Earthquakes

1. Earthquakes Prepared by Betsy Conklin for Dr. Isiorho

2. Introduction • earthquake: a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath the earth’s surface • seismic waves: the waves of energy produced by an earthquake • elastic rebound theory: the sudden release of progressively stored strain in rocks that results in movement along a fault

3. Causes of Earthquakes • movement along faults • explosive volcanic eruptions • mineral transformations within the downgoing rock as pressure collapses one mineral into a denser form

4. Seismic Waves • focus: the point within the earth where seismic waves originate • epicenter: the point on the earth’s surface directly above the focus

5. Body Waves • body waves: seismic waves that travel through the earth’s interior, spreading outward from the focus in all directions • P (primary) wave: a compressional (or longitudinal) wave in which rock vibrates back and forth parallel to the direction of wave propagation; can travel through rock, gas, or liquid

6. Body Waves (cont.) • S (secondary) waves: a slower, transverse wave that travels through near surface rocks at 2 to 5 kilometers per second; the rock vibrates perpendicular to the direction of wave propagation; can travel through rock but not gas or liquid

7. Surface Waves • surface waves: seismic waves that travel on the earth’s surface away from the epicenter • love waves: waves that have no vertical displacement; they move side to side in a horizontal plane that is perpendicular to the direction the wave is traveling or propagating; do not travel through liquids; because of horizontal movement the waves tend to knock buildings off their foundation

8. Surface Waves (cont.) • Rayleigh waves: behave like rolling ocean waves; cause the ground to move in an elliptical path as the wave passes; tend to be incredibly destructive because they produce more ground movement and take longer to pass

9. Measuring Earthquakes • seismograph: a recording device that produces a permanent record of earth motion detected by a seismometer, usually in the form of a wiggly line drawn on a moving strip of paper A seismograph for horizontal motion. Modern seismographs record earth motion on moving strips of paper. The mass is suspended by a wire from the column and swings like a pendulum when the ground moves horizontally. A pen attached to the mass records the motion on a moving strip of paper.

10. Measuring Earthquakes A simple seismograph for detecting vertical rock motion. The pen records the ground motion on the seismogram as the spring stretches and compresses with up and down movement of the spring. Frame and recording drum move with the ground. Inertia of the weight keeps it and the needle relatively motionless.

11. Measuring Earthquakes (cont.) • seismogram: the paper record of each vibration

12. Determining the Location of an Earthquake • travel-time curve: a plot of seismic-wave arrival times against distance A travel time curve is used to determine the distance to an earthquake. Note that the time interval between the first arrival of P and S waves increases with distance from the epicenter.Seismogram X has a 3-minute interval between P and S waves corresponding to a distance of 2,000 km from the epicenter, Y has an interval of 8 minutes, so the earthquake occurred 5,300 km away, and Z an interval of 12 minutes, and is a distance of 9,000 km from the epicenter.

13. Determining the Location of an Earthquake • depth of focus: the distance between focus and epicenter Locating an earthquake. The distance from each of three stations (Denver, St. Johns, and Lima) is determined from seismograms and the travel-time curves shown in figure 16.9. Each distance is used for the radius of a circle about the station. The location of the earthquake is just offshore of Vancouver, British Columbia, where the three circles intersect.

14. Measuring the Size of an Earthquake • intensity: a measure of an earthquake’s effect on people and buildings • modified Mercalli scale: scale expressing intensities of earthquakes (judged on amount of damage done) in Roman numerals ranging from I to XII • magnitude: a measure of the energy released during the earthquake • Richter scale: a numerical scale of magnitudes • moment magnitude: an earthquake magnitude calculated from the strength of the rock, surface area of the fault rupture, and the amount of rock displacement along the fault

15. Effects of Earthquakes • ground motion • fire • landslides • liquefaction (a special type of ground failure) • permanent displacement of the land surface • aftershocks: small earthquakes that follow the main shock Liquefaction of soil by a 1964 earthquake in Niigata, Japan, caused earthquake-resistant apartment buildings to topple over intact. An example of permanent displacement of the land surface - fence compressed by ground movement, Gallatin County, Montana, 1959.

16. Tsunamis • tsunamis (seismic sea waves): huge ocean wave produced by displacement of the sea floor

17. World Distribution of Earthquakes • Circum-Pacific belt: the most important concentration of earthquakes which encircles the rim of the Pacific Ocean. Approximately 80% of the world;s shallow-focus quakes, 90% of the intermediate-focus quakes, and nearly 100% of the deep-focus quakes occur there. • Mediterranean-Himalayan belt: a major concentration of earthquakes which runs through the Mediterranean Sea, crosses the Middle East and the Himalayas, and passes through the East Indies to meet the circum-Pacific belt north of Australia

18. World Distribution of Earthquakes

19. World Distribution of Earthquakes • Benioff zones: distinct earthquake zone that begins at an oceanic trench and slopes landward and downward into the earth at an angle of about 30o to 60o • island arc: Benioff zones that slope under a continent or a curved line of islands

20. First-Motion Studies of Earthquakes • By studying seismograms of an earthquake on a distinct fault, geologists can tell which way rocks moved along that fault. Rock motion is determined by examining seismograms from many locations surrounding a quake. Each seismogram station can tell whether the first rock motion recorded there was a push or a pull. If the rock moved toward the station (a push), then the pen drawing the seismogram is deflected up. If the first motion is away from the station (a pull), then the pen is deflected downward. Map view of two possible solutions for the same pattern of first motion. Each solution has a different fault orientation. If the fault orientation is known, the correct solution can be chosen. The star marks the epicenter, and rock motion is shown by arrows.

21. Earthquakes at Divergent Boundaries • divergent boundaries: where plates move away from each other • the rock motion that is deduced from first-motion studies shows that the faults here are normal faults, parallel to the rift valley

22. Earthquakes at Transform Boundaries • transform boundaries: where plates move horizontally past each other • first-motion studies indicate strike-slip motion on faults parallel to the boundary Narrow band of shallow-focus earthquakes shown as stars along single fault Broad band of earthquakes along a system of parallel faults

23. Earthquakes at Convergent Boundaries • convergent boundaries: where plates move toward each other • two general types - one marked by the collision of two continents, the other marked by subduction of the ocean floor under a continent

24. Pictures All pictures used in this power point presentation were taken from the following: Carlson, Diane H., David McGeary and Charles C. Plummer. Physical Geology: Updated Eighth Edition. New York City, McGraw-Hill Higher Education, 2001.