Earthquakes

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Department
Geological Sciences
Course
GEOL 2390
Professor
William M.Last
Semester
Fall

Description
EARTHQUAKES Associated hazards • ground shaking • surface faulting • ground failure • tsunamis In NA: relatively low $ loss/yr (relative to floods, subsidence etc.) Key point: greatest threat (in terms of $ and life losses) • MODERATE earthquakes ◦ Why? frequency ◦ Calif = 1 Moderate (mag 6-8) eq/3 years -- >8 eq/150 years • most destructive in NA-- see handout In the path of a killer volcano • dormant for 600 years • eruption april-july 1991 • largest eruption of 20th century • 20 x 10 ^6 tonnes SO2 ◦ lowered global mean temp by 1 degree • 150 x larger than Mt. St. Helens • ash flow 200 m thick • 200 m elevation difference • 500, 000 evacuated NB 1. prediction techniques used? 2. hazard types? Earthquake (def'n) • motion/trembling of earth cause by sudden release of accumulated strain ◦ motion is wave, seismic wave ◦ quake severity function of: ■ rupture/fault characteristics ■ factors related to how wave propagated through earth materials ◦ Point: impact not • some places separating, some places meeting, other places sliding past ◦ plate velocity ~1mm/yr to several cm/yr ◦ sometimes continuous/smooth ◦ other times strain accumulates until threshold ◦ when threshold is exceeded - earthquake Physics of seismic waves • 2 basic types - surface waves vs. body waves • surface waves - Rayleigh and Love ◦ R= slowest "ocean" L = faster (back and forth) ◦ strongest motions ◦ greatest threat to structures ◦ slowest velocity • Body Waves ◦ Compressional (P waves, primary) ■ motion indirection of NRG pulse ■ highest velocity ◦ Shear waves (S waves, secondary) ■ displaces material perpendicular to travel direction ■ slower than P waves POINT: • different movement, different hazard impact • surface waves greater threat to structures • S waves used to suggest interior earth properties (solid vs liquid) • P fastest to Rayleigh slowest ◦ P waves; 8-11km/s ◦ S waves; 5-7 ◦ Love; 3.5-4.5 ◦ Rayleigh; 3-4 Richter Scale • best known and most often quoted by press • 1-10 • logarithmic • but NRG released is ~30 factor at each level of magnitude • Richter also showed - larger the NRG of EQ, the larger amount of ground motion at a given distance from the EQ • need at least 3 seismographs to determine epicentre • measure P vs. S wave arrival times • greater the time lag, farther the waves have travelled • measure amplitude of S wave • greater the amplitude …? • only partly measures EQ 'intensity' ◦ other ways to evaluate intensity and magnitude ◦ Mercalli Intensity Scale ◦ I - XII based on visible damage and people's assessment of movement Evaluation of actual energy released by WQ Moment magniftude (Mm) • real magnitude of EQ • function of rock properties, extent of fracture.ovement, amount of displacement, etc. point • slightly different # than Richter Mag Causes of earthquakes • early thoughts ◦ earth sat on the back of an animal, animal moved, Earth move ◦ hot air escaping, widespread belief, Aristotle ◦ God/retribution • Modern thought ◦ majority associated with faulting ◦ most faulting associated with plate margins ◦ divergent, convergent, transform POINT • global distribution of EQ not random • closely related to plate margins Although oat EQ entirely natural process • humans can also induce EQ POINT • our ability to initiate EQ increases significantly in the 20th century • AND increase populations in EQ prone zone • thus EQ risk
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