Earthquakes Stress Relief

Earthquakes & Plate Tectonics Rocks break & move along surfaces called faults Rocks break & move along surfaces called faults When plates move, stress is put on rocks. When plates move, stress is put on rocks. A rubber band (just like rocks) can only be stretched so far until it breaks; this is known as the elastic limit. A rubber band (just like rocks) can only be stretched so far until it breaks; this is known as the elastic limit. Earthquakes are the Earth vibrating. Earthquakes are the Earth vibrating.

Where Do They Occur? Most earthquakes occur along tectonic plate boundaries Most faults are located near plate boundaries 80% of all earthquakes occur along edges of the Pacific Plate Pacific Ring of Fire is an area where earthquakes & volcanoes are very common

Why Do We Have Quakes? As plates bump and grind with one another, deformation takes place. Deformation is the rocks response to stress being applied to it. Forces at Plate Boundaries: 1) Compression 2) Tension 3) Shearing

Faults 3 types of faults Normal Normal Reverse Reverse Strike-slip Strike-slip

Types of Faults- Normal Normal –occur at divergent plate boundaries –force of tension is at work –Rock above the fault surface moves downward in relation to the rock below the fault surface –Example: Sierra Nevadas

Types of Faults- Reverse REVERSE –occur at convergent plate boundaries –force of compression at work –The rocks above the fault surface are forced up & over the rocks below fault surface –Example: Himalayas

Types of faults- Strike-slip –occur at transform fault boundary –force of shearing at work –Rocks move past each other without much upward or downward motion –Rocks snag one another & become twisted & strained –Example: San Andreas fault

Boundary/Force/Fault BoundaryForceFault ConvergentCompressionReverse DivergentTensionNormal Transform Fault ShearingStrike-slip

How Strong are Earthquakes? The strength of an earthquake depends on the type of plate motion involved at the different faults.

Faults & Earthquake Strengths Produced 1. Strike-slip faults: because of the horizontal motion of the plates, earthquakes are typically very SHALLOW and MODERATE in strength 2. Reverse faults: due to compression, earthquakes are usually very DEEP and very STRONG 3. Normal faults: due to tension pulling plates apart, the quakes are SHALLOW and WEAK

Focus vs. Epicenter The focus is the point inside the Earth where primary and secondary waves originate. The focus is the point inside the Earth where primary and secondary waves originate. The epicenter is the point directly above the focus on the surface where surface waves are generated. The epicenter is the point directly above the focus on the surface where surface waves are generated.

Seismic Energy Primary waves –fastest waves –causes particles in rock to move back & forth in same direction Secondary waves Move the earth by causing particles of rock to move at right angles to direction of wave Arrive 2nd at a seismograph station

Surface Waves Surface Waves- Surface Waves- Move by back & forth swaying motion Move by back & forth swaying motion Move by giving particles an elliptical motion Move by giving particles an elliptical motion travels out from epicenter travels out from epicenter Slowest waves Slowest waves Cause the most damage on the earth’s surface Cause the most damage on the earth’s surface

Surface Waves Surface waves travel on the Earth’s surface and cause the most damage to buildings and roads. Their movement resembles ocean waves, moving the ground up and down. They travel the slowest of all seismic waves.

Locating an Epicenter Need 3 seismograph station readings Need 3 seismograph station readings Scientists draw circles around seismograph stations; the radius of the circle is equal to the station’s distance from the earthquake epicenter Scientists draw circles around seismograph stations; the radius of the circle is equal to the station’s distance from the earthquake epicenter The Intersection of the 3 circles is where the epicenter is located The Intersection of the 3 circles is where the epicenter is located The farther apart the arrival of the primary, secondary, & surface waves, the farther away the epicenter The farther apart the arrival of the primary, secondary, & surface waves, the farther away the epicenter

Seismic Waves At certain depths, the speed & path of seismic waves change. At certain depths, the speed & path of seismic waves change. The changes mark the boundaries of earth’s layers. The changes mark the boundaries of earth’s layers. Moho discontinuity- boundary between crust & upper mantle Moho discontinuity- boundary between crust & upper mantle The Shadow Zone is an area where no waves are detected. The Shadow Zone is an area where no waves are detected. Boundaries cause waves to change speed & bend. Boundaries cause waves to change speed & bend.