EARTHQUAKES. Tommy Her, Ben Lee, James Edwards, Chandler Collier, Brent Dorn, and April Bartholomew (photographer)

Transcription

1 EARTHQUAKES Tommy Her, Ben Lee, James Edwards, Chandler Collier, Brent Dorn, and April Bartholomew (photographer) Introduction to Physical Science, EMPACTS Project C. Dianne Phillips, Instructor, NWACC, Bentonville, AR 72712

2 How are Seismic waves generated? Seismic waves are tremors or vibrations in the Earth's crust that are caused by the build up or accumulation of pressure (more correctly termed stress). This accumulation of stress causes the rocks that make up the crust to deform elastically. This is very similar to what happens when you squash or stretch a spring and cause a form of energy to be stored in the rocks of the crust - technically described as elastic potential energy. Strain energy increases and eventually exceeds the ability of the rock to accommodate the stress applied. When this stress gets too large, it exceeds the strength of the rocks in the crust and causes a brittle failure. Brittle failures are failures where fractures form through the material.

3 Elastic Rebound Theory The elastic rebound theory is an explanation for how energy is spread during earthquakes. As rocks on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.

4 Different types of Seismic Waves P waves- P-waves, also known as primary waves or pressure waves, travel at the greatest velocity through the Earth. When they travel through air, they take the form of sound waves. S waves- S-waves, also known as secondary waves, shear waves or shaking waves, are transverse waves that travel slower than P-waves. In this case, particle motion is perpendicular to the direction of wave propagation. Again, imagine a slinky partially stretched, except this time, lift a section and then release it, a transverse wave will travel along the length of the slinky. S-waves cannot travel through air or water but are more destructive than P-waves because of their larger amplitudes.

5 Seismogram A seismogram is a graph output by a seismograph. It is a record of the ground motion Seismograms typically record motions in three cartesian axes (x, y, and z), with the z axis perpendicular to the Earth's surface and the x- and y- axes parallel to the surface.

6 Surface waves are similar in nature to water waves and travel just under the Earth s surface. They are typically generated when the source of the earthquake is close to the Earth s surface. Although surface waves travel more slowly than S-waves, they can be much larger in amplitude and can be the most destructive type of seismic wave. There are two basic kinds of surface waves: Rayleigh waves- these waves travel as ripples like water. Love waves- they cause horizontal sheering of the ground. They are a bit faster than Rayleigh waves.

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8 What can seismic waves tell us? Studies of the different types of seismic waves can tell us much about the nature of the Earth s structure. For example, seismologists can use the direction and the difference in the arrival times between P-waves and S-waves to determine the distance to the source of an earthquake. If the seismographs are too far away from the event to record S-waves, several recordings of P-waves can be crunched in a computer program to give an approximate location of the source.

9 Different kinds of Earthquakes Earthquakes can come in three main forms, depending on the plate movements that occur beneath the earth's surface. They could occur on a Convergent Boundary, Divergent Boundary or a Transform Fault.

10 Convergent boundary: one plate is forced over another plate during movement creating a thrust fault Divergent boundary: plates are force apart each other, using forming a Rift Zone. This kind is coming in ocean floors where new floors are created. Transform fault: The plates slip by each other this is also called Strike-Slip.

11 Faulting? Faulting is a break in the rocks that make up the Earth s crust, along which rocks on either side have moved past each other.

12 Difference between a fault and a tectonic boundary. Fault A fault is a break in the rock, where each side of the rock moves in opposite directions Tectonic boundary A tectonic plate boundary is simply where 2 plates meet

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