What are Earthquakes?

1. What are Earthquakes? • The shaking or trembling caused by the sudden release of energy • Usually associated with faulting or breaking of rocks • Continuing adjustment of the position of rock results in aftershocks

2. Stress and Strain • Most earthquakes occur when rocks fracture, or break, deep within Earth. • Fractures form when stress exceeds the strength of the rocks involved. • Stress is the forces per unit area acting on a material.

3. Stress and Strain • There are three kinds of stress that act on Earth’s rocks: • Compression is stress that decreases the volume of a material. • Tension is stress that pulls a material apart. • Shear is stress that causes a material to twist. • Strain is the deformation of materials in response to stress.

4. Stress and Strain • There is a distinct relationship between stress and strain that can be plotted as a stress-strain curve. • A stress-strain curve usually has two segments: a straight segment and a curved segment. • Low stresses produce the straight segment, which represents the elastic strain of a material. • If the elastic strain is reduced to zero, the deformation disappears.

5. Stress and Strain Ductile Deformation • When stress exceeds a certain value, a material undergoes ductile deformation, shown by the curved segment of the graph. • This type of strain produces permanent deformation, which means that the material stays deformed even if the stress is reduced to zero.

6. Stress and Strain Ductile Deformation • When stress exceeds the strength of a material, the material breaks, or fails, as designated by the X on the graph. • Most rocks, though brittle on the surface, become ductile at the higher temperatures present at greater depths.

7. What is the Elastic Rebound Theory? • Explains how energy is stored in rocks • Rocks bend until the strength of the rock is exceeded • Rupture occurs and the rocks quickly rebound to an undeformed shape • Energy is released in waves that radiate outward from the fault

8. Faults Types of Faults • There are three basic typesof faults: • Reverse faults are fractures that form as a result of horizontal compression. • Normal faults are fractures caused by horizontal tension. • Strike-slip faults are fractures caused by horizontal shear.

9. The Focus and Epicenter of an Earthquake • The point within Earth where faulting begins is the focus, or hypocenter • The point directly above the focus on the surface is the epicenter

10. Benioff Zone Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench downward in the direction of the island arc.

11. Seismograph records earthquakes events. Horizontal Seismograph Vertical Seismograph

12. A seismogram records wave amplitude and time.

13. Where Do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the circum-Pacific belt • most of these result from convergent margin activity • ~15% occur in the Mediterranean-Asiatic belt • remaining 5% occur in the interiors of plates and on spreading ridge centers • more than 150,000 quakes strong enough to be felt are recorded each year

14. Seismic Risk

15. What are Seismic Waves? • Response of material to the arrival of energy fronts released by rupture • Two types: • Body waves • P and S • Surface waves • R and L

16. Body Waves: P and S waves • Body waves • P or primary waves • fastest waves • travel through solids, liquids, or gases • compressional wave, material movement is in the same direction as wave movement • S or secondary waves • slower than P waves • travel through solids only • shear waves - move material perpendicular to wave movement

17. Surface Waves: R and L waves • Surface Waves • Travel just below or along the ground’s surface • Slower than body waves; rolling and side-to-side movement • Especially damaging to buildings

18. How is an Earthquake’s Epicenter Located? Seismic wave behavior • P waves arrive first, then S waves, then L and R • Average speeds for all these waves is known • After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

19. How is an Earthquake’s Epicenter Located? Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

20. Triangulation: Locating an Epicenter • Three seismograph stations are needed to locate the epicenter of an earthquake • A circle where the radius equals the distance to the epicenter is drawn • The intersection of the circles locates the epicenter

21. Clues to Earth’s Interior • Seismic waves change speed and direction when they encounter different materials in Earth’s interior. • P-waves and S-waves traveling through the mantle follow fairly direct paths. • P-waves that strike the core are refracted, or bent, causing P-wave shadow zones where no direct P-waves appear on seismograms. • S-waves do not enter Earth’s core because they cannot travel through liquids and do not reappear beyond the P-Wave shadow zone.

22. Clues to Earth’s Interior

23. Clues to Earth’s Interior • This disappearance of S-waves has allowed seismologists to reason that Earth’s outer core must be liquid. • Detailed studies of how other seismic waves reflect deep within Earth show that Earth’s inner core is solid.

24. Earthquake Magnitude and Intensity • Magnitude is the measurement of the amount of energy released during an earthquake. • The Richter scale is a numerical scale based on the size of the largest seismic waves generated by a quake that is used to describe its magnitude. • Each successive number in the scale represents an increase in seismic-wave size, or amplitude, of a factor of 10. • Each increase in magnitude corresponds to about a 32-fold increase in seismic energy.

25. Earthquake Magnitude and Intensity Moment Magnitude Scale • The moment magnitude scale, widely used by seismologists to measure earthquake magnitude, takes into account the size of the fault rupture, the amount of movement along the fault, and the rocks’ stiffness. • Moment magnitude values are estimated from the size of several types of seismic waves produced by an earthquake.

26. Earthquake Magnitude and Intensity Modified Mercalli Scale • The modified Mercalli scale, which measures the amount of damage done to the structures involved, is used to determine the intensity of an earthquake. • This scale uses the Roman numerals I to XII to designate the degree of intensity. • Specific effects or damage correspond to specific numerals; the higher the numeral, the worse the damage.

27. Earthquake Magnitude and Intensity

28. Earthquake Magnitude and Intensity Modified Mercalli Scale • Earthquake intensity depends primarily on the amplitude of the surface waves generated. • Maximum intensity values are observed in the region near the epicenter; Mercalli values decrease to I at distances very far from the epicenter. • Modified Mercalli scale intensity values of places affected by an earthquake can be compiled to make a seismic-intensity map.

29. Earthquake Magnitude and Intensity Depth of Focus • Earthquake intensity is related to earthquake magnitude. • The depth of the quake’s focus is another factor that determines the intensity of an earthquake. • An earthquake can be classified as shallow, intermediate, or deep, depending on the location of the quake’s focus. • A deep-focus earthquake produces smaller vibrations at the epicenter than a shallow-focus quake.

30. Classification According to Focus Depths

31. How are the Size and Strength of an Earthquake Measured? • Magnitude • Richter scale measures total amount of energy released by an earthquake; independent of intensity • Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

32. What are the Destructive Effects of Earthquakes? Damage in Oakland, CA, 1989 • Building collapse • Fire • Tsunami • Ground failure

33. Ground Shaking Amplitude, duration, and damage increases in poorly consolidated rocks.

34. EARTH DESTRUCTION Seiches - These are the rhythmic sloshing of water in lakes, reservoirs, and enclosed basins. The waves can weaken reservoir walls and cause destruction. Liquefaction - Unconsolidated materials saturated with water turn into a mobile fluid.

35. Earth Destruction • Landslides and ground subsidence – Whenever the land moves quickly, as with landslides, there is the potential for a lot of damage and potential loss of life. • Fire – Ruptured gas lines from earthquakes is one of the major hazards. • Ground Rupture ‐ This refers to areas where the land splits apart causing a rupture. These are often long linear features. • Ground shaking versus material type – More ground shaking occurs in poorly consolidated (loose) sediments than solid bedrock.

36. Destruction and Damage • Factors that determine the destruction accompanying an earthquakes. • Magnitude • Proximity to a populated area • Factors that determine structural damage • Intensity • Duration • The nature of the material upon which the structure rest • The design of the structure

37. Tsunamis • A tsunami is a large ocean wave generated by vertical motions of the seafloor during an earthquake. • These motions displace the entire column of water overlying the fault, creating bulges and depressions in the water. • The disturbance spreads out from the epicenter in the form of extremely long waves that can travel at speed over 800 km/h (500 mph). • When the waves enter shallow water they may form huge breakers with heights occasionally exceeding 30 m (19 ft.).

38. Formation of a Tsunami Tsunami waves can travel at a speed of over 500 mph.

39. Can Earthquakes be Predicted? Earthquake Precursors • changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the ground • seismic dilatancy model • seismic gaps

40. Can Earthquakes be Predicted? Earthquake Prediction Programs • include laboratory and field studies of rocks before, during, and after earthquakes • monitor activity along major faults • produce risk assessments