EARTHQUAKES & SEISMOLOGY

1. EARTHQUAKES & SEISMOLOGY

2. What good are earthquakes?Information about the earth’s interior • Using seismic waves to learn about the interior • Refraction (bending) and reflection • Structure of the earth • Shallow • deep • Composition of the mantle and core

3. Rate x time = Distance Rate = distance/time LA-SF = 360 km 60 sec travel time for P waves 360/60=6 km/sec Seismic waves travel in all directions from focus to seismometers around the world. If we know the location and the time of the quake, we can get the speed (rate) of the waves.

6. Earthquake size measurement Intensity (Mercalli Intensity scale) measures damage to buildings and other structures--Roman Numerals I-XII Richter Magnitude: measures amplitude of 1 seismic wave Moment magnitude: sums amplitudes of all seismic waves (= energy of earthquake); product of fault area, slip, elastic constant

7. - P Waves: primary (push-pull) - Fastest wave (4-7 km/sec in crust) - Travels through solids and liquids

8. - Secondary • - Transverse (shake) • Second fastest (2-5km/sec) • Does not travel through liquid

9. Slowest (1-2 km/sec)

10. Velocity of P, S waves is affected by: • Density of rock or material • Temperature • Pressure • Rigidity (elastic constants for compression, shear) • Cool, rigid rocks make waves travel faster • Hot, soft rocks slow down waves • Liquid (magma) slows down P waves and does not transmit S waves

11. But, the planet is not uniform! Density increases, rigidity increases even more as you get deeper Seismic waves in a hypothetical uniform planet Fig 5.7

12. Sharp velocity changes at layer boundaries (air vs water) cause bending (refraction) and reflection

13. Seismic waves bend when velocity increases graduallywith depth (due to pressure making the rocksmore rigid) Fig 5.7

14. reflected Lower velocity refracted In a layered earth there are abrupt changes in velocity, so waves 1) refract (bend sharply) toward the slower layer, or 2) reflect higher velocity

16. Seismometers at different distances from the quake

17. Time for waves to travel 20° 4 min P waves are strongly bent(refracted) forming theP-wave shadow zonebetween 105° and 140°relative to the earthquake 40° 8 min 60° 10 min 90° 13 min No P received here 5.13 20 min

18. S-wave shadow zone isevidence the outer core is liquid 20° 7 min 40° 14 min 80° 20 min 5.13

19. Layered earth structure--developing a model (hypothesis) • Seismograph stations all around the world receive waves from many earthquakes • Use arrival times of different waves to calculate speed of waves in material and depth of the layers • Check this and modify earth structure with each new set of earthquakes

20. 5.2

21. Shallow structure, continued • Wave speeds decrease in the low-velocity zone (LVZ), at about 100 km depth. • Since S-waves still travel here, we know it’s not liquid, but it might be about 3% melt. • This is evidence for the asthenosphere.

22. Other evidence for the asthenosphere: Glacial rebound (discovered long before plate tectonics and velocity of mantle was known)

23. Density changes with depth Generally, density increases gradually due to increasing pressure on rocks. • Average earth density known from gravity, astronomy =5.5g/cm3 • Crust density = 2.7 g/cm3 • Mantle density = 3.3-5 g/cm3 • Need a dense core to balance this out

24. Evidence for composition of mantle and core • Gravity of earth tells us density of whole earth is greater than any rocks we see, so core must be very dense material. • Density and seismic waves consistent with core made of iron plus some nickel. • Meteorites: some are peridotite, others are metallic iron + nickel. • Earth’s magnetic field generated by convecting molten iron in outer core

25. animations • http://www.pbs.org/wnet/savageearth/animations/earthquakes/index.html