03 lecture ppt

Seismic waves come in two categories: those that can pass through entire Earth body waves and those that move near surface only surface • Body waves : faster than surface waves, have

The Lisbon Earthquake of

1755

• Morning of November 1, 1755: Lisbon experienced

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

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

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

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)

Faults and Geologic

Mapping

Figure 3.4

Figure 3.5

Types of Faults

• 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

Types of Faults

• Use strike and dip to

describe location in 3D space

of deformed rock layer

from horizontal of tilted

layer

horizontal line in tilted layer

Figure 3.7

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

Types of 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

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

Types of Faults

Figure 3.9

Dip-Slip Faults :

• Caused by pushing or pulling force

• Where dominant force is compressional, rev erse fault occurs when hangingwall moves up relative to footwall, and zone of

Types of Faults

Figure 3.10

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

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

– Impossible to identify as foreshock until after ‘the

earthquake’ has occurred

• Smaller events after ‘the earthquake’ are aftershocks

• Left step in right-lateral fault or right

step in left-lateral fault:

– Compression, uplift, hills and

Steps in

Strike-Slip Faults:

• Right step in

right-lateral fault or left

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 v ertical

• 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

• Amplitude : displacement

• Wavelength : distance between successive waves

• Period : time between waves (= 1/frequency)

• Frequency : number of waves in one second

Development of Seismology

Figure 3.17

Seismic waves come in two categories: those that can pass through entire Earth ( body waves ) and those that move near surface only ( surface

• 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

Seismic Waves

P (primary) wav es

• 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

Seismic Waves

Figure 3.18a

S (secondary) wav es

• Exhibit transv erse 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

Seismic Waves

Figure 3.18b

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

Seismic Waves

Figure 3.19

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

Seismic Waves

– P wave speeds

increase gradually

through outer core until

increasing

inner core

(solid)Figure 3.19

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

Seismic Waves

Surface waves:

• Rayleigh waves

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

Seismic Waves

Figure 3.18c

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

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

Locating the Source of an

Earthquake

• (S-P) time indicates how far away earthquake was from station – but in what direction ?

Figure 3.21

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

earthquake’s location

– Method is most reliable when

earthquake was near surface

– Usually computer calculation

Figure 3.23

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

corrections made for distance

Figure 3.24

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

Other Measures of Earthquake Size:

• Richter scale is useful for magnitude of shallow, moderate nearby earthquakes

small-• 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

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

Magnitude of Earthquakes

Moment Magnitude

Scale:

– 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

Magnitude of Earthquakes

Figure 3.25

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

Magnitude of Earthquakes

Magnitude, Fault-Rupture Length, and

Seismic-Wave Frequencies:

• Fault-rupture length greatly influences magnitude:

– 1 km long fault rupture  magnitude 5 earthquake

– 10 km long fault rupture  magnitude 6 earthquake

Magnitude of Earthquakes

Magnitude, Fault-Rupture Length, and

• Seismic wave frequency influences damage:

epicenter but die out quickly with distance from

epicenter

epicenter so do most damage farther away

Magnitude of Earthquakes

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)

Periods of Buildings and Responses of

Foundations:

• Buildings have natural frequencies and periods

• Periods of swaying are about 0.1 second per story

• Building materials affect building periods

• Velocity of seismic wave depends on material through which it is moving

Ground Motion During

Earthquakes

Ground Motion During

• 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

Earthquake Intensity – What We Feel During an

Earthquake

feel during an earthquake

• Used for earthquakes before instrumentation or current earthquakes

in areas without instrumentation

• Assesses effects on people and buildings

quickly after an earthquake using people’s input to the webpage

http://pasadena.wr.usgs.gov/shake

Insert Table 3.6 here

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

Earthquake Intensity – What We Feel During an

Earthquake

• Earthquake magnitude

– Bigger earthquake, more likely death and damage

• Distance from hypocenter

– Usually (but not always), closer earthquake  more damage

– Hard rock foundations vibrate from nearby earthquake body waves

– Soft sediments amplified by distant earthquake surface waves– Steep slopes can generate landslides when shaken

Earthquake Intensity – What We Feel During an

• Duration of shaking

– Longer shaking lasts,

more buildings can

be damaged Insert Table 3.7 here

A Case History of Mercalli

• 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

A Case History of Mercalli

– 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

Building in Earthquake Country

– 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

• Braced Frames

– Bracing with ductile materials offers resistance

• Retrofit Buildings

– Increase resistance to seismic shaking

Building in Earthquake Country

Figure 3.33 Figure 3.34

• 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

Building in Earthquake Country

Figure 3.35

• Retrofit Bridges

– Bridges combine steel with concrete, materials with different properties in earthquakes

– Rebuild with alternating layers of steel and concrete

Building in Earthquake Country

Figure 3.36

– 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