Lithosphere-Earthquakes Unit

What is an earthquake? An earthquake is a sudden, violent shaking of the ground due to rocks fracturing (breaking) deep within the Earth’s crust roun

Where do earthquakes occur? Earthquakes usually occur along plate boundaries, esp. subduction

Faults A fault is an area of stress in the earth where broken rocks slide past each other, causing a crack in the Earth’s surface Fractures form when stress exceeds the strength of the rocks involved

Elastic Rebound Stress causes the ground begins to be deformed. An earthquake occurs when a rock passes its point of maximum elasticity. After it breaks, it regains its original shape (but not position.)

Three Types of Stress Three types of stress: (1) compression-decreases volume (2) tension-pulls apart (3) shear-twists material

Three Types of Faults Reverse faults – form due to compression, shortens crust Normal faults – form due to tension extends crust Strike-slip faults – form due to shear

Faults

Focus and Epicenter Focus – where the fracture occurs to cause an earthquake, beneath Earth’s surface; the closer focus is to Earth’s surface, the more severe the damage is to be expected. Epicenter – the spot on the surface above the focus

Seismograph A device used to measure seismic waves in the Earth’s interior. Seismogram-written record of a seismograph.

Richter Scale Magnitude-a measurement of energy released by an earthquake. Richter scale-a numerical scale based on size of largest seismic wave generated by a quake that describes its magnitude. Higher the number, higher the magnitude.

Historical Earthquakes

Mercalli Scale The Mercalli intensity scale is a seismic scale  used for measuring the intensity of an earthquake.

Seismic Waves Vibrations of an earthquake Three types of waves: Primary waves (P waves) Secondary waves (S waves) Surface waves

P Waves P waves (primary)-push and pull rocks in same direction wave travels; fastest seismic wave. Animation

S Waves S waves (secondary)-moves rocks at right angles to direction of wave’s travel. Animation

Surface Waves Surface waves-travel on Earth’s surface, in both directions of p and s waves; slowest seismic waves, cause most damage.

What P and S waves can tell us By comparing the time difference between the arrival of P and S waves, the epicenter of an earth quake can be located It has also been determined that Earth’s outer core is molten through observation of P and S waves

P Wave Shadow Zone P and S waves can both travel through solids and move in direct patterns in the mantle. P waves that strike the liquid outer core alter direction, creating an area where no P waves appear on a seismogram called the P wave shadow zone.

P Wave Shadow Zone S waves that strike the core do not pass through. S waves cannot travel through liquid, which led scientists to believe the Earth’s outer core was molten. Create an earthquake

Finding the Epicenter Three seismograph stations are needed. Why? Each location gives us a distance, so we can draw a circle there. Then the intersection of 3 circles gives us the epicenter.

How could you tell the epicenter if you only had two seismographs?

Let’s look at the Tonga Trench Deepening earthquakes at a subduction zone

Tsunami A series of ocean waves caused by sudden displacement in the ocean floor .

Tsunami travel times to Hawaii

Old Tsunami Photo