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Earth Sciences
Earth Sciences 1022A/B

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 - Occurs below the spot plotted on a map called the epicentre - Slippage along a fault in the crust at the focus - Elastic-rebound builds up strain and the break/ rupture is the strain release  rocks bent until they rupture then stress builds up and they start bending again - Foreshocks and aftershocks common with main event (mini earthquakes before and after the real thing) 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, (like slinky toy: expands and contracts…) shear: slower, secondary s waves travel perpendicular to the direction of initiation, (starts with up and down/ wave motion, then this motion travels) 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 - Record p, s wave arrival times, time difference on time-travel curves give the distance from the station to the epicentre - Plot distances from at least 3 stations as arcs; 3 arcs intersect at the epicentre (Seismic Station Examples: Montreal, Paris, Sao Paulo)
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