Global seismicity

2. Global seismicity Earthquake epicenters (locations) are shown by the colored dots. Note locations and concentrations of activity. Compare with volcano locations.

3. Plate collisions and volcanoes Ocean-ocean Ocean-continent Continent-continent

4. India-Asia continent-continent collision

5. Rock Behavior • How rocks respond to applied forces (stress) • Stress – force per unit area (lbs per sq in) • Response is termed “strain” • Elastic deformation (reversible) • Ductile deformation (flow) • Brittle deformation (fracture)

6. Rock breakage • Fracture • Separation only; no vertical movement • Fault • Vertical and/or horizontal motion

7. Note offset in rocks

8. Footwall block Hanging wall block

9. Note different fault motions Normal or dip-slip fault Strike-slip fault Reverse fault

10. Strike-slip motion on the plates

11. Note relative position of features

12. Subduction process Shallow-focus: 0 to 70 km depth Intermediate-focus: 70 to 300 km depth Deep-focus: 300 to about 700 km depth

13. P- and S-wave motion

14. Note changes in amplitude of the three wave arrivals

15. Seismogram of EQ near recording station

16. Selected seismic stations in the US

17. Earthquake magnitudes • Measures the “size” of an EQ • Four types of measurements • Local (ML) – original Richter scale • Body-wave (Mb) – P-wave amplitude • Surface-wave (Ms) – Rayleigh-wave amplitude • Moment (Mw) – considers amount of strain energy release along entire fault rupture.

18. Comparison of magnitude scales

19. The Big Ones Japan Mar 2011 9.0

20. Intensity scale • Measures damage caused by seismic energy • Established by Mercalli (Italian) in 1902; modified in 1931 to reflect enhanced building standards in US • Uses Roman numerals (I – XII) • Values depend on EQ magnitude, distance from source, bedrock type, building material and style, duration of shaking

21. Isoseismal map of Dec 1811 EQ near New Madrid, MO

22. Earthquakes don’t kill people- buildings do • Many deaths in older regions on Earth due to poor quality construction, especially through trans-Mediterranean/Asiatic belt • Secondary events (aftershocks) destroy already weakened structures • Surface waves produce the greatest damage

23. Benefits of EQs • Changing natural resource paths • Groundwater • Oil and natural gas • Exposures of minerals • Natural mitigation • Small events lessen likelihood of large events

24. Short-term predictions • Precursors • Events that imply an EQ; usually small magnitude events, often in swarms • Foreshock (sometimes) – main shock – aftershock • Ground deformation • Water level changes in wells • Seismic gaps • Greatest potential for large events in the gap

25. Sample seismogram showing P, S, and surface waves

27. Effects of EQs • Shaking and ground rupture • Liquefaction • Regional elevation changes • Landslides • Fires • Disease

28. Humans cause earthquakes by • Crustal loading by dams and reservoirs • Injection of liquid waste • Underground nuclear explosions

29. Human caused Eqs near Denver, CO

30. Earthquakes in the United States Where do most EQs occur in US and why? What’s happening in eastern & central US

31. Subduction zones in western North America • Alaska • Subduction of Pacific Plate • Pacific Northwest (BC-WA-OR-CA) • Subduction of Juan de Fuca Plate and smaller Gorda Plate • Cascadia Subduction Zone • Volcanoes on land

32. Subduction Zone Earthquakes • Largest EQs worldwide • 9 of the 10 largest earthquakes (1904-2008) were related to plate subduction • One in Tibet was due to India hitting Asia • These 9 occurred along Circum-Pacific “Ring of Fire” • Five EQs were located in northern Pacific [Japan-Kamchatka-Aleutians]

33. Examples of Subduction Zone Earthquakes • Chile 1960 (Mw = 9.5): Nazca Plate diving under South American Plate; tsunami producer • Alaska 1964 (Mw = 9.2): Pacific Plate dives beneath North American Plate; tsunami producer • Mexico 1985 (Ms = 8.1): Cocos Plate dives beneath North American Plate • Indonesia 2004 (Mw = 9.1): India Plate dives under Burma Plate; major tsunami producer • Japan 2011 (Mw = 9.0): Pacific Plate dives under North American Plate; major tsunami producer

34. Seismic Gap, Mexico, Sept 19,1985 Ms = 8.1 • Mexico earthquake filled Michoacan seismic gap • Guerrero gap remains • Major aftershocks of Ms =7.5 and 7.3 within a month

35. Fig 2.20a

37. Average annual worldwide frequency of EQs magnitude 6.0 or greater • M 6.0 – 6.9 Strong 100 • M 7.0 – 7.9 Major 15-20 • M 8.0+ Great 1 every 2-3 years

38. San Andreas Fault System • Movement occurs on many faults • Displacement is distributed over a wide zone • Right-lateral strike-slip motion

39. San Andreas: Earthquake Probabilities • Probability of major earthquake (1988-2018) • Use historical and sag-pond data to calculate recurrence intervals

40. Loma Prieta, Oct 17,1989 [World Series EQ] • Magnitude 6.9; 67 killed • Epicenter at Loma Prieta, highest peak in Santa Cruz Mountains • 100 km SSE of San Francisco • Section of San Andreas that moved in 1906 EQ ruptured again • Marina district in SFO was built on rubble from 1906 EQ; mud was pumped in to fill holes; very unstable “land” • Game 3 halted by Commissioner; after 10-day recess, series continued in Oakland – Oakland swept series 4 games to 0.

42. Fig 2.18a

43. Seismic Wave Amplification near Oakland

46. Plate boundary is not a discrete line, but rather a zone the width of the Bay Area Notice the many major faults that are parallel to the San Andreas fault

47. Location of Hayward fault

48. Univ of California stadiumin Berkeley • The trace of the Hayward fault runs through goal posts • Left side is moving N [top of image]

50. 30 yrs of activity; crosses are epicenters