Earthquakes, geology, and why it matters

1. Earthquakes, geology, and why it matters Stress causes studentsrocks to crack with disastrous consequences Dr. Brendan Duffy

2. Things to think about • Why does rock break to form faults? • How does faulting rock cause earthquakes? • What do faults tell us about the processes that build our land? • How do earthquakes shape our world? • What are the effects of these processes for society • How do earthquakes shake our world?

3. Why do rocks break to form faults? Stress!!! Rocks break under compression, extension and shear

4. Types of Faults • Faulting accommodates movement of one part of the earth relative to another • Faults Are Classified According to the Kind of Motion That Occurs on Them • Joints - No Movement • Strike-Slip - Horizontal Motion • Dip-Slip - Vertical Motion • Typically accommodate either extension or shortening

5. Dip-Slip faults Normal – increase the surface area of crust Vertical stess maximum – the weight of the earth on your shoulders Thrust – decrease the surface area of the crust Horizontal stress maximum – caught between a rock and a hard place σ1 σ1

6. Strike-Slip Faults Left lateral Right lateral

7. Why and how do earthquakes shape our world • Where does the stress come from in the first place? • How is the stress manifest in earthquakes • What are the geomorphological effects (ie the effects on the shape of the earth’s surface) • Different scales • How do we measure them? • What can we understand from them?

8. Fundamental processes like plate motion Ridge push Heat transfer introduces the energy, gravity does the work via: Slab pull, ridge push, slab suction

9. The source of our stress: NZ’s tectonic setting extension Strike slip shortening Relative motion of the Pacific and Australian plate Crust getting deformed differently in different parts of the country Different earthquakes and faults in different parts of the country

10. GPS Geodesy Opening of the Whakatane rift Alpine fault New Zealand is deforming continuously Arrows show movement of parts of NZ relative to Australian plate Small movements are on the Australian plate Large movements are on the Pacific Plate

11. Complicating factors Inherited faults from 60 million years ago

12. Continuously deforming then rebounding abruptly Fault Stable zone during stretching Increasing amount of stretching (strain) prior to snapping Reid, 1910

13. Edgecumbe This rift… Was built like this

14. A few major South Island faults… Pacific Plate Australian plate Clarence fault Seddon Awatere fault • The right lateral plate boundary • Right side moves toward us • Left side gets pushed away and up over the top

15. You can’t break all the rock at once! shortening extension extension shortening Creates localized zones of extension and shortening Zones of enhanced stress Followed by more earthquakes somewhere else

16. Recent seismic sources Elliot et al 2012 Concentrated areas of slip on as many as 8 individual fault segments; surface rupture only on 3 with Mw ≥6.6

17. Mountains like these grow… One earthquake at a time Big and small increments

18. Continuously deforming then rebounding abruptly Fault Stable zone during stretching Increasing amount of stretching (strain) prior to snapping

19. Earthquakes occur due to frictional stick-slip instability When elastic forces exceed the static friction, an earthquake occurs. F is the resistive force of friction μ is the coefficent of friction μs is the coefficient of static friction for the two surfaces μkisthe coefficient of sliding friction, when the fault blocks are moving N is the normal or perpendicular force pushing the two objects together

20. Map view = rupture Fault Time taken to rupture • The slip travels along the fault at rupture velocity vr, so there is also a finite “rupture time” TR • At one point on the fault slip takes a finite time (called “rise time” TD): Slip USGS Time TD

21. Southern California Earthquake Centre simulation 450 km

22. We mostly feel earthquakes that aren’t rupturing the ground Only 3 segments ruptured the surface, and all in one earthquake! Most earthquake hazards are associated with shaking, not ground rupture

23. The most important characteristics of strong ground motion are… • 1) Seismic wave intensity (amplitude) • 2) Seismic wave frequency content (how amplitude is distributed amongst different frequencies) • 3) Duration of shaking

24. Geologic influences on shaking Seismic source effects • Fault type (ss, rev, norm) • Fault size (Magnitude of earthquake) • Rupture depth and location • Rupture directivity • Fault strength / stress drop / radiated energy • Bedrock lithology fracture density? • Basin effects • Topography (elevation, geometric effects) • Soil type, fluid content, and depth-spatial distribution • Others? Path effects Site effects

25. Wave characteristics A given frequency wave passing into a lower velocity (softer) medium will reduce its wavelength To conserve its energy it is amplified Sedimentary basins have lower velocities than bedrock and can therefore produce intense amplification

26. Amplification controls ground accelaration Stiffness Hz • Can be estimated using 1D site response software • Input data includes: • Soil physical properties • Soil dynamic properties • Soil thickness • Input rock motion at the base of the soil column (a function of source and path effects) • These factors can be used to estimate site amplification

27. Spectral acceleration Amplification Period is inverse of frequency For a given wavelength, long period waves are associated with low velocity material

28. Buildings damaged by horizontal waves close to their own natural freq.

29. Some useful links • GNS Science Active Faults Database • http://maps.gns.cri.nz/website/af/viewer.htm • Slinky seismometer • http://cgiss.boisestate.edu/~kasper/geoph297wiki/index.php/How_To_Build_a_Slinky_Seismometer

30. So what if you come and do geology here? Castle Hill 2012 4 Sept 2010 - Darfield San Francisco 2011 Kaikoura 2011 Timor 2009, 10 and 13