Earthquakes A Violent Interlude

1. EarthquakesA Violent Interlude

2. Definition of earthquake • -vibration of the Earth's by shock waves generated by energy released from rocks rupturing under stress • -most (not all) earthquakes and ruptures occur along faults • -most faults and stress occurs along active plate tectonic boundaries

3. Fault Types Dip-Slip [normal] Strike-Slip [right-lateral]

4. Elastic rebound theory Crustal blocks move through time -“locked” segment along the fault: no slip Rocks are strained: deformation of shape -the strain can be released catastrophically Energy released causes earthquake Analogy: breaking rubber band

6. Other causes of earthquakes: • Landslides, rockslides, or slumping of rocks. • Movement of magma, gases, or rocks associated with volcanism

7. Morphology of earthquakes and related terms • Focus or Hypocenter • -exact location of the earthquake disturbance-with few exceptions is located below the surface of the Earth • Epicenter • -the position on the • surface of the Earth • directly above the focus • Slip: the distance of • displacement along the • fault

10. Foreshock(s) • -Earthquake(s) generated at or very near the focus of the main earthquake prior to the main shock • -smaller magnitude than the main shock and precede main quake by a short or substantial time interval

11. Main shock • -is the main earthquake disturbance generated at the focus • Aftershock(s) • -Earthquake(s) generated at or very near the focus of the main earthquake following the main shock • -smaller in magnitude than the main shock. Follow the main earthquake by a short or substantial time interval

12. Seismic waves • -waves which travel within the Earth • -P (primary) wave-moves in a straight line path with alternating compression and expansion • -S (secondary) wave-moves in a sinusoidal motion along its path of movement

13. Principles of P and S Waves

14. P-Wave Propagation P waves Compressional waves – similar to sound waves – Series of contractions and relaxations – Fastest, ~5 km/sec (depends on rock type) – Travel through solid, liquid and gas

15. S-Wave Propagation S waves Shear waves – motion is right angles to direction of wave – Half speed of P waves – Travel only through solid

16. P-wave S-wave

17. Surface Waves -move along the Earth’s surface radiating out from epicenter – two types: Rayleigh & Love

20. Seismographs -seismograph is the machine that detects seismic waves. -seismogram is the printed record.

22. Seismograms from multiple Seismographs around San Francisco Bay Area

23. Locating a focus/epicenter: 1. Need a seismogram from at least 3 different seismic stations. 2. Examine the seismograms and determine the elapsed time between the arrival of the first P-wave and the first S-wave.

24. 3. Use a travel-time graph -knowing the S- & P-wave travel-time, you can determine the distance to the focus/epicenter from the seismic station.

25. 4. On a map, draw a circle around the seismic station. 5. Repeat for two other seismic stations. (Triangulation.)

26. Measuring Earthquakes • Several different scales exist to measure earthquakes. • Modified Mercalli Scale • Measures earthquake intensity. • -describes damage to structures.-ranges from I (felt by only a few) to XII (total destruction). • Intensity depends on: • -Distance to epicenter • -Building materials and design • -Type of ground material - soil, rock, etc.

27. Modified Mercalli Intensity Scale • I. Not felt except by a very few under especially favorable conditions. • II. Felt only by a few persons at rest, especially on upper floors of buildings. • III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. • IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. • Dishes, windows, doors disturbed; walls make cracking sound. • V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects • overturned. • VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. • Damage slight. • VII. Damage negligible in buildings of good design and construction; slight to moderate in • well-built ordinary structures; considerable damage in poorly built or badly designed structures. • VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial • buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, • factory stacks, columns, monuments, walls. Heavy furniture overturned. • IX. Damage considerable in specially designed structures; well-designed frame structures • thrown out of plumb. Damage great in substantial buildings, with partial collapse. • Buildings shifted off foundations. • X. Some well-built wooden structures destroyed; most masonry and frame structures • destroyed with foundations. Rails bent. • XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. • XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

28. Richter Scale • -Measures earthquake magnitude.-determined by measuring the amplitude of the largest waves on the seismogram. • -logarithmic scale.-each number on the Richter Scale is ten times greater in wave amplitude. • -each number on the Richter Scale involves an energy release about 32 times as great.

30. Moment magnitude scale -measures the amount of energy released • Designed to differentiate large earthquakes • May be used to calculate energy of old events by slip along fault • Moment = µ A D • µ = shear modulus A = LW = areaD = average displacement during rupture

31. Moment Magnitude for Indonesian Quake [2004]

32. Locations of Earthquakes • Most occur along tectonic plate boundaries: • -around the Pacific Ocean (circum-Pacific belt is major site of large earthquakes) • -Mediterranean Sea area through Iran and on to the Himalayas • -Asia (Indonesia, Himalayan region) • -Mid-ocean ridges • Some occur far from plate edges • -intraplate earthquakes