Earthquakes and the Earth s Interior

Transcription

1 Earthquakes and the Earth s Interior San Francisco 1906 Magnitude 7.8 Charleston 1886

2 California s Notorious San Andreas Fault fault trace

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4 Earthquakes are the release of energy stored in rocks. Most earthquakes are associated with faults.

5 The focal point is place of the first release of energy. The epicenter is the point on the ground immediately above the focal point. Locate a quake by specifying lat. and long. of epicenter and depth to focal point

6 Energy travels outward from the focal point in a series of spherical waves fronts. Earthquakes are explained using the elastic-rebound theory.

7 Elastic-Rebound Theory I Energy is stored over a long period of time in rocks undergoing elastic deformation along a fault plane. No movement along fault, however, until frictional resistance (Fr) along fault is overcome.

8 Elastic-Rebound Theory II Sudden movement when Fr is overcome. Stored energy released instantaneously as seismic waves propagating through the earth.

9 Slippage along fault after earthquake

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11 Energy traveling outward from the focal point can be detected around the world by recording instruments known as seismographs.

12 Energy travels outward from the focal point as a series of seismic waves. The waves are elastic disturbances. The waves of differing energies are known as Primary, Secondary and Surface waves

13 Characteristics of Seismic Waves For each type of seismic wave you should know: the name and symbol the type of motion the approximate velocity the material it is propagated through the shadow zone (if applicable)

14 Primary Waves the name and symbol = primary or P-wave the type of motion = compressional body wave change volume, not shape approximate velocity = 8-13 km/sec km/min 28,800-46,800 km/hr 35,000 km/hr 22,000 mph travels through = solids, liquids and gases shadow zone = direct arrivals no direct arrivals refracted arrivals

15 Motion of P-Waves alternating compactions and rarefactions A propagating wave is an elastic disturbance. The material returns to its original shape after the waves has passed through it.

16 Shadow Zone for P-waves arrivals and their interpretation

17 Secondary Waves the name and symbol = secondary or S-wave the type of motion = shear waves change shape, not volume approximate velocity = 5-7 km/sec km/min 18,000-25,200 km/hr 20,000 km/hr = 12,428 mph travels through = travel only in solids shadow zone = direct arrivals no direct arrivals

18 Motion of S-Waves particle motion is at right angle to wave motion A propagating wave is an elastic disturbance. The material returns to its original shape after the waves has passed through it.

19 Shadow Zone for S-waves arrivals and their interpretation

20 Surface Waves the name and symbol = surface or L-wave the type of motion = pitching and rolling causes motion felt during quake (and damage) approximate velocity = 4-5 km/sec km/min 14,400-18,000 km/hr 15,000 km/hr 9,000 mph travels through = travel in solids and liquids earthquakes and tidal waves (tsunamis)!

21 Motion of L-Waves

22 tsunamis in the open ocean and shallow water 518 mph 211 mph 31 mph Note how the tidal waves slow down, bunch up and grow taller in shallow water. Does this remind you of anything else we ve studied? Energy distributed through entire depth of water column (unlike wind waves).

23 Seismic Sea Wave Associated with 1964 Alaska Earthquake

24 Distribution of Earthquakes shallow quakes = focal point < 100 km deep quakes = focal point > 100 km NB: I will use deep to include both intermediate and deep on this diagram.

25 The magnitude and frequency of earthquakes are inversely related. There are 1 or 2 earthquakes a year with a Richter magnitude of 8 or higher. There are approximately 1,000,000 earthquakes per year with a magnitude of 3 or less.

26 Earth s Interior Known mostly from indirect evidence, especially the behavior of seismic waves.

27 Method is simple in theory, but complex in application.

28 homogenous interior ugly reality! velocity increases with depth

29 B decrease in speed at greater depth Variation in Seismic Velocity within the earth A increase in speed at shallow depth C D

30 The marked increase in speed at varying depths beneath oceans and continents is called themohorovicic discontinuity (or Moho). The Moho is taken to indicate the base of the crust. It marks the crust-mantle boundary, which is a change in chemical composition. Continental Crust 35 km thick granitic, rich in Si, Al, K average density 2.8 g/cc Oceanic Crust 7-10 km thick basaltic, rich in Si, Mg, Fe average density 3.0 g/cc Taken together oceanic and continental crust make up 1.4% of Earth s volume and 0.7% of its mass.

31 beneath the Moho is the mantle extends down to 2900 km chemical composition rich in Fe and Mg (ultramafic) mineral and chemical composition peridotite 50% Earth s radius 83% Earth s volume 67% Earth s mass The mantle contains the LVZ at a depth of km. The asthenosphere lies entirely within the upper mantle.

32 beneath the mantle is the core extends from 2900 km to the center of the earth (6371 km) roughly similar chemical composition throughout composed almost entirely of Fe liquid outer core solid inner core 50% Earth s radius 16% Earth s volume 32% Earth s mass

33 Compositional layering: crust, mantle, core

34 Geothermal Gradient Melting T of peridotite Melting T of iron NB: melting T increases with increasing P

35 Behavioral Layering Lithosphere solid, brittle slab 100 km thick crust and mantle includes the Moho earthquakes! Asthenosphere solid, nonbrittle from km mantle material only includes LVZ from km no earthquakes!

36 Layering Compositional crust mantle core Behavioral lithosphere asthenosphere