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1 Earthquakes

2 An earthquake is a sudden motion or shaking of the Earth's crust, caused by the abrupt release of stored energy in the rocks beneath the surface. Here earthquake faulting has caused an offset of rows in a lettuce field near El Centro, in Southern California. Such a fault with purely horizontal displacement to the right as one looks across the fault is known as a right lateral strike slip fault.

3 Earthquake Terms Focus the point along a fault where an earthquake begins (usually below surface). Epicenter the point on earth s surface directly above the focus of an earthquake

4 Earthquake Terms Magnitude it s calculated from the amplitude of the largest seismic wave recorded on a Richter scale. A = amplitude t = distance correction factor M = magnitude = log 10 (A) + 3log 10 (8 t) 2.92

5 Earthquake Terms Logarithmic scale an increase of one magnitude on the Richter scale means a seismic wave amplitude that s ten times larger M = 4 A 2 = 10 x A 1 M = 3 A 1 = amplitude

6 How much bigger is a magnitude 8.7 earthquake than a magnitude 5.8 earthquake? since it s logarithmic ( )/( ) = 10 ( ) = 794 times bigger wave Since energy equals E = M where M equals magnitude E = x ( ) = = 22,387 times more energy It would take about 23,000 quakes with a magnitude of 5.8 to release the energy in one 8.7 quake.

7 How big is a magnitude 6.0 earthquake? Richter Magnitudes Mercalli Scale Earthquake s effect Less than 3.5 Generally not felt except by instruments Often felt, but rarely causes damage Over small regions little damage to well built buildings, but major damage to poorly built buildings Can be destructive in areas 100 km away Major earthquake can cause serious damage over large areas Greater than 8 Greater than 9 Great earthquake can cause serious damage in areas several hundred kilometers across Can be felt worldwide

8 How far away does an earthquake occur? p s t = s p (in seconds) distance equals d = (8 km/s) x ( t) for t = 5s, d = (8 km/s) x (5s) = 40 km away The 8 km/s is an average velocity difference for p and s waves The distance is from the focus and not the epicenter 3 separate seismographs are needed to fix an earthquake s position on a map

9 Earthquake Terms Seismic waves an earthquakes produces several different types of sound waves that travel through the earth to reach a recording station p wave s wave surface waves

10 Earthquake waves p wave s wave surface waves p waves primary waves pressure waves longitudinal waves that compress and expand rock particles as the wave passes Expands Compresses (squeezes) p waves travel the fastest so arrive first p waves can travel through solids and liquids

11 Earthquake waves p wave s wave surface waves s waves secondary waves shear waves transverse waves that move rock particles perpendicular to the direction the wave moves Particle motion Direction wave moves s waves travel slower than p waves and arrive second s waves can t travel through fluids such as air or water

12 Earthquake waves p wave s wave surface waves s wave shadow zone can t go through molten outer core p wave shadow zone p wave velocity slowed in outer core

13 Earthquake waves p wave s wave surface waves surface waves Rayleigh and Love waves Waves travel in a rolling motion along the surface Vertical circular particle motion Horizontal circular particle motion < > surface waves are a little slower than s waves (L faster than R), maybe the largest seismic waves (especially for shallow earthquakes), and the only waves recorded for far away earthquakes

14 Earthquake Terms Normal Faults rocks in the hanging wall drop down Tensional forces such as divergent plate boundaries Normal faults in felsic volcanic rock, Death Valley, California. Note the back-tilted layering

15 Earthquake Terms Reverse Faults rocks in the hanging wall pushed up Compressional forces convergent plate boundaries Stress Thrust faults and associated fold. Near Klamath Falls, OR.

16 Earthquake Terms Transform Faults horizontal or strike-slip faults Rocks slide past each other horizontally Transform fault boundary Aerial view of right-lateral fault. Near Las Vegas, Nevada.