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earthquakes learning goals.docx

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Earth and Ocean Sciences
EOSC 114
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Earthquakes learning goals Earthquakes: 1. Where do earthquakes happen worldwide? They happen near plate boundaries and intraplate. 2. How frequently do earthquakes happen? Earthquakes happen very frequently, small ones are very common and are often un felt while large ones are rare. 3. Types of tectonic plate boundaries Divergent play boundaries is where the plates move apart leading to tension from stretching and this causes smallish earthquakes, Convergent plate boundries, there are 2 kinds; subduction zone and continental collision. Continental collision is where continental plates collide and subduction zone is where the ocean subducts under a continental plate or another ocean plate. Also there are transform plate boundaries which are plates that move past each other resulting in a shearing motion. 4. Types of earthquakes that occur at these plate boundaries For convergent plate boundaries the earthquakes can be extensively small or very large. Subduction zones are where the largest earthquakes occur. For divergent plate boundaries only smallish earthquakes occur. Transform plate boundaries produce many earthquakes from their shearing motion they are moderate to large but not as big as convergent margins. 5. Describe how the Earth builds, stores, and releases energy in earthquakes (elastic rebound) The earth builds up energy as elastic stresses build up as rock deforms slightly overtime, the energy is stored as the elasticity of the fault strains and then the energy is released when the elastic stresses exceed what the fault can bear this causes rocks along fault to spring back to undeformed state which is known as elastic rebound. 6. Understand that stress causes strain, and differences between plastic, elastic, and brittle deformation Stress causes strain because the stress strains the fault to bend in one way or another, Plastic deformation is like putty it is where the material strains in response to stress but permanent also no energy is stored in plastic deformation and so it does not bounce back when the stress is removed. Elastic deformation has relatively small stress and because of these small stresses it is not permanent because the materials shape is restored when force is removed due to the stored energy and this released energy can pass as waves. In Brittle deformation the material does store elastic energy but at some point the material will break this results in catastrophic release of energy, Brittle deformation also includes repeated breaking of pre-existing weak surfaces (faults) 7. Describe how a fault slips in an earthquake and why shaking and damage are not always greatest at the epicenter Faults are weak surfaces (weaker than surrounding rock) they break repeatedly and may accumulate hundreds of km of slip over millions of years. Faulting occurs when the friction along he boundary line may temporarily slow down the rough lithospheric plates moving past each other and this braking action exerts forces on the rocks near the plate boundaries and as a result the rocks undergo strain or deformation and when the stress on them exceeds their breaking point the rock suddenly moves along a fault which is how a fault slip occurs. Shaking and damage are not always the greatest at the epicentre because when a rupture begins at the hypocenter and travels away, the rupture propagates away from the hypocenter at about 2-3 km/sec and shaking is greater in the direction the rupture travels. 8. Describe the different types of seismic waves and how they move through the Earth The 2 categories of seismic waves are body waves and surface waves. Body waves travel inside the earth while surface waves travel along boundaries between materials. There are 2 types of Body waves which are P and S waves. P waves move by compression and extension of the solid which is similar to a sound wave it is the fastest type of seismic wave and particles move the same way the wave propagates. S waves move by shearing distortion of the solid, Slower than P wave, it is about 3.5 km per sec as compared to the P waves 6km per sec. S waves also cannot pass through fluids while P waves can. Surface waves require an interface to move such as ground-air, water-air, mantle-liquid outer core. Surface waves are slower than body waves but cause more damage. The 2 types of surface waves are the Rayleigh wave which is a vertical and horizontal motion parallel to the wave direction (like an ocean wave) and then there is a Love wave which has horizontal movement perpendicular to wave travel direction. 9. Understand the principle behind early warning systems, and know how much warning time they can give. The principle behind early warning system is that through the use of S-P lag times and it would also be based on the principle that radio waves travel faster than seismic waves. It would work by having a network from of seismometers which would sense the first motion from a large earthquake which would then send a warning to the city and critical facilities the warning time would be from as short as 15 seconds to as long as 1 min 10. Describe how an earthquake is recorded and how we locate the epicenter An earthquake is recorded by a seismograph which analyzes its S-P lag time to figure out the distance of the epicenter of the earthquake from the seismograph. The exact epicenter can be located by using 3 or more seismographs at various locations that are apart from one another to triangulate where the epicenter is which works by creating a radius around each seismograph and the point where the 3 radii touch is where the epicentre is located. 11. Predict how local ground conditions will affect the duration and amplitude of shaking Local ground conditions will affect the duration and amplitude of the shaking because if the local ground conditions are unconsolidated sediment or if the ground has high water content the seismic waves would slow which would cause some of the P and S Wave`s forward directed energy to be transferred to surface waves which would amplify the amount of ground motion because surface waves cause more shaking. 12. Compare and contrast the meanings and uses of magnitude and intensity scales Magnitude indicates how much energy was released and Intensity is how strong the ground motion is at the felt location. Magnitude scales indirectly estimate the magnitude they use seismic
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