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Lecture Outlines Natural Disasters, 7 th edition

Lecture Outlines Natural Disasters, 7 th edition. Patrick L. Abbott. Some Earthquakes in Western North America Natural Disasters, 7 th edition, Chapter 6. Plate tectonic history of Western North America. Opening of Atlantic Ocean basin pushes North and South America west

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Lecture Outlines Natural Disasters, 7 th edition

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  1. Lecture OutlinesNatural Disasters, 7th edition Patrick L. Abbott

  2. Some Earthquakes in Western North AmericaNatural Disasters, 7th edition, Chapter 6

  3. Plate tectonic history of Western North America • Opening of Atlantic Ocean basin pushes North and South America west • Farallon plate subducts beneath North America • By 28 million years ago, enough of the Farallon plate has subducted under North America for the Pacific spreading center to make contact with North America Figure 6.1

  4. Plate tectonic history of Western North America • Remaining spreading centers to the north and south are connected by a transform fault  ancestor of San Andreas fault • In last 5.5 million years, Gulf of California has opened 300 km, tearing peninsular California (Baja California and California west of the San Andreas fault) away from the North American plate and on the Pacific plate Figure 6.1

  5. Plate tectonic history of Western North America • Eventually peninsular California willmove north to Alaska(can not sink into Pacific Ocean or be subducted) • Consider size of Pacific and North American plates as they move past one another  stress is not just along San Andreas fault but over broad area of western North America Figure 6.1

  6. Subduction Zone Earthquakes

  7. Subduction Zone Earthquakes • Subduction of Pacific plate under Alaska creates truly large earthquakes (not San Andreas fault) • The Good Friday Earthquake, Alaska, 1964 • Major subduction movement created magnitude 9.2 earthquake, as Pacific slab shoved under Alaska in seven lurches • Seven minutes of shaking induced avalanches, landslides, ground settling and tsunami, killing 131 people • Relatively low loss of life because area is sparsely inhabited, few people downtown, low tide and warm weather

  8. Subduction Zone Earthquakes Mexico City, 1985 • Subduction of Cocos plate under North America created magnitude 8.1 earthquake, with magnitude 7.5 and 7.3 aftershocks • Earthquake was 350 km from Mexico City, but intense damage • 8,000 people were killed by building collapses • Earthquake was expected Michoacan seismic gap • Guerrero seismic gap still exists Figure 6.3

  9. Subduction Zone Earthquakes Mexico City, 1985 • Earthquakes Don’t Kill, Buildings Do • 5,700 buildings in Mexico City were damaged, while small towns closer to the epicenter had much less damage Figure 6.2 Figure 6.4

  10. Subduction Zone Earthquakes Mexico City, 1985 • Earthquakes Don’t Kill, Buildings Do • Resonance between seismic waves, soft lake-sediment foundations and improperly designed buildings, all vibrating most intensely at 1 to 2-second periods, destroyed many buildings Figure 6.6 Figure 6.5

  11. Subduction Zone Earthquakes Pacific Northwest, The Upcoming Earthquake • Cascadia subduction zone – small plates subducting under North America • Why no major earthquakes in last 200 years in Pacific Northwest? • Has subduction stopped? • No, because volcanism still occurs • Are plates subducting so smoothly that earthquakes don’t occur? • Probably not, since subducting lithosphere is so young Figure 6.7

  12. Subduction Zone Earthquakes Pacific Northwest, The Upcoming Earthquake • Similar subduction setting to Japan and Chile • Japan’s subduction zone relieved stress with two magnitude 8.1 earthquakes in 1944 and 1946 • Chilean subduction zone generated world’s largest measured earthquake in 1960, magnitude 9.2 • 1960 Chilean events would be comparable to earthquake series that shifts entire Cascadian subduction zone • Could Cascadia have a magnitude 9 earthquake? • Yes, it has and will again

  13. Subduction Zone Earthquakes Earthquake in 1700 • Last major earthquake in Pacific Northwest was about magnitude 9, in 1700 • Trees in drowned forests died by salt-water flooding when ground dropped in 1700 • Tsunami hit Japan in 1700 – might have been caused by magnitude 9 earthquake along Washington-Oregon coast • Effects of magnitude 9 earthquake: • Three to five minutes of intense shaking • Tsunami 10 m high, 15 to 40 minutes after earthquake Figure 6.8

  14. Spreading-Center Earthquakes • Spreading center underlies Gulf of California, extends north into U.S., including Salton Sea, Coachella and Imperial Valleys • Salton Trough: • High heat flow, glassy volcanic domes, boiling mud pots, geothermal reservoirs, earthquake swarms from moving magma • One of most earthquake-active U.S. areas • Brawley seismic zone – often hundreds of small earthquakes over few days (swarms), continually releasing stress rather than building for big earthquake • Other nearby faults generate larger earthquakes, up to magnitude 7 Figure 6.9

  15. Transform Fault Earthquakes in California San Francisco, 1906 • Home to about 400,000 people • Intense ground shaking for about one minute in early morning hours • Damage much worse in areas constructed on artificial fill rather than rock or consolidated sediment • Many fires broke out and water lines were destroyed, making fire-fighting impossible – ten times more damage than shaking

  16. Transform Fault Earthquakes in California San Andreas Fault Earthquakes • In 1906 earthquake, 430 km of San Andreas fault between Cape Mendocino and San Juan Bautista shifted up to 6 m • This section of the fault has experienced no major earthquakes since 1906: ‘locked’ section of fault • Stress is stored until large rupture releases energy Figure 6.10

  17. Transform Fault Earthquakes in California San Andreas Fault Earthquakes Figure 6.12 Figure 6.13

  18. Transform Fault Earthquakes in California San Andreas Fault Earthquakes • Different sections of the fault behave differently: • South of 1906 section, between San Juan Bautista and Cholame: • Frequent small to moderate earthquakes and creep (millimeters per year of ongoing offset) • South of creeping section, between Cholame and San Bernardino: • Locked section with recent deficit of earthquake activity • Most recent large rupture in 1857 Fort Tejon earthquake of magnitude 7.8 • South of locked Fort Tejon section: • Complex zone with locked sections

  19. In Greater Depth:Neotectonics and Paleoseismology • Compressive bends in strike-slip cause land to uplift; pull-apart bends cause land to drop down • Down-dropped areas accumulate sediment from • Sand washing in from heavy rains • Clays settling in ponded water • Vegetation buried by clay and sand Figure 6.15

  20. In Greater Depth:Neotectonics and Paleoseismology • Fault movements can result disturbances and offsets in sediment deposition in down-dropped basins or fault-dammed areas • Determine size of earthquakes by amount of offset • Dates of prehistoric earthquakes from radioactive carbon Figure 6.17 Figure 6.16

  21. In Greater Depth:Neotectonics and Paleoseismology Earthquake Prediction – Intermediate and Long Term • Rockwell classifies fault-movement timing into: • Quasi-periodic timing, detectable with paleoseismology • Clustered movements, such as North Anatolian fault in Turkey • Random movements, inherently unpredictable, such as San Andreas fault in California

  22. Transform Fault Earthquakes in California • December 1988: Using paleoseismology, assessed 30% probability of magnitude 6.5 earthquake on Loma Prieta segment of San Andreas within 30 years • Loma Prieta earthquake occurred 10 months later • In 2003: Assessed 62% probability of magnitude 6.7 earthquake in San Francisco Bay region before 2032, and 85% probability of magnitude 7 or higher earthquake in southern California before 2024 Figure 6.19

  23. Transform Fault Earthquakes in California World Series (Loma Prieta) Earthquake, 1989 • Just prior to scheduled start of Game 3 of the World Series, at Candlestick Park of San Francisco • Rupture of southernmost 42 km of 1906 section • Fault rupture was deep, did not offset ground surface • Fault movement was vertical and horizontal, not just horizontal • Rupture lasted only 7 seconds (short for magnitude 7.0 earthquake) Figure 6.21

  24. Transform Fault Earthquakes in California World Series (Loma Prieta) Earthquake, 1989 • Occurred at bend in San Andreas fault near Santa Cruz Mountains’ highest peak, Loma Prieta • Bend in fault uplifted Santa Cruz Mountains • At bend, fault plane is inclined at 70 degrees (not vertical), so fault movement shifted Pacific plate 1.9 m to north, and uplifted Santa Cruz Mountains 36 cm Figure 6.20

  25. Transform Fault Earthquakes in California World Series (Loma Prieta) Earthquake, 1989 • Mainshock was magnitude 7.0, numerous aftershocks • Loma Prieta region was seismic gap before earthquake • Mainshock and aftershock filled inseismic gap • Destruction was much less than could have been, as shaking lasted only 11 seconds • 67 people killed, 3,757 people injured, 12,000 people homeless • $6 billion in damages Figure 6.22

  26. Transform Fault Earthquakes in California World Series (Loma Prieta) Earthquake, 1989 • Earthquakes don’t kill, buildings do • Near epicenter, damage was mostly to older and poorly designed buildings • Seismic waves were amplified by soft muds and artificial-fill foundations around San Francisco Bay • Ground motion on soft-sediment sites was 10 times stronger than at nearby rock sites • Farther from epicenter, considerable damage to poorly sited or poorly designed structures • Marina District • Interstate 880

  27. Transform Fault Earthquakes in California World Series (Loma Prieta) Earthquake, 1989 • Marina District Figure 6.23

  28. Transform Fault Earthquakes in California World Series (Loma Prieta) Earthquake, 1989 • Marina District • Built on fill fromdebris from 1906 earthquake • Amplified shaking • Deformation and liquefaction of artificial-fill foundations • Soft first-stories – ground floors of buildings cleared out to make room for parking, leaving building without adequate support so rest of building pancakes onto first floor

  29. Transform Fault Earthquakes in California World Series (Loma Prieta) Earthquake, 1989 • Interstate 880 • 42 people killed when upper deck collapsed onto lower deck Figure 6.24

  30. Transform Fault Earthquakes in California World Series (Loma Prieta) Earthquake, 1989 • Interstate 880 • Section where collapse occurred was built on soft muds (other sections on sand and gravel survived) • Freeway’s natural resonance was amplified by shaking of muds • Could have been predicted by collapses of freeways in 1971 San Fernando earthquake Figure 6.25

  31. Transform Fault Earthquakes in California Bay Area Earthquakes – Past and Future • Historic records for last 225 years (good records only last 150 years) • Major earthquake recurrence interval is more than 150 years • 19th century earthquake history quite different than 20th century • 19th century: Seven greater than 6 magnitude earthquakes before 1906 earthquake • 20th century: No large earthquakes until 1970s Figure 6.27

  32. Transform Fault Earthquakes in California Bay Area Earthquakes – Past and Future Pattern 1: common large earthquakes vs. rare giant earthquakes • Plate tectonic movement is satisfied by either one magnitude 8 earthquake in a century or one magnitude 6-7 earthquake every decade • Pattern 2: East-west pairings of earthquakes • In 1836, Monterey Bay area had earthquake about magnitude 6.5; in 1838, San Francisco peninsula had earthquake of magnitude 6.8 • In 1865, Santa Cruz had a large earthquake; in 1868, Hayward had a magnitude 6.9 earthquake Figure 6.27

  33. Transform Fault Earthquakes in California Bay Area Earthquakes – Past and Future Pattern 3: northward progression of earthquakes • Earthquakes of 1865 and 1868 on the Hayward fault were preceded by five moderate earthquakes that moved northward up the Calaveras fault • From 1974 to 1988 four moderate earthquakes occurred from south to north on the Calaveras fault • Area of Hayward fault today is populated by 2 million people, with schools, hospitals, city halls, houses and the University of California built on the fault itself

  34. Transform Fault Earthquakes in California Kobe, Japan, 1995 Versus Oakland, California, 20?? • Most expensive earthquake in history • Magnitude 6.9 earthquake with 50 km long rupture of Nojima fault and 100 seconds of shaking • Tile roofs and little lateral support caused collapse of buildings causing 6,308 fatalities • More than 140 fires resulted Figure 6.29

  35. Transform Fault Earthquakes in California Kobe, Japan, 1995 Versus Oakland, California, 20?? • Hayward fault has 27% probability of causing magnitude 6.7 or greater earthquake before 2032 • San Francisco Bay region, 62% probability of 6.7 magnitude earthquake on a fault before 2032 • Hayward fault and Nojima fault: • About 50 km long, densely populated areas, with large areas of weak ground materials • Both capable of generating magnitude 7 earthquakes Figure 6.31

  36. How Faults Work How Faults Work: Old View Elastic-rebound theory • Movement takes place on both sides of a fault but not on the fault itself, until buildup of stress on the fault overcomes friction on the fault and the rocks on both sides of the fault move quickly in an earthquake • After earthquake, all stress on the fault has been removed and buildup begins again Figure 6.32

  37. How Faults Work How Faults Work: Newer View Windows of opportunity • Fault movement begins at hypocenter and radiates outward • How much of stored stress is relieved depends on how long the fault moves • Opening gate for line of people: how long the gate is open determines how many people will get through • Moving a carpet by making a ripple and propagating ripple across floor: ripple may be small (small offset  little relieved stress) but may travel great distance (large area of fault slips  large earthquake)

  38. How Faults Work Landers, California, 1992 • Sequence began in 1992 with magnitude 6.1 Joshua Tree earthquake, followed two months later by magnitude 7.3 Landers earthquake, then followed few hours later by magnitude 6.3 Big Bear earthquake • Followed in 1999 by magnitude 7.1 Hector Mine earthquake Figure 6.33

  39. How Faults Work Landers, California, 1992 • What was learned about how faults move?: • Unlike most fault movements, Landers rupture front slowed when it reached right-step but did not stop • Rupture velocity was initially 3.6 km/sec, slowed almost to stop then continued northward at varying speeds Figure 6.34

  40. How Faults Work Landers, California, 1992 • What was learned about how faults move?: • Amount of slip varied from centimeters to 6.3 m • Amount of fault movement at the ground surface differs from that at depth • Fault movement and shaking at the hypocenter were much less than later, farther from hypocenter Figure 6.35

  41. How Faults Work Landers, California, 1992 • What was learned about how faults move?: • Only small portion of fault slipped at any one time • Fault movement lasted 23 seconds but no spot on the fault moved for more than 4 seconds • When the rupture front slowed at the fault step, stress built up behind the front until it moved through the step • Triggered other earthquakes in areas to the north (directivity), fortunately for Los Angeles to the south • 1992 Landers series, faults moved from south to north; 1999 Hector Mine event, fault moved from north to south • Patches of the faults without much movement became origin for later earthquakes

  42. How Faults Work Southern San Andreas Fault • Coachella Valley segment was assigned 40% probability of magnitude 7.5 earthquake before 2018 • Neotectonic analyses indicate earthquakes about 250 years apart, with last one 315 years ago • Recent large earthquakes at both ends of the Coachella Valley segment of the San Andreas fault (like before 1906 earthquake on northern San Andreas) • Might be not just Coachella segment (6 m unreleased stress) • Could continue on San Bernardino Mountains segment (4.3 m unreleased stress) creating magnitude 7.8 earthquake, or even on Mojave segment (4.7 m unreleased stress), creating magnitude 8 earthquake

  43. In Greater Depth: Earthquake Prediction – Short Term • Plate tectonics: can know why and where earthquakes occur • Neotectonics: can know how big and how often earthquakes occur on a given fault • Short-term prediction of earthquakes? • Detailed behavior of faults may be too unpredictable to ever allow short-term prediction of earthquakes • False lead: Parkfield, California, with magnitude 6 earthquakes every 22 years, should have allowed prediction of next earthquake to occur in 1988, +/- five years (finally occurred in 2004)

  44. Thrust Fault Earthquakes in Southern California • Big Bend: left step in San Andreas fault in southern California creates thrust (reverse) faults • Thrust faults uplift mountains in Transverse Ranges • Blind thrust faults do not reach surface, difficult to detect • Los Angeles region is shortening by 10-15 mm/yr • Detectable by global positioning system (GPS) • Could generate magnitude 6 earthquake every six years Figure 6.36

  45. Thrust Fault Earthquakes in Southern California Northridge, California, 1994 • Pico blind thrust ruptured in magnitude 6.7 earthquake under Northridge (shaking fortunately directed away from Los Angeles) • 61 people killed, 9,000 people injured, $20 billion in damages Figure 6.37

  46. Thrust Fault Earthquakes in Southern California Northridge, California, 1994 • Similar to 1971 San Fernando Valley earthquake except shaking directed away from Los Angeles in 1994 Figure 6.39

  47. Thrust Fault Earthquakes in Southern California Northridge, California, 1994 • Los Angeles region seems to be experiencing fewer earthquakes than it could • GPS shows deformation that should result in twice as many magnitude 6 earthquakes and three times as many magnitude 7 earthquakes • Radar interferometry indicates that ground is warping without large earthquakes being generated

  48. The “Big One” • What is meant? • Comparable earthquake in Alaska, 2002 on Denali fault • Over 140 seconds, ruptured 340 km in magnitude 7.9 earthquake with offsets up to 8.8 m • Significant directivity – triggered earthquake swarms up to 3,660 km to the southeast as far as Utah • Unpopulated area, minimal effects on people • 1857 Fort Tejon earthquake on San Andreas fault • Over 130 seconds, ruptured 360 km in magnitude 7.8 earthquake with offsets up to 9.5 m • Relatively unpopulated area at that time • San Bernardino section of San Andreas fault now houses 3 million people

  49. The “Big One” Earthquake hazard studies in southern California evaluate: • San Andreas fault, along eastern and northern side of Los Angeles metropolitan area, last movements in 1812, 1857 • Thrust faults (caused 1971 San Fernando, 1994 Northridge earthquakes) “Big One” would be comparable to 1857, 1971 and 1994 southern California events occurring simultaneously

  50. The “Big One” Worst-case scenario for southern California involves both: • 1957 Mongolian earthquake sequence • Major strike-slip fault moved in magnitude 8 earthquake at same time as thrust faults moved in magnitude 7 earthquakes Figure 6.41

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