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John G. Anderson Professor of Geophysics

Earthquake Engineering GE / CEE - 479/679 Topic 3. Faulting and Paleoseismology 1 January 29, 2008. John G. Anderson Professor of Geophysics. Brittle behavior Lower temperatures Lower pressures Lithified materials High strain rates. Ductile behavior Higher temperatures Higher pressures

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John G. Anderson Professor of Geophysics

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  1. Earthquake EngineeringGE / CEE - 479/679Topic 3. Faulting and Paleoseismology 1January 29, 2008 John G. Anderson Professor of Geophysics

  2. Brittle behavior Lower temperatures Lower pressures Lithified materials High strain rates Ductile behavior Higher temperatures Higher pressures Loose materials Low strain rates Factors favoring Consequently, the crust of the Earth has a shallow brittle layer, where earthquakes occur, above deeper ductile materials. In some places, there is a thin ductile layer near the surface.

  3. What we need from the geologist • Fault location and geometry • Is the fault active? • Slip rate • Repeat time (or recurrence interval) • Magnitude of “characteristic earthquake” or Mmax • Distribution of earthquake sizes

  4. Strike slip faulting can be challenging to recognize because the slip is horizontal, and the dominant markers also are primarily horizontal.

  5. Blind thrust fault Anticlinal fold forms above a fault Examples: 1983 Coalinga, California earthquake 1987 Whittier, California earthquake 1994 Northridge, California earthquake (?)

  6. Faulting in a Recent Earthquake • Normal faulting - “typical” Basin and Range

  7. Borah Peak EarthquakeOct 28, 1983Ms=7.3 Modified Mercalli Intensity Map

  8. Modified Mercalli Intensity Scale • Gives a local characteristic of the earthquake at a site. • Based on response of people and structures. • MMI is generally larger near the epicenter of an earthquake, and decreases with distance. • However, site effects can cause anomalies in this trend.

  9. See the “Appendix” at the end of this presentation for a complete description of the Modified Mercalli Intensity. • The following show a few representative examples.

  10. Modified Mercalli Intensity Scale of 1931 • IV. Felt indoors by many, outdoors by few. • Awakened few, especially light sleepers.Frightened no one, unless apprehensive from previous experience.Vibration like that due to the passing of heavy or heavily loaded trucks.Sensation like heavy body striking building or falling of heavy objects inside.Rattling of dishes, windows, doors; glassware and crockery clink and clash.Creaking of walls, frame, especially in the upper range of this grade.Hanging objects swung, in numerous instances.Slightly disturbed liquids in open vessels. Rocked standing motor cars noticeably.

  11. Modified Mercalli Intensity Scale of 1931 • VI. Felt by all, indoors and outdoors. • Frightened many, excitement general, some alarm, many ran outdoors.Awakened all.Persons made to move unsteadily.Trees, bushes, shaken slightly to moderately.Liquid set in strong motion.Small bells rang -- church, chapel, school, etc.Damage slight in poorly built buildings.Fall of plaster in small amount.Cracked plaster somewhat, especially fine cracks; chimneys in some instances.Broke dishes,.Fall of knick-knacks, books, pictures.Overturned furniture in many instances.Moved furnishings of moderately heavy kind.

  12. Modified Mercalli Intensity Scale of 1931 • VIII. Fright general -- alarm approaches panic. • Disturbed persons driving motor cars.Trees shaken strongly -- branches, trunks, broken off, especially palm trees.Ejected sand and mud in small amounts.Changes: temporary, permanent; in flow of springs and wells; dry wells renewed flow; in temperature of spring and well waters.Damage slight in structures (brick) built especially to withstand earthquakes. • Considerable in ordinary substantial buildings, partial collapse: racked, tumbled down, wooden houses in some cases; threw out panel walls in frame structures, broke off decayed piling.Fall of walls.Cracked, broke, solid stone walls seriously.Wet ground to some extent, also ground on steep slopes.Twisting, fall, of chimneys, columns, monuments, also factory stacks, towers.Moved conspicuously, overturned, very heavy furniture.

  13. Borah Peak EarthquakeOct 28, 1983Ms=7.3 Modified Mercalli Intensity Map

  14. Eyewitness Account • On the morning of October 28, 1983, Don Hendrickson and John Turner were on a dirt road in Arentson Gulch, Idaho, in a 4-wheel drive vehicle, looking for elk, when Don, after feeling light-headed and dizzy, saw the road fall away in front of his vehicle, as if a sinkhole had formed. This was followed by the formation of a surface rupture about 20 m in front of the vehicle, with the only sound being the crumbling of earth in front of them. This was followed by violent shaking and a deafening rumbling noise, the entire episode lasting 10 or 15 seconds. • Quoted from Yeats et al, 1997.

  15. Eyewitness Account • “At the same time, Mrs. Lawana Knox was seated not too far away on a slope north of Thousand Springs Valley, Idaho, looking for her husband, a hunter, when a 1- to 1.5 meter-high fault scarp formed in front of her at about 300 meters distance, reaching its full height in about one second. The scarp seemed to tear from the northwest to the southeast along the flank of the mountain just as though one took a paint brush and painted a line along the hill. The scarp took only a few seconds to extend several miles along the range front, but it did not form until the peak of strong shaking had begun to subside, at least 10 seconds afterwards…” • Quoted from Yeats et al, 1997.

  16. Photo courtesy of Craig dePolo

  17. Photo courtesy of Craig dePolo

  18. Photo courtesy of Craig dePolo

  19. Photo courtesy of Craig dePolo

  20. Some things to notice • Faults are not necessarily simple, linear features at the surface. • Faults at the surface are strongly affected by the (complex) surface materials. • Faults change character along the length of the earthquake. • Smaller scarps might be easily overlooked.

  21. Displacements during the 1983 Borah Peak, Idaho, earthquake surface rupture (frome Crone and others, 1987)

  22. Map of aftershocks. Note: aftershocks in the first 24 hours are generally considered to be the best measure of the extent of faulting in the main shock. Over time, the aftershock zone tends to expand.

  23. Wells and Coppersmith, 1994

  24. The fault dips at about a 45o angle. Most aftershocks are between 4-12 km depth, so the upper 4 km is not brittle enough to support elastic rebound by itself. The main shock is deeper than almost all aftershocks. (after Richens et al, 1987)

  25. Focal Mechanisms show that aftershocks had a predominantly normal faulting mechanism.

  26. Seismic Moment • Definition: M0=μAD • Where μ = shear modulus (often G in engineering) A = fault area D = average slip on the fault

  27. Example, determine the seismic moment of the 1983 Borah Peak, Idaho, earthquake from the surface rupture

  28. Example, determine the seismic moment of the 1983 Borah Peak, Idaho, earthquake from the surface rupture Length of surface rupture, L = 33 km

  29. Example, determine the seismic moment of the 1983 Borah Peak, Idaho, earthquake from the surface rupture Average surface displacement D = 100 cm Length of surface rupture, L = 33 km

  30. W

  31. dip = 45o W Z = 16 km W=Z/sin(dip) = 22.6 km

  32. Moment calculation M0=μAD μ = 3.3 x 1011 dyne/cm2 A = LW L = 33 km = 33 x 105 cm W = 22.6 km = 22.6 x 105 cm D = 100 cm M0 = 2.5 x 1026 dyne-cm

  33. Moment Magnitude • Definition MW=(2/3) log10 (M0)-10.73 The units of M0 are dyne-cm • For the Borah Peak example For M0 = 2.5 x 1026 dyne-cm MW = 6.9

  34. Comments on Seismic Moment • M0 can also be measured by seismologists using seismograms. • Therefore, M0 and MW, provides a link between the geologist, the seismologist, and the earthquake engineer. • By determining M0 or MW using seismograms, one gets an estimate of the amount of deformation.

  35. What did seismologists find for the Borah Peak earthquake? • I went to the web page: www.seismology.harvard.edu • Searched their Harvard CMT catalog • Result: M0 = 3.12 x 1026 dyne-cm • This gives MW=6.9 • For this earthquake, mb=6.2, and MS=7.3

  36. Comparison: geology & seismology • Geology: M0 = 2.5 x 1026 dyne-cm • Seismology: M0 = 3.12 x 1026 dyne-cm • They differ by 25% • Is this a significant difference? • No, as the uncertainties in both measurements are a factor of 2 or more. • Geology - surface strain that is not concentrated on the fault, or slip at depth that does not reach the surface. • Seismology - Variability due to wave propagation.

  37. Discussion • Based on careful mapping the geologist can estimate the magnitude of a large historical earthquake. • That estimate may also be used as the estimate of the magnitude of a future earthquake, based on the “characteristic earthquake” hypothesis.

  38. Earthquake parameters are correlated • For instance, in general, as the rupture length increases, the slip also increases. Two earthquakes drawn to scale, from Scholz, 1981

  39. Wells and Coppersmith, 1994

  40. Magnitude prediction equations • Best known: Wells and Coppersmith (1994). • Distributed to the class. • Recommendation: read the article, and learn how to use these relations.

  41. Wells and Coppersmith, 1994

  42. Wells and Coppersmith, 1994

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