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Lecture 3

Environmental Hazards Week 3 Notes.docx

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Environmental Science
Mandy Meriano

Natural Hazards Week 3 Notes Earthquakes are the result from rupture of rocks along a fault -> energy released in the form of seismic waves. They are mapped according to their epicenter. They are measured by seismographs. An earthquake is the sudden motion of a rock body along a fault. A fault is a plane of fracture in the lithosphere, where rock masses can move past each other. A strike-slip fault is one in which the motion is horizontal. A dip-slip fault is where the motion is vertical. Normal fault – Rocks on the hanging wall drop relative to the rocks on the footwall. Reverse Fault – The rocks on the hanging wall are uplifted relative to those on the footwall. Strike slip faults can be classified as either right or left stepping laterals. When a strike slip earthquake occurs, additional stresses occur anywhere there is a step (bend). Some steps interfere with motion, others do not. Active faults – movement during the past 11,600 years Potentially active faults – Movement during the past 2.6 million years. Inactive faults – No movement during the past 2.6 million years Tectonic creep occurs when movement is so gradual that earthquakes are not felt. (They can slowly damage infrastructure). Body waves travel through the body of the earth. P-waves (primary or compressional waves) – move fast with a push/pull motion. Can move through solids, liquids and gasses. S-waves (secondary or shear waves) – Move slower with an up/down motion. Can travel only through solids. Surface waves (Love and Rayleigh waves) travel along the surface of the earth. These are the slowest travelling waves, but they have the highest amplitude and cause the most damage. As p-waves travel faster than s-waves, there is a relationship between their arrival times and the distance between the earthquake and the seismic station (the differences in seconds correspond to distance). Finding the distance between earthquake and three separate seismic stations allow the epicenter of the earthquake to be determined. The Richter magnitude scale measures the amplitude of the seismic waves. The moment magnitude scaled measures the total energy released by an earthquak
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