Transcript Earthquakes
We’ve looked at plate tectonics... Now lets look at a possible outcome: Earthquakes What are Earthquakes? • The shaking or trembling caused by the sudden release of energy • Usually associated with sudden movements of tectonic plates • Continuing adjustment of position results in aftershocks • • The point within the Earth where faulting begins is the focus, or hypocenter The point directly above the focus on the surface of the earth is the epicenter http://www.earthquakecountry.info/roots/basics.html Wadati-Benioff zone • A dipping flat zone of earthquakes that is produced by the interaction of a downgoing oceanic crustal plate with a continental plate. Seismographs The instrument used to record and measure vibrations caused by the breakage of rock along a fault zone. http://www.youtube.com/watch?v=DX5VXGmdnAg&feature=related http://www.youtube.com/watch?v=83GOKn7kWXM&feature=related Using Waves • The shock waves spreading out from an earthquake are called seismic waves. • There are two general types of seismic waves: body waves and surface waves. • Surface waves travel just beneath the Earth's surface. • Body waves travel through the Earth's interior. • There are two types of body waves, P waves and S waves. • S waves have much higher amplitude than P waves, travel more slowly, carry more destructive force, but cannot travel through the Earth's liquid core, while P waves can. • P waves arrive first, and then S waves (followed by L and R waves). • Because we know the average speeds for the waves, after an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter. • http://www.youtube.com/watch?v=yOGoK CK17a4 How is an Earthquake’s Epicenter Located? • Three seismograph stations are needed to locate the epicenter of an earthquake • A circle where the radius equals the distance to the epicenter is drawn • The intersection of the circles locates the epicenter Measuring the Strength of Earthquakes – The Richter scale measures total amount of energy released by an earthquake – Measurements on the Richter Scale increase by factors of 10. – An earthquake of magnitude 6 is 10 times stronger than one of magnitude 5 and 100 times stronger than one of magnitude 4 Earthquakes Occurrence • 80% of all earthquakes occur in the circum-Pacific belt • 15% occur in the Mediterranean-Asiatic belt • 5% occur in the interiors of plates and on spreading ridge centers More than 150,000 quakes strong enough to be felt are recorded each year Destructive Effects of Earthquakes • Ground Shaking – Damage increases in poorly consolidated rocks – Best place to be is on solid bedrock • http://www.youtube.com/watch?v=CtBXTv tFaCU&feature=fvwrel Earthquake Damage Mexico City 1985 Alaska 2002 Indonesia 2005 New Zealand 1987 San Francisco 1989 Effects • Landslides • Fires (from damaged electrical or gas lines) • Soil liquefaction (sand temporarily transforms from a solid to a liquid, causing buildings/bridges to tilt or sink) • Tsunamis • Flooding • Injury, disease, loss of life • Property damage (higher insurance rates) Can Earthquakes be Predicted? Clues that an Earthquake may happen: – changes in elevation or tilting of land surface, – fluctuations in groundwater levels and/or magnetic fields, – electrical resistance of the ground, – opening of gaps or fissures We know where they can happen, and we can produce risk assessments, but we can’t warn public. Geologist can say its coming “soon”. But “soon” could be next week or in 123 years!!! Earthquake Prediction Programs – laboratory and field studies of rocks before, during, and after earthquakes – monitor activity along major faults – produce risk assessments • Predicting earthquakes