Urban Earthquakes

1. Urban Earthquakes

2. Faults Faults are fractures in the crust along which appreciable displacement has occurred. No movement = a joint Sudden movements along faults = earthquakes

3. Dip-Slip Normal Reverse Thrust Strike-Slip Right lateral Left lateral Transverse Types of Faults

4. Faults

5. Dip Slip Faults • Movement is primarily vertical and parallel to the fault plane • May produce long, low cliffs called fault scarps • Parts • Hanging wall (rock surface above the fault) • Footwall (rock surface below the fault)

6. Dip Slip Faults • Normal Faults • Hanging wall block moves down relative to the footwall block • Accommodate lengthening or extension of the crust • Most are small with displacements of a meter or so • Larger scale normal faults are associated with structures called fault-block mountains

7. Dip Slip Faults • Normal Faults • Sierra Nevada fault • March 27, 1872 Owens valley earthquake • 8.5 M - California’s greatest • 29 killed • Cities on normal faults • Bishop, Reno, Carson City

8. Dip Slip Faults • Reverse faults • Hanging wall block moves up relative to the footwall block • Reverse faults have dips greater than 45° • Thrust faults have dips less then 45° • Accommodate shortening of the crust (compression)

9. Thrust faults • The southern boundary of the transverse range is a series of east trending thrust faults • Santa Monica fault system • Sierra Madre fault system • These faults may be characterized by very high horizontal acceleration

10. Thrust faults • Thrust faults may be buried beneath parts of the Los Angeles basin and not visible on the surface • Cities on or very near thrust faults • Pasadena • all foothill communities

11. Strike-Slip Faults • Dominant displacement is horizontal and parallel to the strike of the fault • Right-lateral • Left-lateral • Transform fault

12. Strike-Slip Faults • These faults dominate the geology of California • Active for 30 m.y. • 10's to 100's of miles of displacement • These are the most active faults in California • 6 out of 10 earthquakes occur on these faults

13. Strike-Slip Faults • Right lateral faults are most common • They dominate the peninsular range and coast range • They trend NW-SE • Mostly active

14. Strike-Slip Faults • San Andreas-San Jacinto faults are most important • Right lateral • 600 miles in length • 300+ miles of displacement • Fault makes a major bend through the transverse range

15. Strike-Slip Faults • San Andreas-San Jacinto faults are most important • Studies by Caltech indicate 9 major San Andreas earthquakes since the 6th century • Past events ~ 575, 665, 860, 965, 1190, 1245, 1470, 1745, 1857 • Average repeat interval of 160 years • 2 major historic earthquakes • (1) 1857 Fort Tejon - 8 M • (2) 1906 San Francisco - 8.3 M

16. Recognition of faults • Earthquakes • Geological observation • Offset of the ground surface • Linear fault scarps • Sag ponds, offset drainages • Fracture with different rocks on opposite sides • Zones of crushed rocks • Get professional help . . .the geological kind!

17. What are earthquakes? • An earthquake is the vibration of Earth produced by the rapid release of energy • Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome

18. Earthquake Locations Depths range from 5 to nearly 700 km

19. What Happens During an EQ? • Stress = forces acting upon the rock • Strain = the changes in the rock in response to stress Image from Natural Hazards & Disasters 1/e, Hyndman & Hyndman

20. What Happens During an EQ? 1. Tectonic stress 2. Crustal rocks deform 3. Elastic energy stored 4. Energy released (seismic waves) = earthquake

21. Terminology • Focus • aka Hypocenter • Faults • Epicenter • Elastic Rebound • Foreshocks • Mainshock • Aftershock

22. Seismic Waves • Two main types of seismic waves • Surface waves • Body waves

23. Earthquake Waves

24. Seismographs

25. Measuring Earthquakes • Two measurements that describe the size of an earthquake: • Intensity – a measure of the degree of earthquake shaking at a given locale based on the amount of damage • Magnitude – estimates the amount of energy released at the source of the earthquake

26. Measuring Earthquakes • Intensity Scales • Modified Mercalli Intensity Scale • Developed using CA buildings as its standard • Rates from I (not felt) to XII (total damage) • The destruction caused may not be a true measure of the earthquakes actual severity Image from Natural Hazards & Disasters 1/e, Hyndman & Hyndman

27. Measuring Earthquakes • Magnitude Scales • Richter magnitude - concept introduced by Charles Richter in 1935 • Based on the amplitude of the largest seismic wave recorded • Accounts for the decrease in wave amplitude with increased distance • Used primarily for local/nearby earthquakes • ML

28. Measuring Earthquakes • Magnitude Scales • Moment magnitude • Developed to more adequately estimate the size of very large earthquakes • Derived from the amount of displacement that occurs along a fault • Often used by seismologists • MW Image from Natural Hazards & Disasters 1/e, Hyndman & Hyndman

29. Image from Natural Hazards & Disasters 1/e, Hyndman & Hyndman

30. Largest Earthquakes in the WorldSince 1900 http://wwwneic.cr.usgs.gov/neis/eqlists/10maps_world.html

31. Historic California Earthquakes (+7 M) 1857 –Fort Tejon (8.25) 1972 – Owens Valley (7.2) 1906 – San Francisco (8.25) 1922 – W. of Eureka (7.3) 1923 – Cape Mendocino (7.2) 1927 – SW of Lompoc (7.3) 1940 – Imperial Valley (7.1) 1952 – Kern County (7.7) 1971 – San Fernando (6.5) 1980 – W. of Eureka (7.2) 1989 – Loma Prieta (7.1) 1991 – W. of Crescent City (7.1) 1992 – Cape Mendocino (7.2) 1992 – Landers (7.3) 1994 – Northridge (6.7) 1999 – Hector Mine (7.1) (http://www.scecdc.scec.org/clickmap.html)

32. Map of southern California, with epicenters of historic earthquakes (as far back as 1812) of particular note plotted over the background topography. Shown, too, are major highways (in tan) and the surface traces of major faults (in greenish-blue). (http://www.scecdc.scec.org/clickmap.html)

33. Earthquake Hazards • Ground Shaking • Surface Faulting • Fires • Tsunami

34. EQ Hazards – Ground Shaking • Severity of ground shaking depends on: • Total energy released • Distance from the source • Acceleration • Nature of the ground material • Total time of shaking

35. EQ Hazards – Ground Shaking Factors that affect the strength of ground shaking: • the softness of the surface rocks and • the thickness of surface sediments. Source: http://earthquake.usgs.gov/image_glossary/amplification.html

36. EQ Hazards – Ground Shaking • Acceleration • The rate of increase in velocity. • Expressed as a proportion of the acceleration of gravity (g) • Most EQs are less than 1.0 g

37. EQ Hazards – Ground Shaking • Nature of the Ground Material • Loose sediment = amplification of wave energy • Bedrock = dampening of wave energy • Anticlines = energy dissipated • Synclines = energy focused Image from Natural Hazards & Disasters 1/e, Hyndman & Hyndman

38. EQ Hazards – Ground Shaking • Results: • Liquefaction • Landslides • Ground Subsidence • Building Collapse

39. EQ Hazards – Ground Shaking • Building Collapse Factors: • Intensity of the earthquake • Duration of the vibrations • Nature of the material upon which the structure rests • The design of the structure • National Geographic’s Make Your Own EQ

40. Notable CA EQs • 1906 San Francisco • 1989 Loma Prieta • 1971 Sylmar • 1994 Northridge

41. 1906 San Francisco When: April 18, 1906 Fault: San Andreas Epicenter: 27 miles north of San Francisco Magnitude: 8.3 (estimated) Duration of Shaking: 50 seconds of extreme shaking

42. source 1906 San Francisco • Geology of Region: • Bedrock in hills • Alluvium and wet bay muds everyplace else

43. 1906 San Francisco Damages directly caused by earthquake: • Surface rupture 270 miles long extended north and south of San Francisco • Surface offsets of 20 feet (Right-lateral displacement) • Extensive ground shaking • Liquefaction, mainly in San Francisco

44. 1906 San Francisco

45. 1906 San Francisco Isoseismal map for the San Francisco, California, earthquake of April 18, 1906. Isoseismals are based on MM intensity estimates from data. source

46. 1906 San Francisco Damage to man-made structures: Unreinforced brick and combination brick-frame buildings collapsed Mission Dolores on Dolores at 16th street, and the wrecked Parish church next to it. From the Archives of The Museum of the City of San Francisco

47. 1906 San Francisco Damage to man-made structures: The great fire • Water mains were broken to complicate an already inadequate system • 500 blocks burned (28,000 buildings) • Fire burned for 3 days and 3 nights Crowds gather at Market and Laguna streets to flee the Great Fire. Building at lower center right still survives as do along Laguna. Almost all others pictured here burned. From the Archives of The Museum of the City of San Francisco

48. 1906 San Francisco Fatalities: • The official 1906 city death count - only 260 people • The official San Francisco government policy of 1906 was to deny the importance of the earthquake • Why? See Galveston Hurricane (1900) Image Source

49. 1906 San Francisco Fatalities: • 1987 review by the San Francisco city archivist

50. 1906 San Francisco • Excellent book on the 1906 earthquake - “The San Francisco earthquake” by Thomas Gordon • Also visit: Museum of the City of San Francisco - 1906 Earthquake and Fire • 16 Views of the Great Earthquake and Fire (in PowerPoint)