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Chapter 3 Earthquake Geology and Seismology Lecture PowerPoint

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1 Chapter 3 Earthquake Geology and Seismology Lecture PowerPoint
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 The Lisbon Earthquake of 1755
Morning of November 1, 1755: Lisbon experienced two major earthquakes in close succession, the first of which caused widespread fires and the second of which caused sea waves which swept many away A few hours later, Lisbon was again shaken by an earthquake in Fez, Morocco (550 km away) 70,000 people killed and 90% of structures destroyed or damaged Changed people’s attitudes about the world

3 What is an Earthquake? An event in which the Earth quakes, and vibrations are felt or recorded Caused by volcanic activity, meteorite impacts, undersea landslides, explosions of nuclear bombs or most commonly, by movement of the Earth across a fault Fault: fracture in the Earth across which the two sides move relative to each other Stresses build up until enough to cause rocks to fracture and shift, sending off waves of seismic energy, felt as earthquake Figure 3.2

4 Faults and Geologic Mapping
19th century recognition that fault movements cause earthquakes led to identification of earthquake-hazard belts Understanding faults begins with understanding rock relationships, formalized by Steno: Law of original horizontality: sediments are originally deposited in horizontal layers Law of superposition: in undeformed sequence of sedimentary rock layers, each layer is younger than the layer beneath it and older than the layer above it

5 Faults and Geologic Mapping
Law of original continuity: sediment layers are continuous, ending only against a topographic high, by pinching out from lack of sediment, or by gradational change from one sediment to another If sedimentary layer ends abruptly, may have been eroded by water action or truncated by fault passing through layer Identifying truncated sedimentary layers and recognizing their offset continuation allows determination of fault length Length of fault determines size of earthquake possible on fault (longer fault ruptures create bigger earthquakes) Understanding fault offset can also have financial rewards, if ore-bearing unit exists two different places on either side of fault (example of gold-bearing gravels 840 km apart in New Zealand)

6 Faults and Geologic Mapping
Figure 3.5 Figure 3.4

7 Types of Faults Jointing – brittle lithospheric rocks fracture and crack Large stress differential on either side of a fracture results in movement: fracture becomes a fault Movement ranges from millimeters to hundreds of kilometers, resulting in tilting and folding of layers Use strike and dip to describe location in 3D space of deformed rock layer

8 Types of Faults Use strike and dip to describe location in 3D space of deformed rock layer Dip: angle of inclination from horizontal of tilted layer Strike: compass bearing of horizontal line in tilted layer Figure 3.7

9 Types of Faults Dip-slip faults:
Dip-slip faults are dominated by vertical movement Ore veins often form in fault zones, so many mines are actually dug out along faults Miners refer to the block beneath them as the footwall (block beneath the fault) and the block above them as the hangingwall (block above the fault) Figure 3.8

10 Types of Faults Dip-Slip Faults: Caused by pushing or pulling force
Where dominant force is extensional (pulling), normal fault occurs when hangingwall moves down relative to footwall, and zone of omission results Figure 3.9

11 Types of Faults Dip-Slip Faults: Caused by pushing or pulling force
Where dominant force is compressional, reverse fault occurs when hangingwall moves up relative to footwall, and zone of repetition results Figure 3.10

12 Types of Faults Strike-slip faults: Dominated by horizontal movement
When straddling a fault, if right-hand side moved towards you, it is a right-lateral fault When straddling a fault, if the left-hand side has moved towards you, it is a left-lateral fault Convention works in either direction Figure 3.11

13 Types of Faults Faults are complex zones of breakage with irregular surfaces, many miles wide and long Stress builds up until rupture occurs at weak point and propagates along fault surface Point where rupture first occurs is hypocenter or focus Point directly above hypocenter on surface is epicenter Figure 3.12

14 Types of Faults Fault rupture is series of events over weeks to months to years, with largest event referred to as ‘the earthquake’ Smaller events preceding ‘the earthquake’ are foreshocks Impossible to identify as foreshock until after ‘the earthquake’ has occurred Smaller events after ‘the earthquake’ are aftershocks

15 Types of Faults Steps in Strike-Slip Faults:
Earth does not rupture along clean, straight line but with several breaks that stop and start and bend Left step in right-lateral fault or right step in left-lateral fault: Compression, uplift, hills and mountains Figure 3.13

16 Types of Faults Steps in Strike-Slip Faults:
Right step in right-lateral fault or left step in left-lateral fault: Extension, down-dropping, basins and valleys Figure 3.14

17 Types of Faults Transform Faults:
As oceanic plates spread apart at mid-ocean ridges, they must slide past other plates, along transform faults Transform faults link spreading centers, or connect spreading center to subduction zone Between two spreading centers, transform fault motion is same as strike-slip fault Outside two spreading centers, plates are moving at same rate so there is no offset – fracture zone Figure 3.15

18 Development of Seismology
Seismology: study of earthquakes Earliest earthquake device: China, 132 B.C. Instruments to detect earthquake waves: seismometers Instruments to record earthquake waves: seismographs Capture movement of Earth in three components: north-south, east-west and vertical One part stays as stationary as possible while Earth vibrates: heavy mass fixed by inertia in frame that moves with the Earth, and differences between position of the frame and the mass are recorded digitally Figure 3.16

19 Development of Seismology
Waves: Amplitude: displacement Wavelength: distance between successive waves Period: time between waves (= 1/frequency) Frequency: number of waves in one second Figure 3.17

20 Seismic Waves Seismic waves come in two categories: those that can pass through entire Earth (body waves) and those that move near surface only (surface waves) Body waves: faster than surface waves, have short periods (high frequency – 0.5 to 20 Hz), most energetic near earthquake hypocenter Two types of body waves: P waves and S waves

21 Seismic Waves P (primary) waves Fastest of all waves
Always first to reach a recording station (hence primary) Move as push-pull – alternating pulses of compression and extension, like wave through Slinky toy Travel through solid, liquid or gas Velocity depends on density and compressibility of substance they are traveling through Velocity of about 4.8 km/sec for P wave through granite Can travel through air and so may be audible near the epicenter Figure 3.18a

22 Seismic Waves S (secondary) waves
Second to reach a recording station (after primary) Exhibit transverse motion – shearing or shaking particles at right angles to the wave’s path (like shaking one end of a rope) Travel only through solids S wave is reflected back or converted if reaches liquid Velocity depends on density and resistance to shearing of substance Velocity of about 3.0 km/sec for S wave through granite Up-and-down and side-to-side shaking does severe damage to buildings Figure 3.18b

23 Seismic Waves Seismic Waves and the Earth’s Interior
Waves from large earthquakes can pass through the entire Earth and be recorded all around the world Waves do not follow straight paths through the Earth but change velocity and direction as they encounter different layers Figure 3.19

24 Seismic Waves Seismic Waves and the Earth’s Interior
From the Earth’s surface down: Waves initially speed up then slow at the asthenosphere Wave speeds increase through mantle until reaching outer core (liquid), where S waves disappear and P waves suddenly slow P wave speeds increase gradually through outer core until increasing dramatically at inner core (solid) Figure 3.19

25 Seismic Waves Surface waves:
Travel near the Earth’s surface, created by body waves disturbing the surface Longer period than body waves (carry energy farther) Love waves Similar motion to S waves, but side-to-side in horizontal plane Travel faster than Rayleigh waves Do not move through air or water

26 Seismic Waves Surface waves: Rayleigh waves
Backward-rotating, elliptical motion produces horizontal and vertical shaking, which feels like rolling, boat at sea More energy is released as Rayleigh waves when earthquake hypocenter is close to the surface Travel great distances Figure 3.18c

27 Seismic Waves Sound Waves and Seismic Waves:
Seismologists record and analyze waves to determine where an earthquake occurred and how large it was Waves are fundamental to music and seismology Similarities: More high frequency waves if short path is traveled Trombone is retracted, short fault-rupture length (small earthquake) More low frequency waves if long path is traveled Trombone is extended, long fault-rupture length (large earthquake)

28 Locating the Source of an Earthquake
P waves travel about 1.7 times faster than S waves Farther from hypocenter, greater lag time of S wave behind P wave (S-P) Figure 3.22

29 Locating the Source of an Earthquake
(S-P) time indicates how far away earthquake was from station – but in what direction? Figure 3.21

30 Locating the Source of an Earthquake
Need distance of earthquake from three stations to pinpoint location of earthquake: Visualize circles drawn around each station for appropriate distance from station, and intersection of circles at earthquake’s location Method is most reliable when earthquake was near surface Usually computer calculation Figure 3.23

31 Magnitude of Earthquakes
Richter scale: Devised in 1935 to describe magnitude of shallow, moderately-sized earthquakes located near Caltech seismometers in southern California Bigger earthquake  greater shaking  greater amplitude of seismogram lines Defined magnitude as ‘logarithm of maximum seismic wave amplitude recorded on standard seismogram at 100 km from earthquake’, with corrections made for distance Figure 3.24

32 Magnitude of Earthquakes
Richter scale: For every 10 fold increase in recorded amplitude, Richter magnitude increases one number With every one increase in Richter magnitude, the energy release increases by about 45 times, but energy is also spread out over much larger area and over longer time Bigger earthquake means more people will experience shaking and for longer time (increases damage to buildings) Many more small earthquakes each year than large ones, but more than 90% of energy release is from few large earthquakes Richter scale magnitude is easy and quick to calculate, so popular with media

33 Magnitude of Earthquakes
Other Measures of Earthquake Size: Richter scale is useful for magnitude of shallow, small-moderate nearby earthquakes Does not work well for distant or large earthquakes Short-period waves do not increase in amplitude for bigger earthquakes Richter scale: 1906 San Francisco earthquake was magnitude 8.3 1964 Alaska earthquake was magnitude 8.3 Other magnitude scale: 1906 San Francisco earthquake was magnitude 7.8 1964 Alaska earthquake was magnitude 9.2

34 Magnitude of Earthquakes
Other Measures of Earthquake Size: Body wave scale (mb): Uses amplitudes of P waves with 1 to 10-second periods Surface wave scale (ms): Uses Rayleigh waves with 18 to 22-second periods All magnitude scales are not equivalent Larger earthquakes radiate more energy at longer periods which are not measured by Richter scale or body wave scale, so large or distant earthquake magnitudes are underestimated

35 Magnitude of Earthquakes
Moment Magnitude Scale: Seismic moment (Mo) Measures amount of strain energy released by movement along whole rupture surface; more accurate for big earthquakes Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault Moment magnitude scale uses seismic moment: Mw = 2/3 log10 (Mo) – 6 Figure 3.25

36 Magnitude of Earthquakes
Foreshocks, Mainshock and Aftershocks Large earthquakes are not just single events but part of series of earthquakes over years Largest event in series is mainshock Smaller events preceding mainshock are foreshocks Smaller events following mainshock are aftershocks Large event may be considered mainshock, then followed by even larger earthquake, so then re-classified as foreshock

37 Magnitude of Earthquakes
Magnitude, Fault-Rupture Length, and Seismic-Wave Frequencies: Fault-rupture length greatly influences magnitude: 100 m long fault rupture  magnitude 4 earthquake 1 km long fault rupture  magnitude 5 earthquake 10 km long fault rupture  magnitude 6 earthquake 100 km long fault rupture  magnitude 7 earthquake

38 Magnitude of Earthquakes
Magnitude, Fault-Rupture Length, and Seismic-Wave Frequencies: Fault-rupture length and duration influence seismic wave frequency: Short rupture, duration  high frequency seismic waves Long rupture, duration  low frequency seismic waves Seismic wave frequency influences damage: High frequency waves cause much damage at epicenter but die out quickly with distance from epicenter Low frequency waves travel great distance from epicenter so do most damage farther away

39 Ground Motion During Earthquakes
Buildings are designed to handle vertical forces (weight of building and contents) so that vertical shaking in earthquakes is usually safe Horizontal shaking during earthquakes can do massive damage to buildings Acceleration Measure in terms of acceleration due to gravity (g) Weak buildings suffer damage from horizontal accelerations of more than 0.1 g At isolated locations, horizontal acceleration can be as much as 1.8 g (Tarzana Hills in 1994 Northridge, California earthquake)

40 Ground Motion During Earthquakes
Periods of Buildings and Responses of Foundations: Buildings have natural frequencies and periods Periods of swaying are about 0.1 second per story 1-story house shakes at about 0.1 second per cycle 30-story building sways at about 3 seconds per cycle Building materials affect building periods Flexible materials (wood, steel)  longer period of shaking Stiff materials (brick, concrete)  shorter period of shaking Velocity of seismic wave depends on material through which it is moving Faster through hard rocks, slower through soft rocks

41 Ground Motion During Earthquakes
Periods of Buildings and Responses of Foundations: When waves pass from harder to softer rocks, slow down Must therefore increase their amplitude in order to carry same amount of energy  greater shaking Shaking tends to be stronger at sites with softer ground foundations (basins, valleys, reclaimed wetlands, etc.) If the period of the wave matches the period of the building, shaking is amplified and resonance results Common cause of catastrophic failure of buildings

42 Earthquake Intensity – What We Feel During an Earthquake
Mercalli intensity scale was developed to quantify what people feel during an earthquake Used for earthquakes before instrumentation or current earthquakes in areas without instrumentation Assesses effects on people and buildings Did You Feel It?: Maps of Mercalli intensities can be generated quickly after an earthquake using people’s input to the webpage Insert Table 3.6 here

43 In Greater Depth: What To Do Before and During an Earthquake
Before an earthquake: Inside and outside your home, visualize what might fall during strong shaking, and anchor those objects by nailing, bracing, tying, etc. Inside and outside your home, locate safe spots with protection – under heavy table, strong desk, bed, etc. During an earthquake: Duck, cover and hold Stay calm If inside, stay inside If outside, stay outside

44 Earthquake Intensity – What We Feel During an Earthquake
Mercalli scale variables: Earthquake magnitude Bigger earthquake, more likely death and damage Distance from hypocenter Usually (but not always), closer earthquake  more damage Type of rock or sediment making up ground surface Hard rock foundations vibrate from nearby earthquake body waves Soft sediments amplified by distant earthquake surface waves Steep slopes can generate landslides when shaken

45 Earthquake Intensity – What We Feel During an Earthquake
Mercalli scale variables: Building style Body waves near epicenter are amplified by rigid short buildings Low-frequency surface waves are amplified by tall buildings, especially if on soft foundations Duration of shaking Longer shaking lasts, more buildings can be damaged Insert Table 3.7 here

46 A Case History of Mercalli Variables
San Fernando Valley, California, Earthquake of 1971 Earthquake magnitude Magnitude 6.6, with 35 magnitude 4.0 or higher aftershocks in 7 minutes after main shock Distance from epicenter Bull’s-eye damage pattern Building style ‘Soft’ first-story buildings were major problem Hollow-core bricks at V.A. Hospital caused collapse Collapse of freeway bridges Figure 3.27

47 A Case History of Mercalli Variables
San Fernando Valley, California, Earthquake of 1971 Duration of Shaking Strong ground shaking lasted 12 seconds Lower Van Norman Reservoir failed by landslides until stood only 4 ft above water – had shaking continued 5 seconds longer, dam would have failed, homes of 80,000 would have flooded Learning from the Past, Planning for the Future 1994 Northridge earthquake caused same kinds of damage Figure 3.30

48 Building in Earthquake Country
Eliminate resonance: Change height of building Move weight to lower floors Change shape of building Change building materials Change attachment of building to foundation Hard foundation (high-frequency vibrations)  build tall, flexible building Soft foundation (low-frequency vibrations)  build short, stiff building

49 Building in Earthquake Country
Shear Walls Designed to receive horizontal forces from floors, roofs and trusses and transmit to ground Lack of shear walls typically cause structures like parking garages to fail in earthquakes Figure 3.32 Figure 3.31

50 Building in Earthquake Country
Braced Frames Bracing with ductile materials offers resistance Retrofit Buildings Increase resistance to seismic shaking Figure 3.33 Figure 3.34

51 Building in Earthquake Country
Base Isolation Devices on ground or within structure to absorb part of earthquake energy Use wheels, ball bearings, shock absorbers, ‘rubber doughnuts’, etc. to isolate building from worst shaking Figure 3.35

52 Building in Earthquake Country
Retrofit Bridges Bridges combine steel with concrete, materials with different properties in earthquakes Rebuild with alternating layers of steel and concrete Figure 3.36

53 Building in Earthquake Country
Houses Modern 1-, 2-story wood-frame houses perform well in earthquakes Additional support can be given by building shear walls, bracing, tying walls and foundations and roof together Much damage as interior items are thrown about Bolt down water heaters, ceiling fans, cabinets, bookshelves, electronics Figure 3.37

54 End of Chapter 3


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