Natural Hazards and Disasters Chapter 3 Earthquakes and Their Causes

1. Natural Hazards and Disasters Chapter 3 Earthquakes and Their Causes

2. Earthquake Devastates South Asia • October 8, 2005: violent earthquake in Pakistani Kashmir • Buildings collapsed, crushed by landslides, boulders from steep mountainsides • Highways and roads closed by rockfalls and landslides • Delays in relief, repairs: many died over following cold winter • In all, 87,000 people died, tens of thousands injured, 3.5 million people lost homes

3. 222,570

4. Faults and Earthquakes Faults: fractures in crust along which rocks on one side move past rocks on the other side Measured according to amount of displacement: over several million years, west side of San Andreas fault has moved about 450 km to north Some faults produce earthquakes when they shift, others produce almost none Most faults along plate boundaries Few faults in stable interior regions of continents

5. Three Types of Faults Normal faults Reverse faults Strike-slip faults

6. Faults and Earthquakes • Normal faults • Crustal extension • Rocks above steeply-inclined fault surface slip down past rocks beneath fault surface

7. Faults and Earthquakes • Reverse faults • Crustal compression • Rocks above steeply-inclined fault surface slip up and over rocks beneath fault surface • Thrust faults are same as reverse faults, but have much less steeply-inclined surface

8. Faults and Earthquakes • Strike-slip faults • Vertical surface • Rocks on one side of fault slip laterally past rocks on other side of fault • Where rocks on far side of fault slip to right: right-lateral strike-slip fault • Where rocks on far side of fault slip to left: left-lateral strike-slip fault

9. Causes of Earthquakes • Commission appointed after great San Francisco earthquake of 1906 to find cause of earthquakes, headed by Andrew Lawson • Lawson and students had mapped San Andreas fault (SAF), but had no idea it caused earthquakes • After 1906 earthquake, they found that west side of SAF had shifted north by as much as 7m • Abraham Himmelwright and the post-disaster building survey • Social vulnerability and building codes matter

10. Causes of Earthquakes Commission theorized that the two sides of SAF had been stuck for years, accumulating energy, while surrounding areas shifted Stuck segment of fault finally slipped, released energy in form of earthquake Elastic rebound theory

11. Causes of Earthquakes Stress: forces imposed on rock Strain: change in shape of rock, in response to stress Rocks deform elastically under low stress: revert to former shape when stress is removed Rocks deform plastically under high stress: permanently change shape or flow Fault ruptures under brittle failure

12. Causes of Earthquakes Size of earthquake is related to amount of movement on fault, in terms of offset and rupture length Offset is distance of movement across fault Surface rupture length is total length of break

13. Causes of Earthquakes Some faults move continuously, rather than suddenly snapping Strain is released by creep Central SAF undergoes creep Hayward fault in northern California undergoes creep and generates large earthquakes

14. Tectonic Environments of Faults • Earthquakes and faults are associated with plate boundaries • Sense of motion across fault is dictated by relative motion across plate boundary • Strike-slip faults along transform boundaries • Thrust faults and reverse faults along convergent boundaries • Normal faults along divergent boundaries

15. Notice Something? • Hint: How many of the earthquakes in the left table do you see in the right table? • Is there a direct relationship between size of an earthquake and the number of people killed? Why or why not?

16. Transform Faults • San Andreas Fault • Continental transform fault marks relative motion between Pacific plate (moving northwest) and North American plate (moving slightly south of west) • Movement between plates would generate several magnitude 7 earthquakes in century – usually occur in clusters

17. Transform Faults • San Andreas Fault • Compression of Los Angeles basin (at bend in SAF) produces blind thrusts – do not break surface • Seventeen earthquakes greater than magnitude 4.8 hit Los Angeles between 1920 and 1994, from thrust faults and blind thrusts adjacent to SAF • SAF south of Parkfield has been stuck since 1857 Fort Tejon earthquake

18. Transform Faults

19. Subduction Zones • Largest earthquakes on record, including • 1960 Chile, magnitude 9.5 • 1868 Peru, magnitude 9 • 2001 Peru, magnitude 8.4 • 2007 Peru, magnitude 8.0 • 2004 Sumatra, magnitude 9.1, generated megatsunami • 2007 Sumatra, magnitude 8.4

20. Subduction Zones Absence of large earthquakes along Cascadia subduction zone in Pacific northwest worried seismologists Record now shows that large earthquakes occur there every few hundred years

21. Continental Spreading Zones • Basin and Range of Nevada • North-trending normal faults separate raised mountain ranges from down-dropped valleys • Eastern margin is Wasatch Front, edge of Salt Lake basin • Many small earthquakes, but no large ones since Salt Lake City founded in 1847 • Fault zone capable of generating magnitude 7 earthquakes

22. Intraplate Earthquakes Within-plate earthquakes New Madrid, Missouri: December 1811, January 1812, February 1812 Great earthquakes felt as far as Boston

23. Intraplate Earthquakes • The Humboldt Fault, part of the Midcontinent Rift System • Largest Kansas earthquake ever? • Near Wamego, 1867, 5.5 • Less than a 3% chance of a 7.0 or higher, which would rupture the dam if the focus were close enough

24. Intraplate Earthquakes • Charleston, South Carolina, 1886 • East Coast fault system • Buried boundary between Piedmont continental crust and Atlantic oceanic crust • Eastern U.S. earthquakes less frequent, but more widely felt • Older, stable crust that transmits earthquake wave energy more efficiently

25. Intraplate Earthquakes

26. Earthquake Waves • When fault slips, released energy travels outward in seismic waves • Outward from point on fault that first slipped: focus (or hypocenter) • Epicenter is point on surface of Earth directly above focus

27. Earthquake Waves

28. Types of Earthquake Waves • First arrival is P wave (P = Primary, first) • Compressional motion • Feels like sudden jolt • Travels at 5-8 km/s through rock • Sometimes heard as low rumbling

29. Types of Earthquake Waves • Second arrival is S wave (S = Secondary) • Shear motion • Feels like wiggling, difficult to stand • Travels at 3.5-4.5 km/s through rock • More destructive than P waves • Both P and S waves are body waves

30. Types of Earthquake Waves • Last arrivals are surface waves • Include Love and Rayleigh waves • Long, rolling motion • Travel slower than P or S waves • Can be more destructive than P or S waves

31. Types of Earthquake Waves

32. Types of Earthquake Waves • Wave terminology • Period: time for one complete cycle between successive wave peaks to pass • Wavelength: distance between wave crests • Amplitude: amount of positive, negative wave motion • Frequency: number of peaks per second

33. Seismographs • Record shaking of earthquake waves on seismogram • How do we stand still and watch the Earth move? • Heavy weight suspended from rigid column anchored to ground: stays still from its own inertia while ground moves • Pen attached to weight marks paper attached to ground • North-south, east-west and vertical components Chinese Dragon Jar

34. Locating Earthquakes Use time interval between P wave arrival and S wave arrival on seismogram  approximate distance to earthquake

35. Locating Earthquakes Arrival times of P waves and S waves indicate distance and direction of earthquake Wave arrival times needed from at least three stations to pinpoint location of earthquake Three circles should intersect at one location

36. By the Numbers Earthquake-Wave Velocities Surface waves travel with constant velocity through shallow rocks: arrival times increase linearly with distance P waves and S waves travel faster as they go deeper through the Earth: waves at greater depths reach seismograph fastest along curving paths

37. Earthquake Size and Characteristics • How “big” was the earthquake? • Answer in terms of perceived effects, what is noticed and felt by people: Intensity • Answer in terms of amount of energy released, the physics of the tectonic event: Magnitude

38. Earthquake Intensity Mercalli Intensity Scale developed in 1902 Based on how strongly people feel shaking and severity of damage caused Modified Mercalli Intensity Scale used today

39. Earthquake Intensity • Mercalli Intensity Scale maps reflect subjective observations of people and objective descriptions of damage • Strongest intensities near epicenter and where ground conditions favor strong ground shaking • Computer-generated ShakeMaps show maximum ground movement for potential earthquakes • Used to send emergency response to areas with likely greatest damage

40. Earthquake Magnitude • Richter Magnitude Scale (ML) derived by Charles Richter in 1935 • Based on maximum amplitude of earthquake waves measured on Wood-Anderson seismograph (adjusted as though 100 km from earthquake) • Logarithmic scale to compare earthquakes • Amplitude 10 times greater on seismographs corresponds to magnitude greater by 1.0

41. Earthquake Intensity • Surface-wave magnitude MS • Similar calculation as for ML • Surface waves usually have largest amplitude on seismogram • Body-wave magnitude MB • Measured from amplitude of body waves • Can calculate magnitude from nomograph • Straight line between P – S time and S-wave amplitude intersects central line at magnitude of earthquake

42. Earthquake Intensity • Larger earthquakes (above 6.5): strongest shaking with lower frequency, sometimes below frequency range of seismograph • Richter magnitude less accurate above 6.5 • Moment magnitude is measure of total energy expended during earthquake • Determined from long-period waves • Moment equals (shear strength of rocks) x (surface area of rupture) x (slip distance on fault)

43. Earthquake Intensity • Many small events, fewer large ones, rare giant event • Gutenberg-Richter frequency-magnitude relationship: straight-line plot of all earthquakes of certain size against frequency of occurrence • Most energy release from largest earthquakes • 1 magnitude 8 earthquake ~= 1000 magnitude 6 earthquakes

44. Ground Motion and Failure During Earthquakes • How much, how long ground shakes during earthquake depends on • How much fault moved • Where fault moved • Relationship between earthquake magnitude and ground motion • Local conditions amplify shaking and increase damage

45. Ground Acceleration and Shaking Time • Acceleration – percentage of acceleration of gravity (g) • Duration of shaking depends on size of earthquake • Increase above magnitude 6 does not cause stronger shaking • Instead increases area and total time of shaking • Amount of shaking decreases with distance from earthquake • Severity of shaking also depends on type of material waves travel through • Softer material  more intense shaking

46. Secondary Ground Effects • Earthquakes often trigger landslides • Can also cause liquefaction • Soils become almost liquid when shaken, solidify when shaking stops • Significant damage to structures atop liquefied sediments

47. Case in Point A Major Earthquake on a Blind Thrust Fault: Northridge Earthquake, California, 1994 Moment magnitude 6.7 Epicenter 20 kilometers southwest of similar 1971 San Fernando Valley earthquake Pico fault was unknown before earthquake Acceleration 1g many places, almost 2g at one site Anomalously strong shaking on some ridge tops and edges of sedimentary basins Buildings collapsed from weak first stories $15 billion damage, 61 deaths

48. Case in Point A Major Subduction-Zone Earthquake: Chile, 1960 Moment magnitude 9.5 1000 km stretch of subduction zone ruptured Large foreshock 30 minutes earlier saved many lives, as people stayed out in streets Dozens of magnitude 6 and 7 aftershocks 15 minutes later, tsunami struck shore in 3 waves 15 hours after earthquake, tsunami struck Hawaii 22 hours after earthquake, tsunami struck Japan

49. Case in Point Paleoseismology Provides a Record of a Giant Event: Pacific Northwest Earthquake, 1700 Radiocarbon dating of peat and dead trees narrowed timing of large earthquake to 1699-1700 Tsunami records in Japan indicate wave on January 27, 1700, with no other earthquakes to account for it Subduction zone ruptured along 1200 km stretch of coast  probably about magnitude 9 Intervals between comparable earthquakes on Cascadia subduction zone were 300-900 years

50. Case in Point Damage Mitigated by Depth of Focus: Nisqually Earthquake, Washington, 2001 Struck Seattle-Tacoma area February 28, 2001 with magnitude 6.8 Fairly widespread minor damage One death, 250 injured Indirectly caused by stress from Cascadia subduction zone Less damage because earthquake was deep (50 km), not much surface wave motion