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Western University
Earth Sciences
Earth Sciences 1022A/B
Stephen R Hicock

Earth Sciences 1022b Lecture Brief 11 Feb 14 EARTHQUAKES Cause: mainly by elastic-rebound of the crust as stress builds up - rocks bend, then eventually rupture at the focus and energy radiates in all directions, then the cycle starts over as rocks start bending again; commonly happens at plate boundaries - foreshocks and aftershocks accompany the main event with some earthquakes Seismology: uses tremor-detecting seismographs to record vibrations as surface waves, and body waves that travel through rock: surface waves: slowest, long l waves travel on the surface like ripples on water compressional: push-pull - faster, primary p waves travel in the direction of initiation shear: slower, secondary s waves travel perpendicular to the direction of initiation Locating the source of an earthquake: differences in arrival times of p and s waves yield wave travel distances from the epicentre - plot these distances from 3 stations as arcs on a map and their intersection gives the location of the epicentre - seismic stations around the world indicate that earthquakes occur mainly at plate boundaries and are shallow, intermediate, deep (>300m); shallow most destructive Measuring the size of earthquakes: the Richter scale measures earthquake magnitude based on the amplitude of the largest wave on the seismogram which depends on the energy released; largest earthquakes recorded in history ~8.9 - each nu
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