1. Earthquakes

2. Earthquake
An earthquake is the vibration of Earth produced by the rapid release of elastic energy accumulated in rocks. Earthquakes occur when:
Elastic energy exceeds rock strength and the rock breaks forming a fault.
Elastic energy accumulated in the rock exceeds the friction that holds rock along an existing fault line.

3. Earthquakes and Faults
Fault: large fracture in earth’s crust caused by plate motion. Faults are often, but not always associated with plate boundaries.
Scarp: vertical offset caused by faulting ( in this diagram it’s actually the mountain range).

4. Earthquakes and Faults
The earthquake begins at the focus, which is the initial point of rupture along the fault, at depth.
The epicenter is the location on the earth’s surface directly above the focus.
Vibrational energy radiates from the focus in all directions, in the form of waves.

5. Earthquake energy
Interior forces (from heat, etc) push tectonic plates.
at plate boundaries, frictional forces hold plates stationary. While rocks along plate boundary are “stuck”, elastic energy is stored and causes elastic deformation.

6. Energy release – Elastic rebound
Frictional resistance holding the rocks together is overcome.
Resulting movement releases stored energy as rocks return to original shape.

7. Energy Storage and Release
Released energy is in the form of waves, which cause movement on the earth’s surface and interior

8. Energy storage and release
Energy release may also result in fractures in the earth’s crust.

9. Seismology: Study of Wave Energy in the Earth
Types of seismic waves
Body waves travel entirely through earth’s interior
Primary (P) waves
Push-pull (compress and expand)
Travel through solids, liquids, and gases
Secondary (S) waves
Slower velocity than P waves
Slightly greater amplitude than P waves
Travel though solids only

10. Seismology Types of seismic waves (cont’d) Surface waves
Travel along surface of Earth
Cause greatest destruction
Waves exhibit greatest amplitude and slowest velocity

11. Seismic Wave Motion
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12. Earthquake Waves
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13. Seismograph
Instrument used to record surface and body waves passing through the earth
More than one type of seismograph is needed to record both vertical and horizontal ground motion
Records obtained are called seismograms

14. Seismographs
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15. Figure 14.6

16. Figure 14.8

17. Figure 14.9

18. How Big
Modern measurement scales (Richter, moment magnitude) measure the amount of energy released by a quake.
This value is the same no matter how far away from the epicenter the measurement is taken

19. How Big
An earthquake occurs in Pakistan. Pakistanis report a magnitude of Scientists in California would probably measure a magnitude of:
Greater than 6.2
6.2
Less than 6.2

20. Magnitude Scales
measure energy released (objective, rather than intensity (subjective)
Logarithmic type scale – the energy released by the earthquake increases by a factor of about 30 for each increment on the scale (i.e. a magnitude 6 earthqake releases roughly 30 times as much energy as a magnitude-5 earthquake.

21. Measurement of Energy Released during an Earthquake
Richter Magnitude (ML)
introduced by Charles Richter in 1935
Based on the amplitude of the largest seismic wave recorded during earthquake
Moment Magnitude (Mm)
Measures the amount of movement and surface area of a fault that moved during earthquake
More accurate than ML, especially for very large earthquakes

22. The table at the right shows the amount of energy released (in terms of TNT needed) for Richter Scale measurements (corrected to account for saturation)

23. Most earthquakes occur at tectonic plate boundaries

24. Transform boundaries, e.g. San Andreas Fault Zone
Strike-slip fault; movement along fault is mainly horizontal
Fault creep – small, slow movements along fault
Stick-slip movement – fault moves in a series of jolts with no movement in between. Significant energy buildup possible, resulting in is large-magnitude damaging earthquake

25. Figure 14.21

26. Convergent boundaries – one plate sliding under another
Benioff zone – upper part of sinking plate, where it scrapes past opposing plate, causing earthquake activity along the down-plunging contact zone
Pacific NW evidence
India-Pakistan border, 2005

27. Tsunami
When an earthquake occurs beneath the sea, the sea floor rises and falls, due to rupture and elastic rebound.
Resulting water displacement forms a fast-moving wave.
In the deep water of the open ocean, tsunami are barely detectable.
In shallow water near shore, the wave speed decreases as it drags against the bottom.
The water “stacks up”, causing a large wall of water to make landfall.

28. December 2004 Sumatra-Andaman Tsunami
This shows a portion of the convergent boundary between the Eurasian plate and the Indo-Australian plate. The latter is moving northward, pushing against the former
Green star shows epicenter of earthquake that caused tsunami.
Red arrows show plate motion
Red dots show earthquakes > magnitude 5.0, from 1965 to 2004.

29. Sea floor moves due to slippage; radiating energy results in tsunami

30. Tsunami
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31. Often the water retreats before the wave hits

32. Figure 14.17

33. Earthquakes along divergent boundaries
Friction along sliding blocks (transform faults)

34. Earthquakes in plate interiors
, New Madrid, MO, site of aborted divergent boundary
This area is still seeing active movement along the fault.

35. Table 14.2

36. Table 14.3

37. Figure 14.21