Barajlar002

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Barajlar002

  1. 1. Seismic hazard: quake hazards & forecasting
  2. 2. Structure of talk <ul><li>The range of seismic hazards </li></ul><ul><ul><li>primary (e.g.ground shaking, surface rupture, landslides) </li></ul></ul><ul><ul><li>secondary (e.g. fire-following, toxic contamination) </li></ul></ul><ul><li>Forecasting earthquakes </li></ul><ul><ul><li>definitions </li></ul></ul><ul><ul><li>types of forecasting </li></ul></ul><ul><ul><li>probabilistic forecasting </li></ul></ul><ul><li>Earthquake prediction </li></ul><ul><ul><li>setting the scene </li></ul></ul>
  3. 3. Hazardous phenomena generated by earthquakes <ul><li>Primary </li></ul><ul><ul><li>ground shaking </li></ul></ul><ul><ul><li>ground lurching & displacement </li></ul></ul><ul><ul><li>ground settlement </li></ul></ul><ul><ul><li>liquefaction </li></ul></ul><ul><ul><li>landslides, mudslides and avalanches </li></ul></ul><ul><ul><li>tsunami & seiches </li></ul></ul><ul><li>Secondary </li></ul><ul><ul><li>structural collapse </li></ul></ul><ul><ul><li>fire-following </li></ul></ul><ul><ul><li>falling material </li></ul></ul><ul><ul><li>floods from dam bursts & levée failures </li></ul></ul>
  4. 4. Ground shaking <ul><li>A number of aspects of ground shaking affect severity of damage </li></ul><ul><ul><li>peak ground acceleration </li></ul></ul><ul><ul><li>average ground acceleration </li></ul></ul><ul><ul><li>duration </li></ul></ul><ul><li>Duration is critical </li></ul><ul><ul><li>100 s quake would be far more destructive than 30 s quake with similar shaking levels </li></ul></ul><ul><ul><li>Kobe (1995) - strong motion just 11 s long </li></ul></ul><ul><ul><li>lengthened to 100 s in areas of soft soils </li></ul></ul>Kobe 1995
  5. 5. More on duration of strong motion <ul><li>Quake M affects duration much more than it affects max acceleration </li></ul><ul><li>Larger the M, the longer the fault rupture, and the larger the area from which seismic waves generated </li></ul><ul><li>Duration of shaking increases with distance from fault but intensity is less </li></ul><ul><li>Due to dispersion effects of seismic waves </li></ul><ul><li>Higher frequencies are attenuated more than lower ones </li></ul>F Intensity of shaking Duration of shaking Increasing distance from fault
  6. 6. Duration and peak acceleration of strong motion
  7. 7. Local influences on ground shaking <ul><li>Physical properties of soil and rock </li></ul><ul><ul><li>competent bedrock may transmit peak accelerations > 2 g </li></ul></ul><ul><ul><li>sands can transmit up to ~ 0.6 g </li></ul></ul><ul><ul><li>gravels much higher </li></ul></ul><ul><ul><li>clays only capable of transmitting up to 0.15 g </li></ul></ul><ul><li>Geological structure </li></ul><ul><ul><li>focus waves </li></ul></ul><ul><li>No simple correlation between M, acceleration,and distance from quake source </li></ul>
  8. 8. More on ground conditions Ground conditions can also affect seismic intensity
  9. 9. The clay problem (Mexico City 1985) <ul><li>M s 7.9 quake </li></ul><ul><li>370 km away in Central American Trench </li></ul><ul><li>Little damage over intervening distance </li></ul><ul><li>High frequency ground motions attenuated out </li></ul><ul><li>Mexico City struck by low frequency ground motions close to natural vibration frequencies of underlying saturated lake-bed clays </li></ul><ul><li>Clays amplified motions up to 50 times compared to adjacent solid rock </li></ul><ul><li>Clay up to 40 m thick </li></ul><ul><li>Damage correlated with thickness </li></ul>Mexico City 1985
  10. 10. Ground lurching & displacement <ul><li>May be major problem close to source fault </li></ul><ul><li>Displacement may be vertical, horizontal or oblique </li></ul><ul><li>Movements may be very large </li></ul><ul><ul><li>San Francisco 1906 </li></ul></ul><ul><ul><li>in places horizontal displacement of 6.5 m </li></ul></ul><ul><li>Can be severely damaging for buildings close to faults </li></ul><ul><li>Particularly damaging for roads, railway lines, canals and pipelines that cross fault </li></ul>San Francisco 1906
  11. 11. Differential ground settlement <ul><li>May involve uplift or subsidence </li></ul><ul><li>Often fault related </li></ul><ul><li>Alaska 1964 </li></ul><ul><ul><li>shorelines uplifted by ~10m in places </li></ul></ul><ul><ul><li>subsided by ~2m in others </li></ul></ul><ul><ul><li>>250,000 sq km affected </li></ul></ul><ul><li>May result in inundation by sea </li></ul><ul><ul><li>Alaska; Izmit (1999) </li></ul></ul><ul><li>Damage to ‘lifelines’ </li></ul>Izmit 1999
  12. 12. Liquefaction <ul><li>Sands and silts </li></ul><ul><ul><li>undergo temporary loss of strength </li></ul></ul><ul><ul><li>behave as viscous fluid </li></ul></ul><ul><li>Seismic waves cause void collapse resulting in densification </li></ul><ul><li>Drainage of pore water cannot be achieved rapidly enough resulting in excessive pore pressures </li></ul><ul><li>End point is development of a QUICK condition </li></ul><ul><ul><li>material behaves as heavy liquid with virtually no shear strength </li></ul></ul>Void collapse Pore pressure increases result Original sediment structure
  13. 13. More about liquefaction <ul><li>When pore water pressures exceed normal stress imposed by weight of sediment column the material behaves like a fluid </li></ul><ul><li>Water moves upwards from void spaces to surface forming sand boils </li></ul><ul><li>Particularly at risk </li></ul><ul><ul><li>loosely packed sands and silts used for land fill & reclamation </li></ul></ul><ul><li>Results of liquefaction </li></ul><ul><ul><li>foundering of buildings </li></ul></ul><ul><ul><li>damage to utilities </li></ul></ul><ul><ul><li>mass movements </li></ul></ul>Sand boils result from expelled water
  14. 14. Examples of liquefaction <ul><li>Niigata (Japan 1964) </li></ul><ul><ul><li>wholesale foundering of apartment blocks </li></ul></ul><ul><li>Kobe (Japan 1995) </li></ul><ul><ul><li>severe damage to port facilities built on reclaimed land around Osaka Bay </li></ul></ul><ul><li>Loma Prieta (Calif. 1989) </li></ul><ul><ul><li>liquefaction of sub-surface wet sand layers in fill over 100y old caused serious damage in Marina District of SF </li></ul></ul>Niigata (Japan 1964)
  15. 15. Maximum possible distances for liquefaction M s 8.5 20 km 125 km 650 km 1500 km M s 8 M s 7 M s 6
  16. 16. Seismogenic mass movements <ul><li>Strong ground motion commonly results in gravitational slides </li></ul><ul><li>Lituya Bay (Alaska 1958) </li></ul><ul><ul><li>large landslide triggered 60m surge with run-up of >500m </li></ul></ul><ul><li>Liquefaction of material on a slope will trigger rapid flow failure </li></ul><ul><ul><li>killed 200,000 in Kansu (China 1920) quake </li></ul></ul><ul><li>Lateral spreads </li></ul><ul><ul><li>can be locally v. damaging </li></ul></ul><ul><ul><li>Alaska 1964; damaged 200 bridges </li></ul></ul>Lateral spread San Francisco 1906
  17. 17. Lituya Bay (Alaska 1958)
  18. 18. Other major seismogenic landslides Huascaran (Peru 1970) Sherman glacier (Alaska 1964)
  19. 19. Seiches & tsunami <ul><li>Seiches: oscillations set up in enclosed bodies of water by distant quakes </li></ul><ul><li>Observed in English lakes in 1950 due to Assam (India) quake </li></ul><ul><li>May cause damage to retaining walls of reservoirs and flooding </li></ul><ul><li>Tsunami: </li></ul><ul><ul><li>common feature of submarine earthquakes; enhanced by submarine landslides </li></ul></ul><ul><ul><li>also from sub-aerial seismogenic landslides </li></ul></ul>Hilo (Hawaii) 1946
  20. 20. Some notable seismogenic tsunami
  21. 21. Destructive capacity of tsunami <ul><li>High velocities </li></ul><ul><ul><li>700 - 800 kph in deep water </li></ul></ul><ul><ul><li>impact velocities 50 kph or more </li></ul></ul><ul><li>Long wavelengths </li></ul><ul><ul><li>150 - 250 km </li></ul></ul><ul><li>Wave period </li></ul><ul><ul><li>tens of minutes </li></ul></ul><ul><li>Run-up heights in excess of 20m </li></ul><ul><li>Ocean-basin extent possible </li></ul><ul><li>Multiple waves in a tsunami ‘wave train’ </li></ul>Aitape (PNG) 1998
  22. 22. Tsunamigenic earthquakes <ul><li><10% of submarine quakes generate tsunami </li></ul><ul><li>Magnitude 6.5 or greater </li></ul><ul><li>Focal depth < 50km </li></ul><ul><li>Most destructive have depths of < 25 km </li></ul><ul><li>Vertical uplift of large area of sea bed </li></ul><ul><li>Link between average run-up and quake size </li></ul><ul><li>May be strongly focused </li></ul><ul><li>Submarine landslides enhances tsunami potential </li></ul>Submarine slide simulation (PNG 1998)
  23. 23. Tsunami magnitude scale (Japan)
  24. 24. Structural collapse <ul><li>‘ Buildings not earthquakes kill people’ </li></ul><ul><li>Structural collapse depends upon </li></ul><ul><ul><li>how well buildings constructed </li></ul></ul><ul><ul><li>how well maintained </li></ul></ul><ul><ul><li>magnitude, duration & acceleration of strong ground motion </li></ul></ul><ul><ul><li>the ground response (type & character of underlying soil & rock) </li></ul></ul><ul><ul><li>distance from epicentre </li></ul></ul>Pancake collapse of concrete building (Gujarat 2001)
  25. 25. More on structural collapse <ul><li>Structural collapse primarily due to inflexible response of buildings </li></ul><ul><ul><li>low tensile and shear strength </li></ul></ul><ul><ul><li>high rigidity </li></ul></ul><ul><ul><li>low ductility </li></ul></ul><ul><ul><li>low capacity for redistributing loads </li></ul></ul><ul><li>Particularly poor adobe (mud bricks) and unreinforced masonry </li></ul><ul><li>Common problem: not ‘tying’ walls and floors together </li></ul><ul><li>Lack of enforcement of codes </li></ul>Collapse of rock and cement buildings (Gujarat 2001)
  26. 26. Solutions to structural collapse <ul><li>Design buildings to dissipate vibrational energy by inelastic (plastic) response at key locations (beam-column joints) </li></ul><ul><li>Aim is to have buildings that are strong and flexible; and that yield but don’t collapse </li></ul><ul><li>Ensure and enforce appropriate building codes </li></ul><ul><li>Continuing problem in developing countries </li></ul>
  27. 27. Earthquake loss-susceptibility by construction type NSD = Non-seismic design Modified Mercalli Intensity Average damage (%)
  28. 28. Fire-following quake <ul><li>Major problem in 20th century quakes </li></ul><ul><ul><li>San Francisco 1906 </li></ul></ul><ul><ul><li>Tokyo 1923 </li></ul></ul><ul><li>More recently Kobe 1995 </li></ul><ul><li>Factors contributing to fires </li></ul><ul><ul><li>wooden buildings </li></ul></ul><ul><ul><li>narrow streets </li></ul></ul><ul><ul><li>inadequate water provision </li></ul></ul><ul><ul><li>fractured gas mains & power lines </li></ul></ul><ul><ul><li>car fires </li></ul></ul><ul><ul><li>discarded cigarettes </li></ul></ul><ul><ul><li>overturned stoves and water heaters </li></ul></ul><ul><ul><li>chemical and petroleum leaks and spills </li></ul></ul><ul><ul><li>strong winds </li></ul></ul>San Francisco 1906
  29. 29. Fire following Japanese quakes <ul><li>Great Kanto quake (Tokyo-Yokohama) 1923 </li></ul><ul><ul><li>killed at least 140,000 </li></ul></ul><ul><ul><li>tens of thousands burnt to death </li></ul></ul><ul><li>Great Hanshin quake (Kobe) 1995 </li></ul><ul><ul><li>fires started in old, cramped parts of city </li></ul></ul><ul><ul><li>many wooden buildings </li></ul></ul><ul><ul><li>146 fires started </li></ul></ul><ul><ul><li>23,000 homes destroyed </li></ul></ul><ul><li>Tokyo today </li></ul><ul><ul><li>~ 1 million wooden homes </li></ul></ul>Kobe Tokyo
  30. 30. Floods & dam bursts <ul><li>New Madrid area </li></ul><ul><ul><li>liquefaction during 1811-12 quakes caused banks of Mississippi to fail </li></ul></ul><ul><ul><li>Levee failure could be a problem in next quake </li></ul></ul><ul><li>Lower San Fernando dam (California) </li></ul><ul><ul><li>failed during quake in 1971 </li></ul></ul><ul><ul><li>12 s of strong shaking </li></ul></ul><ul><ul><li>peak acceleration of ~ 0.5 g </li></ul></ul><ul><ul><li>upstream section of dam collapsed but held - just! </li></ul></ul>Lower San Fernando Dam (California) 1971
  31. 31. Forecasting earthquakes <ul><li>Forecasting is not prediction </li></ul><ul><ul><li>less precise </li></ul></ul><ul><ul><li>based upon analysis of earthquake return periods rather than identification of pre-cursor y signs </li></ul></ul><ul><li>Active faults or fault segments do not rupture in a random manner </li></ul><ul><ul><li>they have characteristic return periods (or at least return period envelopes ) </li></ul></ul><ul><ul><li>these reflect strain accumulation along the fault and the capacity of the fault to resist strain up to a given characteristic point - for that fault or fault segment </li></ul></ul><ul><li>There are complications: </li></ul><ul><ul><li>Rupture will not occur according to a rigid timetable - there is a return period envelope rather than specific date </li></ul></ul><ul><ul><li>Strain may be released by one large quake or a number of smaller ones (e.g. Marmara Sea south of Istanbul) </li></ul></ul><ul><ul><li>this has implications for risk assessment </li></ul></ul>
  32. 32. San Andreas example <ul><li>Prior to 1906 M 8.25 San Francisco quake ~ 3.2m displacement across fault in 50 years </li></ul><ul><li>Post-quake rebound on the fault was ~ 6.5m </li></ul><ul><li>Amount of time for strain released in quake to accumulate </li></ul><ul><ul><li>(6.5/3.2) x 50 ~100 y </li></ul></ul><ul><li>Return period until next comparable quake = 100y </li></ul><ul><li>Assumes </li></ul><ul><ul><li>uniform strain accumulation </li></ul></ul><ul><ul><li>quake did not alter </li></ul></ul><ul><ul><li>fault properties </li></ul></ul>
  33. 33. Problems with forecasting <ul><li>Forecasts only as good as the available catalogues </li></ul><ul><li>Historical catalogues good for well studied regions such as California, Japan, Europe, China </li></ul><ul><li>Poor for regions of low frequency-high magnitude seismicity </li></ul><ul><ul><li>Cascadia subduction zone </li></ul></ul><ul><ul><li>New Madrid </li></ul></ul><ul><ul><li>Jamaica </li></ul></ul><ul><ul><li>Western Europe </li></ul></ul><ul><li>Catalogues need to go back further; requires geological studies </li></ul>Cascadia subd. zone
  34. 34. The Seismic Gap concept <ul><li>Defined as an area in an earthquake-prone region where there has been a below average level of seismic energy release </li></ul><ul><li>The 1989 Loma Prieta quake filled a gap that had been aseismic since 1906 </li></ul><ul><li>Other gaps exist in </li></ul><ul><ul><li>Aleutian arc (Alaska) </li></ul></ul><ul><ul><li>south of Istanbul </li></ul></ul><ul><ul><li>Tokyo </li></ul></ul><ul><ul><li>southern California </li></ul></ul>Istanbul seismic gap
  35. 35. Seismic intensity forecasting <ul><li>Other parameters can be usefully forecast than just timing of a quake </li></ul><ul><li>Forecasting seismic intensity at a particular site is vital for: </li></ul><ul><ul><li>siting structures such as dams, schools, hospitals & emergency centres </li></ul></ul><ul><ul><li>constructing seismic hazard maps </li></ul></ul><ul><li>Requires detailed information on geology, ground conditions </li></ul>Seismic intensity forecast map - Tokai (Japan)
  36. 36. Probabilistic forecasting <ul><li>Most useful way of expressing a forecast of a future quake is in terms of probabilities </li></ul><ul><li>Most people are familiar with probabilities as a result of gambling </li></ul><ul><li>Example from San Francisco area (Bolt, 1999) </li></ul><ul><ul><li>5 quakes > M = 6.75 in 155 y between 1836 & 1991 </li></ul></ul><ul><ul><li>if events are random, another quake of  6.75 can be expected in 155/5 y = 31 y with high probability </li></ul></ul><ul><li>Problem: quakes not entirely random. On a particular fault system may be clustered (due to stress transfer) or follow certain trends </li></ul><ul><li>Alternative method of probabilistic forecasting is based on the ELASTIC-REBOUND model </li></ul><ul><li>Based upon estimates of strain accumulation across fault </li></ul>
  37. 37. Strain measurement and forecasting <ul><li>Geological mapping undertaken to define active fault segments </li></ul><ul><li>Assumption made that a discrete segment will rupture in one go </li></ul><ul><li>As Seismic moment links magnitude with rupture length this gives measure of maximum expected earthquake </li></ul><ul><li>Relationship between M s and fault rupture length L: M s = 6.10 + 0.70 log L </li></ul>
  38. 38. Calculating probabilities <ul><li>Next: determine slip history of each segment </li></ul><ul><li>Calculate strain accumulation rate for each segment </li></ul><ul><li>Slip history for fault segment can then be plotted against time </li></ul><ul><li>As slip is related to quake magnitude allows recurrence intervals between quakes greater than a given magnitude to be determined </li></ul>Magnitude 6 Amount of slip Time
  39. 39. The quake probability histogram <ul><li>Construct histogram showing No. of quakes that occur with each specified recurrence time </li></ul><ul><li>Most probable recurrence interval is that which divides histogram into two equal areas </li></ul><ul><li>If time since last quake in the magnitude range is T1, the probability of the next quake occurring in T1 - T2 years = ratio of red area to yellow area </li></ul><ul><li>As recurrence time T2 increases ratio approaches 1 and a quake becomes virtually certain </li></ul><ul><li>The more consistent the recurrence time the better the forecast </li></ul>Quake frequency Recurrence time T1 T2 T1 T2
  40. 40. The quake probability histogram & the San Andreas <ul><li>Suited to California & San Andreas fault system because active faults exposed at surface </li></ul><ul><li>Enables displacements to be measured easily and strain to be monitored </li></ul><ul><li>Method crucially depends on constraining well the number of potentially destructive quakes in historic time and their ages </li></ul><ul><li>For more discussion of problems see Bolt (1999) p228 - 229) </li></ul>
  41. 41. Predicting earthquakes <ul><li>A highly controversial issue in seismology </li></ul><ul><li>Involves giving a precise warning about the timing and size of a future quake </li></ul><ul><li>Reliant upon the occurrence of pre-cursory signs in advance of a quake </li></ul><ul><li>Method must be shown to be repeatable in order to be of any use </li></ul><ul><li>In a zone of high seismicity, any prediction is going to have greater than chance than zero of being right </li></ul><ul><li>On the other hand - a prediction that is not fulfilled ensures that the method is invalid </li></ul>
  42. 42. Proposed earthquake precursors <ul><li>Changes in seismic velocities </li></ul><ul><li>Crustal deformation </li></ul><ul><li>Groundwater changes </li></ul><ul><li>Gas release </li></ul><ul><li>Atmospheric effects </li></ul><ul><li>Anomalous animal behaviour </li></ul><ul><li>Changes in magnetic and electrical properties of the rocks </li></ul><ul><ul><li>the so-called VAN method </li></ul></ul>

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