2 What is an Earthquake?A sudden trembling of the ground caused by movement happening in the crust
3 Where Do Earthquakes Happen? Most earthquakes happen at the edges of plates, where plates meetAbout 80 percent of all earthquakes happen along the edge of the Pacific PlateAlong the edges, plates can collide, they may pull apart, or they may slide past each otherThese motions can cause the rocks to bend and stretch until they breakAs a result, faults form at or below the surface of the crust
6 What is a Fault?Fault- huge cracks inthe Earth’s crust
7 Three Kinds of Faults 1. Normal Fault Produced at divergent boundaries Rocks above the faultsurface move down
8 Three Kinds of Faults 2. Reverse Fault Produced at convergent boundariesRocks above the faultmove upward
9 Three Kinds of Faults 3. Strike Slip Fault Produced at transform boundariesRocks slide past each other indifferent directions
10 How Earthquakes StartEarthquakes start the moment rocks begin to scrape past each other along a fault.
11 How Earthquakes Start Focus The point where the earthquake starts, where rocks begin to slide past each otherIt is usually below the surfaceThe sudden movement causes vibrations to spread out from the focusThese vibrations travel through the crust in the form of waves
12 How Earthquakes Start Seismic Waves A vibration that spreads out away from a focus when an earthquake startsThey soon reach the Earth’s surface at a point directly above the focus
13 How Earthquakes Start Epicenter The point on the Earth’s surface directly above the focus of an earthquakePeople can first feel the ground shaking at the epicenter
14 How Earthquakes Start Epicenter This shaking is what causes most earthquake damageDamage is usually greatestat the epicenter becauseit is so close to the focusAs waves travel away fromthe focus they get weaker
15 AftershocksAfter the first shaking, it may be quiet, and then there are aftershocksThis is the shaking of the Earth’s crust after the initial shaking of an earthquake
16 Earthquake WavesThere are different kinds of seismic waves which travel differently and at different speeds.
17 Earthquake Waves Primary Waves, or P Waves Accordion-like motion shakingRock material squeezes together and spreads apart repeatedlyThis motion produces seismic waves that move in the same direction that the rock is shakingThey are the fastest seismic wave- the first to reach any faraway location
18 Earthquake Waves Secondary Waves, or S waves Shaking like a ruler held off the edge of a desk and “twanged”This motion in rocks creates seismic waves that move in a perpendicular direction from the vibrationThese waves travel slower than primary wavesThey are second to arrive at any given faraway location.
19 Earthquake Waves Surface Waves This shaking causes Earths surface to heave up and down like an ocean wave, or sway from side to sideTears apart structures built on the surface