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Earth’s Interior Processes Ch. 15 - Earthquakes & Plate Tectonics Earthquakes What are earthquakes? Earthquakes What are earthquakes? • Shaking of Earth’s crust caused by a release of energy due to movement of lithospheric plates • Usually associated with faulting or breaking of underground rocks • Continuing adjustment of position results in aftershocks Earthquakes • Focus: The point within the earth where the first movement occurs during an earthquake • Epicenter: The point on the Earth’s surface directly above the focus (this is usually the location reported in the media) Earthquakes Causes of earthquakes: Earthquakes can occur due to many reasons: – Volcanic eruption: • Earthquakes can occur as rock under the earth is moving to fill in spaces where magma is no longer present – Meteor impact: • Very rarely a great meteorite will hit, causing the ground to shake – However most earthquakes are triggered by the movement of lithospheric (tectonic) plates Earthquakes • Most of the time, friction between the plates prevents movement along the plate boundary. • However, this causes strain to build up and and cause the plates deform or change shape • Eventually, strain becomes great enough to overcome frictional forces Plates move suddenly, and snap back to their original shape but at a new location creating an earthquake This explanation for the cause of earthquakes is called the Elastic Rebound Theory Earthquakes Seismic Waves • Earthquakes release energy in waves called seismic waves. • There are two types of seismic waves Earthquakes Seismic Waves Two types of seismic waves: 1. Body waves – travel from the focus of an earthquake through the Earth’s interior 2. Surface waves – travel along the Earth’s surface (on the ground) Earthquakes Seismic Waves: Two types of BODY waves a) Primary Waves (P-waves): – Squeeze and stretch rocks parallel to the direction of travel – Also called compressional waves – Travel through solids, rocks, and liquids – Travel with a high velocity (5-8 km/s) Rocks & Minerals Seismic Waves: Two types of BODY waves b. Secondary waves (S-waves) – Move rocks perpendicular (90 degree angle) to the direction of wave movement – Much slower than P-waves (3-6 km/s) – Travels through solids but not liquids and gases Rocks & Minerals • P and S waves also help us understand the Earth’s interior • Both waves speed up when travelling through more dense material and slow down through less dense material Earthquakes • At 2900km deep, P waves slow down and S waves stop At the core-mantle boundary Remember: S waves can’t travel through liquid (outer core) • At 5200km deep, P wave velocity increases At the outer-inner core boundary P waves move faster through solid than liquid Therefore the inner core must be solid Earthquakes • When P waves cross from one layer into another, they are refracted • This causes a “shadow zone”, where seismograph stations can’t detect P or S waves from an earthquake Earthquakes Seismic Waves: Two types of SURFACE waves: a) Love waves travel from side to side b) Rayleigh waves travel in elliptical patterns Earthquakes Locating an Earthquake: • A seismograph is an instrument that detects and record earthquake waves onto sheets • This produces a record of seismic activity called a seismogram More about this next class For now, see p. 274 Earthquakes