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Earthquakes: How do we measure their impact? Review: Forces and Faults • What are 3 types of forces responsible for earthquakes? – Compression – Tension – Shear • What faults are associated with each type of force? Describe. – Compression – reverse fault – Tension – normal fault – Shear – strike – slip fault Reverse Energy and Seismic Waves • When rocks move and break along a fault, energy is released and this energy creates damage – Energy is released from the focus, located under the surface of the Earth – This energy travels away from the focus as seismic waves – “Seismic” is from the Greek word for shaking – Click to see an animation of the seismic waves Seismic Waves • Primary waves (P – waves) – Make rocks vibrate back and forth – Fastest waves at about 6km/second • Secondary waves (S – waves) – Shake rocks from side to side – Half the speed of P waves about 3 km/second • Energy that reaches the surface is in the form of waves called surface waves – Cause the most damage but are the slowest…why? Surface Waves • The point on the Earth’s surface directly above an earthquake’s focus is the epicenter • Surface waves travel outward from the epicenter and move up, down and side to side • Can cause buildings to move in different directions and fall apart How do we measure the strength of an earthquake? • Seismologists use the different speeds of the seismic waves to calculate the distance to the epicenter of the earthquake • A seismograph measures the energy released, or magnitude of the earthquakes • Which types of waves arrive first to a station? 1)Primary 2)Secondary 3)Surface How do we measure the strength of an earthquake? • Earthquake magnitude is usually measured using the Richter scale – 1.0 – 9.0 – Each 1 step increase = 10 x stronger Ex: From 6.0 to 8.0 = 100x Earthquake Occurrences Richter Magnitude Number expected/year 1.0 – 3.9 > 949 000 4.0 – 4.9 6200 5.0 – 5.9 800 6.0 – 6.9 120 7.0 – 7.9 18 8.0 – 8.9 <2