ground shaking caused by sudden release of energy stored in rocks beneath surface

Transcription

1 earthquakes

2 earthquakes what is an earthquake? ground shaking caused by sudden release of energy stored in rocks beneath surface tectonic forces produce stresses on rocks that exceed elastic limits and cause brittle failure (rupture along a fault plane) seismic waves released from breaking point

3 what happens along the fault? elastic rebound theory 3 m offset 1906 San Francisco earthquake

4 offset lettuce rows - El Centro, CA

5 earthquakes hypocenter (focus) location of first rupture along fault epicenter point on Earth s surface above rupture seismic waves radiate from focus (hypocenter)

6 earthquakes seismic waves -- energy released from earthquake two types of seismic waves body waves travel outward from focus through body of Earth surface waves travel away from epicenter on surface of Earth

7 body waves P (primary) waves compressional body wave vibration is back and forth in direction wave travels (slinky) fast (4-7 km/s); first to arrive pass through solids and fluids S (secondary) waves shear body wave vibration is perpendicular to direction the wave travels (rope) slower (2-5 km/s); secondary arrival pass only through solids

8 body waves P wave displacement parallel to wave motion S wave displacement normal to wave motion from: P (primary) waves faster than S (secondary) waves

9 body waves why are P waves faster than S waves? why don t S waves travel through fluids? look at equations for velocities V (primary wave) = k + 4/3µ " " " V (secondary wave) = µ " " 1/2 1/2 " density µ shear modulus (rigidity) k bulk modulus (compressibility) numerator of P wave velocity > numerator of S wave velocity µ, or shear modulus, is zero for a fluid (fluids cannot support shear) velocity of S wave in fluid is zero

10 surface waves slowest seismic waves Love waves side to side motion of ground surface cannot travel through fluids Rayleigh waves ground moves in elliptical path In direction opposite to direction of travel of wave very destructive to buildings

11 measuring earthquakes instruments record arrival of seismic waves at sites calculation of size (magnitude) and location (focus) seismometers instruments that detect seismic waves seismographs devices that record motion detected by seismometers seismograms paper or digital records of seismic wave vibrations

12 vertical component seismometer measures vertical motion of Earth s surface very heavy weight on spring ground moves, but weight stays at same level pencil attached to weight writes on rotating paper

13 horizontal component seismometer measures horizontal motion of Earth s surface

14 sample seismogram P, S, L, R: arrivals of P, S, Love, Rayleigh waves, respectively from same earthquake at same seismometer note that P is first S is second surface waves (Love and Rayleigh) are last

15 locating earthquakes P and S waves leave focus (hypocenter) at same time and head toward seismograph stations A and B P waves travel faster than do S waves P waves get farther and farther ahead of S waves with distance and time from earthquake difference in arrival time is greater at station B than at A

16 locating earthquakes we know relationship of velocities of P and S waves thus we can generate a travel-time curve to estimate distance from focus (hypocenter) travel-time curve S wave curve separation between 2 curves increases with time and distance P wave curve 3 minute difference at 2000 kms 8 minute difference at 5300 kms

17 locating earthquakes how do we determine location of epicenter? use 3 stations determine distance to epicenter for each station draw circles with center at station and distance to epicenter as the radius constrain location of epicenter by intersection of 3 circles

18 locating earthquakes example use arrival times of S and P waves on 3 seismograms "and travel-time curve from:

19 3 seismograms from Japan/ S. Korea Akita Pusan Tokyo from:

20 travel-time curve find time of arrival of S and P waves for each site calculate difference in P and S wave arrival time for each site from:

21 3 seismographs S-P interval: "Tokyo: 44 sec "Pusan: 56 sec "Akita: 71 sec distance "Tokyo: 434 km "Pusan: 549 km "Akita: 697 km from:

22 draw circles whose centers are stations and radii are distances from S-P difference distance "Tokyo: 434 km "Pusan: 549 km "Akita: 697 km epicenter is intersection of 3 circles (Kobe, Japan) hypocenter (focus) can be determined as well from:

23 measuring earthquake size two approaches intensity measure of earthquake effects on people/structures Modified Mercalli Intensity Scale I (not generally felt) -XII (total devastation) magnitude amount of energy released by earthquake Richter Scale developed for southern California; no upper bound; accurate at > 7; less accurate for small events Moment Magnitude more objective; uses rock strength, area of rupture, and amount of displacement (slip along fault)

24 Modified Mercalli Intensity Index 1886 Charleston, SC earthquake

25 relationship of Mercalli Intensity to Richter magnitude

26 moment magnitude and energy released note for any year # small >>> # big

27 determining magnitude from seismograms measure maximum amplitude of S wave (body wave magnitude) (other methods exist using other waves) from:

28 measure amplitude of S waves on 3 seismograms these are the same as earlier example Akita: Pusan: Tokyo: 30 mm 90 mm 170 mm Akita Pusan Tokyo from:

29 figure to left plots relationship between distance from epicenter magnitude amplitude of wave on seismogram from:

30 distance (from earlier) "Tokyo: 434 km "Pusan: 549 km "Akita: 697 km amplitude Akita: Pusan: Tokyo: 30 mm 90 mm 170 mm magnitude ~ 6.8 from:

31 location and size of earthquakes in the US occur everywhere but more common in Alaska, Pacific Northwest and California note concentration in eastern Arkansas, western Tennessee and Kentucky, southern Illinois, southeastern Missouri New Madrid Seismic Zone

32 earthquake risk in the US assumes that future earthquakes will occur where they have in the past New Madrid Seismic Zone

33 effects of earthquakes ground shaking most familiar, topples buildings fire from broken gas mains, fallen electric wires landslides triggered by shaking liquefaction water-saturated soil behaves like fluid; can no longer support structures displacement of land surface movement of blocks along fault

34 pancaked building Mexico City earthquakes don t kill people - buildings do!

35 ground rupture, Taiwan

36 ground rupture, Taiwan

37 surface displacement Alaska

38 buckled concrete San Fernando, CA

39 soil liquefaction Nigata, Japan shaking disturbs clay particles in soil the clay particles collapse like a house of cards

40 liquefaction animations note color of rock changes to darker gray as soil liquefies

41 another deadly consequence tsunami (harbor waves) produced by displacement of water by vertical motion of sea floor (e.g. dip-slip fault) not like ocean waves have very long wavelengths water keeps rising for many minutes

42 travel across ocean at ~700 km/hour have small amplitudes in ocean ocean (a few feet) reach great heights in coastal regions as they feel the bottom

43 now you know why this won t happen

44 tsunami wave propagation times from Hawaii

45 tsunami Alaska earthquake

46 Banda Aceh tsunami, 2004 Boxing Day

47 rupture area of Sumatra-Banda Aceh event (Boxing Day, 2004) compared to Cascadia subduction zone of western North America from:

48 earthquakes and plate tectonics map of plate boundaries (yellow) earthquakes and plate tectonics

49 global distribution of earthquakes occur along plate boundaries

50 geology in the news California earthquake: 10/30/ (largest in Bay area in 20 years) epicenter north and east of San Jose on Calaveras Fault (branch of San Andreas)

51 a little bit more about earthquakes Sumatra 12/26/ Pakistan 10/06/ deaths 82,321

52 earthquakes largest earthquakes in the world since 1900 all occur along subduction plate boundaries

53 intraplate earthquakes --those not along plate boundaries New Madrid Seismic Zone (NMSZ) is one example

54 New Madrid Seismic Zone 3 large earthquakes in winter of sand blows (liquefaction) flattened trees formation of St. Francis and Reelfoot Lakes reversal of flow locally in Mississippi River some estimates say 8.5 magnitude controversial others say 7.0 magnitude importance of recurrence interval

55 New Madrid Seismic Zone red dots are epicenters (current activity) Reelfoot and Cottonwood Grove are faults

56 Digital elevation model : New Madrid

57 Reelfoot Lake, formed during events

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59 New Madrid Seismic Zone sand blows in eastern Arkansas photo credit: Haydar Al-Shukri

60 Modified Mercalli Intensity map for New Madrid events in

61 earthquakes recurrence interval important in NMSZ controversy what is recurrence interval and how is it determined? average time between successive events geologists dig trenches across active faults and map when sediments were offset during earlier earthquakes use buried organic matter (charcoal) to date layers and bracket ages of earthquakes --paleoseismology--

62 trench from:

63 New Madrid: paleoseismology

64 liquefaction evidence for past earthquakes last four clustered cycles have ~ year recurrence prior cycle has longer recurrence: ~2600 years

65 earthquakes: paleoseismology led to estimate of ~150 years for recurrence of large earthquakes in southern California note that last rupture in southern California was in 1857 (green line) 1906 rupture in northern California was 477 km long! (red line)

66 earthquakes: prediction long-term: seismic gaps--no earthquakes in recent past short-term: changes in water levels in wells increases in radon (gas) emissions along faults expansion of rock volume due to cracking --micro-earthquake swarms --tilt or bulge in rocks --changes in seismic wave velocity of rocks anomalous radio-wave or sound signals unusual animal behavior

67 earthquakes: human-induced injection of water into subsurface "--Rocky Mountain arsenal, 1960 s, M=5.5 construction of dams "--residents complained of earthquakes after " "filling of Hoover Dam in 1930 s "--similar effects from dams in India (M=6.3); " "East Africa; South Carolina oil and hot water extraction (subsidence and fracturing mining operations (blasting and fracturing) nuclear explosions "--use world-wide seismic network to monitor " "global test-ban treaty

68 earthquakes: generate seismic waves use seismic waves to infer internal structure cannot study deep interior directly --deepest drill hole is 12 km-stayed in crust (uppermost layer)

69 earthquakes: generate seismic waves use seismic waves to infer internal structure P waves accelerate when they move through more rigid solids and slow down as they pass through fluids S waves accelerate when they move through more rigid solids and disappear when they encounter fluids

70 seismic waves move through Earth seismic reflection wave approaches boundary between two rock types and reflects off boundary sharp boundary between rocks of different densities will result in reflection reflected wave received at seismic station

71 seismic waves move through Earth seismic refraction wave approaches boundary between two rock types and bends (refracts) through boundary bend to a shallower angle if V 2 > V 1 V 1 V 2 bend to a steeper angle if V 1 > V 2 above example V 2 > V 1

72 wave ray paths above example V 2 > V 1 above example V 2 < V 1 angle (or dip of ray path) decreases across boundary angle (or dip of ray path) increases across boundary density in Earth generally increases with depth thus so does seismic velocity--rays shallow with depth

73 velocities of seismic waves in Earth note general increase with depth

74 radial pattern of ray paths from focus 1) ray path shallows as velocity increases with depth pattern of increasing seismic velocity, Vp and Vs, of rocks with depth 2) ray path flattens as it cannot shallow farther; ray turns up toward surface 3) ray encounters decreasing velocities as it travels upward, causing ray path to steepen toward surface

75 earthquakes not only source of energy shotgun vibraseis (thumper truck) energy does not penetrate very deep deploy small array of geophones to detect returning waves

76 internal Earth structure from seismology crust: 0-60 km thin, outer layer mantle: km dense rock core: km metallic

77 crust two types oceanic higher seismic velocity: 7 km/sec -- dense, mafic (basalt/gabbro); thinner: 0 to 10 km continental lower seismic velocity: 6 km/sec -- less dense, felsic (granite); thicker: 30 to 50 km crust/mantle boundary: Mohorovicic discontinuity (seismic evidence)

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79 mantle: km depth solid rock with isolated pockets of magma seismic velocities of 8 km/sec--ultramafic, dense

80 mantle: km depth uppermost mantle and crust make up lithosphere (brittle, outer shell that forms the plates) seismic wave velocities decrease abruptly below lithosphere low-velocity asthenosphere (plastic; flows slowly)

81 mantle: 3D picture of seismic velocities similar to CAT scan--shows mantle is not homogeneous blue = fast velocities (cold) red = slow velocities (hot)

82 core: 2 sections -- inner and outer core outer core: liquid inner core: solid evidence for solid and liquid comes from P and S waves

83 seismic evidence: P waves P wave shadow zone (from refraction) outer core is liquid P waves slow and bend steeper geometry dictates that no P waves will be received between 103 and 142 from focus shadow zone because P waves pass through liquid, they will reach inner core

84 seismic evidence: S waves S wave shadow zone (from refraction) outer core is liquid S waves cannot pass through geometry dictates that no S waves will be received Greater than 103 from focus shadow zone because S waves cannot go through fluid, none will, reach inner core

85 core: composition inferred from: calculated density "(know bulk density of Earth and know outer " "layers of Earth are light ) electromagnetic properties "(outer core generates magnetic field) composition of meteorites "(have proper densities) physical properties "(from seismic waves) iron metal best fits (common in meteorites) --liquid iron in outer core-- --solid iron in inner core--

86 core-mantle boundary: D layer large changes in seismic velocity, temperature, density topography (bumps) low-velocity zones where hot core may melt mantle from: Garnero, E.J., Ann. Rev. Earth Planetary Sci., 28, , 2000.

87 core-mantle boundary: rocks? pallisite meteorites--may be from core/mantle boundary " " " " " "of asteroid olivine (silicate) (mantle) iron metal alloy (core)