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• Earthquakes: vibrations through earth’s crust – Occur when rocks under stress slip or shift along a fault Normally, rocks are pressed together tightly = not moving Or “locked” Elastic Rebound Theory • Rocks along each side of a fault are moving slowly. • If the rock is “locked”, then stress will increase • When the rock is stressed beyond its capacity, the rocks separate, or fracture, at the weakest point, and then they spring back to their original position – As rocks move they create vibrations, called Seismic Waves • Aftershocks: series of tremors caused by other areas adjusting to stress of the initial earthquake Focus: place where slip first occurs Epicenter: The point on Earth’s surface directly above the focus. Seismic waves radiate out in all directions from the focus. Major Earthquake Zones Where do earthquakes occur? • Along or near the edges of lithospheric plates Journal: Why do think that earthquakes occur in these areas? Explain the cause of the earthquakes! Fault Zones • Groups of interconnected faults along a plate boundary 6.2 Recording Earthquakes • Seismograph: instrument that detects and records seismic waves Types of Waves • The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities. Types of Waves • P wave: The fastest wave, and therefore the first to arrive at a given location. • compression wave, alternately compresses and expands material in the same direction it is traveling. • S wave : is slower than the P wave and arrives next • shakes the ground up and down and back and forth perpendicular to the direction it is traveling. • Surface waves: follow the P and S waves. Locating an Earthquake • Scientists analyze the difference between the arrival of P and S waves . Journal: • What instrument measures earthquakes??? Earthquake Measurement • Magnitude: Measure of energy released – Described as ground motion – Measured using the RICHTER SCALE • Mercalli Scale: describes intensity, or amount of damage an earthquake causes. – Scale I – XII Journal: • Describe the difference between the Richter Scale and the Mercalli Scale. • Reading an Earthquake Wave Graph IN CLASS