What is an Earthquake? A. Earthquake: Sudden slip on a fault produces radiated seismic energy

1. Earthquake Introduction What is an Earthquake? A. Earthquake: Sudden slip on a fault produces radiated seismic energy

2. Earthquakes What is an earthquake? Sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip. Or by volcanic or magmatic activity, or other sudden stress changes in the earth. When seismologists think of an earthquake, this image of a seismogram is what comes to their minds.

3. Faults and earthquakes • A fault is a fracture or a zone of fractures in a rock where the two sides have been displaced relative to each other. • Movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. • The total offset may be centimeters to kilometers. 1999 Izmit earthquake, Turkey

4. Faults and earthquakes Offset road, 1999 Izmit earthquake, Turkey (and collapsed condos)

5. Faults and earthquakes Offset road, 1992 Landers earthquake, California

6. Faults and earthquakes Offset parking lot, 1992 Landers Earthquake, California

7. Why do earthquakes occur? B. Elastic Rebound: the mechanism of earthquake generation 1) Stress in the Earth mostly due to movements of tectonic plates 2) Strain energy accumulates a) stored potential offset of fault b) the fault is locked 3) Yield Point is reached strength of the rock is exceeded 4) Sudden brittle failure, faulting produces earthquake Earthquakes Thrust fault scarp at El Asnam, Algeria. The fault scarp from the earthquake of October 10, 1980, at El Asnam, Algeria, shows a 3-m vertical offset

8. Earthquakes Why do earthquakes occur? B. Elastic Rebound: the mechanism of earthquake generation 4) Sudden brittle failure 5) Deformed rock springs back into place releasing energy 6) Energy is transmitted in all directions as seismic waves. This produces ground shaking

9. Earthquakes B. Elastic Rebound Theory

10. Earthquakes B. Elastic Rebound Theory

11. Earthquakes B. Elastic Rebound Theory

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15. Earthquake hypocenter and epicenter Fault: a zone of weakness in the Earth between two crustal blocks C. Hypocenter or Focus: where slippage along the fault zone results in an earthquakes D. Epicenter: a point on the Earth’s surface directly above the hypocenter

16. Earthquake hypocenter and epicenter C. Focus (Hypocenter) The precise underground spot where the rocks begin to rupture or shift—the weakest point The energy released radiates out in all directions like a stone dropped into a calm pool D.Epicenter The location on Earth’s surface directly above the focus The energy is released from the focus and travels out in all directions like waves. Waves are strongest closest to the epicenter and the damage is greatest there, and dissipates further away

17. II. Seismic Waves—the products of Earthquakes Examples: When you toss a large stone into a quiet lake the energy from the falling stone is transferred to the water on impact. Ripples radiate in all directions, but eventually die out at some distance from the point of impact. Sound energy produced by your vocal cords is transmitted through the air in the form of sound waves. These also die out with great distance from their source

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20. II. Seismic Waves—the products of Earthquakes Seismic waves With brittle failure (due to stress that exceeds their strength) the elastic energy that is released is similarly transmitted at great speeds through the surrounding rocks in all directions. The energy is transmitted in the form of waves: seismic waves

21. Earthquakes Seismic Waves A. Body Waves: transmit energy through the Earth’s interior in all directions from the focus 1. P waves (primary waves) a. Speed: Fastest seismic waves—4 miles/second = 14,400 miles/hour (in cont crust) So they’re the first signal to arrive at an earthquake-recording station b. Movement: compression and contraction in the direction of wave propagation.

22. Earthquakes Seismic Waves A. Body Waves When the rock first slips, it pushes into neighboring rock compressing it. Then the rocks expand elastically (dilate) past their original volume compressing its neighbor. Push-pull waves (slinky analog) Temporarily change volume of rock as they travel through Vibration is parallel to the direction of wave travel c. What we feel: a series ofsharp jolts, like a sonic boom—rattles windows

23. Earthquakes • Seismic Waves • 2. S waves (secondary waves) • Speed: slower than P waves—2 miles/second = 7,800 miles/hour (in cont crust • b. Movement: it shears the rock sideways at right angles to the direction of travel • Imagine a rope tied at one end that was then sharply snapped up and down • Temporarily change the shape of the material they are being transported through • Vibrations are perpendicular to the direction of wave travel c. What we feel: up-and-down and side-to-side motion, shaking the earth vertically and horizontally, continuous wriggling motion

24. Earthquakes • Seismic Waves • B. Surface Waves: waves that travel along the earth’s surface—like lake ripples • Created by the vibration of Body Waves • Travel along the surface so longer time travel to reach a point • Love Waves: move the ground side to side in a horizontal plane (at right angles to the direction of propagation. • Whip objects from side to side.

25. Earthquakes

26. Earthquakes Seismic Waves B. Surface Waves: waves that travel along the earth’s surface—like lake ripples 2.Rayleigh Waves: like rolling ocean waves, move both vertically and horizontally in the direction of wave propagation Similar to a brisk walk across a waterbed, some people get “seasick” Animation of seismic waves

27. III. Measuring Seismic Waves and Locating Earthquakes A. Locating and Measuring size of EQ’s We know that P and S waves travel at different speeds, so we can use them to determine the distance to an earthquake 1) We need a measuring device: Seismograph: earthquake recorders Pendulum, allowed to swing A frame, attached to the ground, supports a recording device When the ground moves, so does the recording device and the pendulum swings

28. III. Measuring Seismic Waves and Locating Earthquakes A. Measuring size of EQ’s seismographs records EW motion NS motion Vertical motion. Used to require three seismometers. Recording device traditionally used a rotating drum of paper. When the earth moves, the pen at the end of the pendulum leaves a trace on the paper around the drum: This record is called a seismogram

29. III. Measuring Seismic Waves and Locating Earthquakes B. Interpreting a Seismogram Time-series: Plot time versus amplitude of wave 1. Record is fairly flat 2. First hint of motion is where the P waves arrive 3. Next large jump in amplitude is the S waves

30. How do seismometers work? A seismograph records the motion of the ground. Originally, a drum covered with paper rotates under a pen. The pen moves from one end of the cylinder to the other creating a helical spiral line around the cylinder. A sensor converts the motion of the ground into an electrical signal which is amplified and converted once again into motion of the pen. Seismic shaking is shown in the top center of these recordings (the pen makes a regular mark every minute). Seismographs are being replaced with digital recorders. Animation of seismometer

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34. III. Measuring Seismic Waves and Locating Earthquakes • C. Locating the epicenter of earthquakes • P and S waves start out from the focus at the same time, but they travel different speeds • So the P waves arrive before S waves, and the further the seismograph is from the focus, the further apart the P and S wave arrival times are. • 2. The media uses the different velocities and arrival times of P and S waves to determine the location of a quake’s epicenter.

35. III. Measuring Seismic Waves and Locating Earthquakes • C. Locating the epicenter of earthquakes • P and S waves start out from the focus at the same time, but they travel different speeds • So the P waves arrive before S waves, and the further the seismograph is from the focus, the further apart the P and S wave arrival times are. • 2. The media uses the different velocities and arrival times of P and S waves to determine the location of a quake’s epicenter.

36. III. Measuring Seismic Waves and Locating Earthquakes • C. Locating the epicenter of earthquakes • a. Measure distance between P and S wave arrival time • b. Use travel time chart to determine distance to city • c. Use compass to draw distance radius around city • Need 3 seismograph stations to determine epicenter location

37. III. Measuring Seismic Waves and Locating Earthquakes • C. Locating the epicenter of earthquakes • Measure distance between P and S wave arrival time • b. Use travel time chart to determine distance to city • c. Use compass to draw distance radius around city • Need 3 seismograph stations to determine epicenter location

38. Question of the week What is the difference between the earthquake focus (hypocenter) and epicenter?