Fig Earthquakes and Earthquake Hazards

3 Seismic waves allow us to look inside the Earth

 = compressibility  = rigidity  = density Seismic Waves

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Fig. 8.12ab W. W. Norton

Fig. 8.13ab W. W. Norton

Fig. 8.13c W. W. Norton

Fig. 8.15

Seismic Refraction i’ i sin(i) sin(i’) = Velocity of A Velocity of B A B

19 Refraction

Seismic wave tomography — a CAT- scan of the interior Colors are seismic wave velocity anomalies

23 Red = slow (hot) Blue = fast (cold) Here is a slice through North America showing the now- subducted Farallon Plate, sinking to the bottom of the mantle. Heating reduces rigidity more than it reduces density, so hotter material have slower seismic velocities

24 How Faults Work — the Mechanics of Earthquakes

Static Friction with a spring FnFn F s =kx At time of sliding  =  s , where  s is the static coefficient of friction  = F n /A (A= area of base)  = F s /A Why Earthquakes (stick-slip behavior)?

FnFn F s =kx FsFs Displacement If friction was this simple — and if the applied forces, coming from plate motions, were constant — would we have stick-slip behavior?  s 

Of course, friction is NOT so simple — the coefficient of friction changes once sliding begins, and if  d <  s, then we should see the idealized stick- slip behavior slip

 Displacement s s  d d  slope is -k (spring constant) Idealized Stick-Slip Behavior  s =  static  d =  dynamic

29 EQ Slip Model

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31 As the rupture grows, more energy is released and a larger magnitude earthquake results. If a big segment of the fault is right near the limit, a small rupture can take off and grow into a huge rupture and a huge earthquake

 Displacement s s  d d  Idealized Stick-Slip Behavior This is equivalent to the “re-loading” time if we assume that the tectonic driving forces are applied at a steady rate

Time Displacement Plate tectonic rate and friction along fault determine recurrence time and average size of earthquakes Earthquake event Time between earthquake — re-loading time plate tectonic rate

Time Displacement Same plate tectonic rate, but low friction (weaker fault plane), so shorter recurrence time and smaller earthquakes Earthquake event Time between earthquake plate tectonic rate

35 rocks in the circles were formerly right next to each other The San Andreas Fault is the plate boundary between NAM and PAC

36 4 cm/yr x 100 yr = 4 m (avg slip for M8 EQ) Slip on the fault occurs in small segments, but over time, every part of the fault has to accommodate the 4 cm/yr plate tectonic rate

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Normal Fault Surface Scarp Borah Peak, Idaho M 7.3 October 28, 1983

1964 Alaskan Earthquake (M~9.2) This side moved up about 6 m

M~8