by movement at faults  Faults: Fracture in the Earth’s crust Faults form when rocks break deep.

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Earthquakes

 Natural vibrations of the ground caused by movement at faults  Faults: Fracture in the Earth’s crust Faults form when rocks break deep within the Earth’s surface due to stress

In conclusion…  Earth’s rocks deform when stress is exerted on them. If the stress exceeds a certain limit, the rocks fracture to form faults. Movement along faults causes most earthquakes.

Earthquake Waves  Seismic Waves – The vibrations of the ground during an earthquake  3 Types of Seismic waves Primary waves (aka P waves)  Push/pull rocks same way waves are traveling  Travels through Earth’s interior

Secondary waves (S waves)  Move perpendicular to the direction of the waves  Travel through interior of Earth Surface waves  Move in 2 directions Like the ocean and like a snake at the same time  Travel on the surface of Earth

Some more vocabulary  Focus Point of failure of the rocks Where the earthquake starts Usually several kms below the surface  Epicenter The point on Earth’s surface directly above the focus

Locating Earthquakes  There are more than 1 million earthquakes each year…  However, 90% of them are not felt by people.  The exact location of the epicenter and the time of occurrence are initially unknown.

 For any distance from the epicenter, the P waves always arrive first.  The further the recording instrument is from the epicenter, the greater the difference in arrival times of the 1 st P wave and the 1 st S wave.  This information is used to determine the exact location of the epicenter. Gathering data….

510 15 2025 P wave S wave

Need data from several locations: At least 3 locations

 A travel-time graph is used to determine the distance from a recording station to the epicenter Based on travel time of the P and S waves The time between when the P wave arrives and the S wave arrives is called the S-P lag

Epicenter Distance (km) S-P lag (min)

Using the info…  Once you figure out the distance of the epicenter to the recording instrument you plot a CIRCLE on a map with the radius being the distance discovered.

Where the 3 intersect is the epicenter

Measuring Earthquakes  There are several different scales used to measure earthquakes. Measure size of seismic waves Measurements of the fault  Size, amount of movement, type of rock occurred in Measure amount of damage done on surface of Earth

CD-ROM video clips  Slides on CD-ROM  Types of seismic waves 369-371  Locating an epicenter 375-387

Factors Influencing Intensity  Size of seismic waves produced  Depth of the focus  Distance from the epicenter of the quake

Using Earthquakes for Info  Most of the knowledge of Earth’s interior comes from the study of seismic waves. These waves change speed and direction when they encounter different materials. Scientists track where they are detected and at what strength.

Understanding Stress  Fractures form when the stress exceeds the strength of the rocks  3 Kinds of Stress: a force Compression – push motion Tension – pull motion Shear – twist motion  Strain – deformation in material due to stress.

Stress Strain Elastic Deformation – not permanent Ductile Deformation- permanent damage Can Vary Based on Temperature! Failure

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