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2. Internal Structure of the Earth Earth’s Internal Structure: Compositional Layers Crust: Continental crust (20-70 km) Oceanic crust (~6 km) Mantle Upper mantle Lower mantle (660km 2900 km) Core Outer core (liquid) Inner core Earth’s Internal Structure: Mechanical Layers Lithosphere Asthenosphere (weak layer) Mesosphere (mantle) Outer Core (liquid) Inner Core Earthquake Seismology I. II. III. Earthquake descriptors Seismic waves Earthquake location What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy released radiates in all directions from its source, the focus (or hypocenter) Energy propagates in the form of seismic (elastic) waves Sensitive instruments (seismometers) around the world record the event What causes an earthquake? Earthquakes are usually caused by sudden movement on faults Basic terminology (hypocenter) Earthquake Descriptors Epicentral angle How are earthquakes generated ? Elastic rebound Earthquake mechanism Stick: stress builds up on rough surfaces that is locked Slip: sudden slip on the locked surface (focus) when stress becomes too high Vibrations (earthquakes) occur as the deformed rock “springs back” to its original shape (elastic rebound) Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome Foreshocks and aftershocks Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years Seismology The study of earthquake waves, seismology, dates back almost 2000 years to the Chinese Seismographs, instruments that record seismic waves Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape Basics of waves frequency(1/T), period(T), amplitude (A), velocity (V) and wavelength (V/f) Seismic waves: Body wave P wave S wave Surface wave Reyleigh wave Love wave Body waves Travel through Earth’s interior Two types based on mode of travel Primary (P) waves Push-pull (compress and expand) motion, changing the volume of the intervening material Travel through solids, liquids, and gases Generally, in any solid material, P waves travel about 1.7 times faster than S waves Secondary (S) waves shear motion at right angles to their direction of travel Travel only through solids Body Waves P and S waves Particle Motion Surface Waves Surface waves are BAD !! Wave Velocity One dimensional wave equation: x x+dx When the hammer hit the bar, a stress imbalance occurs between x and x+dx: ( x x) ( x) Using Taylor expansion, ( x x) ( x) dx o 2 x dx x Recall that: f ma f dA Where dA is the cross-section area of the bar. f dA dxdA ma x But: m dxdA u a 2 t 2 We have: 2u dxdA ( dxdA) 2 x t 2u 2 x t For elastic media, stress is related to strain, which is measured by the gradient of displacement: x E x u x x We have: 2u 2 x t u 2u ( E ) E ( ) E 2 x x x x x 2u 2u E 2 2 x t 2u E 2u 2 2 t x Wave equation: 2 2u u 2 c 2 t x 2 E c