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EARTHQUAKES • Natural, rapid shaking of the ground that results from displaced rocks within the crust. • Breaks in the rock release energy that travel through the Earth as seismic waves. • When the waves reach the surface, the ground moves. EARTHQUAKES • When seismic waves reach the ocean floor, tidal waves (tsunamis) are formed. • Earthquakes tend to occur along plate boundaries, however, an earthquake can occur along ANY fault in the rock anywhere! SEISMIC WAVES • Seismic waves are detected using a seismograph, which uses a series of springs and weights to record vibration. • 3 types of seismic wave: • Primary (P) waves: compressional or longitudinal, like sound waves. Travel fastest and so are detected first – hence the name. Can travel through solids, liquids, and gases. SEISMIC WAVES • 3 types of seismic wave: • Secondary (S) waves: transverse, like ocean waves or light. Particles move perpendicular (at right angles) to the direction of the wave and can only travel through solids. SEISMIC WAVES • 3 types of seismic wave: • Surface waves: produced by interaction of P and S waves, resulting in a rolling motion of the ground. EPICENTER AND FOCUS • The break in the rock which is the source of the waves is called the focus. • Area directly above focus is called the epicenter. EPICENTER AND FOCUS • The location of the epicenter is determined by the arrival of P- and S-waves: • Using a seismograph to record the waves, distance to epicenter can be calculated if you know when waves arrived and how fast each wave type moves. Which we do. • You need three seismograph stations to find an epicenter (triangulation). EPICENTER AND FOCUS • Why three? What if you only had one seismograph station? • What if you had two? • Triangles. Is there anything they can’t do? EARTHQUAKE MEASUREMENT • There are two scales I expect you to know: • Richter scale • Measures energy of earthquakes. • Scale goes from 0-9. Each increase in number means a tenfold increase in magnitude (a logarithmic scale). • So a 6.0 is 10x as large as a 5.0. A 7.0 is 100x as large as a 5.0. EARTHQUAKE MEASUREMENT • Modified Mercalli Intensity scale (MMI) • Measures destruction level at a given location. • Scale goes from I to XII. EARTHQUAKE MEASUREMENT • Image – Richter and MMI scales