Earthquake Engineering notes- module 1 ppt

brief information on earthquake

1. Earthquakes
•Causes -tectonics and faults
•Magnitude -energy and intensity
•Earthquake geography
•Seismic hazards -shaking, etc.
•Recurrence -frequency and regularity
•Prediction?
•Mitigation and preparedness

2. Causes:accumulated strain
leads to fault rupture
-the elastic rebound model

3. North American tectonic regimes
(much simplified)

4. Styles of faulting

5. Theearth’scrustisconstantlyexperiencingpressurefromforces
withinandaroundit.Thispressurebuildsupovertime,and
eventuallycausesthecrusttobreak.Thisbecomesafault.
Causes:
faultmovementreleases
energyasseismicwaves
radiatingfromrupture
Seismic
waves

6. 3. Shear stresscauses rocks
to slide past each other
resulting in strike-slip faults
2. Compressionsqueezes
rock together resulting in
reverse faults
1.Tensionpulls rocks
apart resulting in
normal faults
Fault Types
(Credit: U.S. Geological Survey
Department of the Interior/USGS)
(Credit: U.S. Geological Survey
Department of the Interior/USGS)
(Credit: U.S. Geological Survey
Department of the Interior/USGS)
Faultsurfacesaresurfacesalongwhichrocksmoveunder,over,or
pasteachother.Rocksmayget“stuck”alongthefaultsurface,causing
abuild-upofstrainenergy,andresultinginanearthquakewhenthe
rocksbreakfreeofeachother.Thereare3typesofstressthatcan
affectrocks,resultingin3differenttypesoffaults:

7. Faultsaredividedintothreemaingroups:
-whentwoplatesaremovingapartandone
sideofthefracturemovesbelowtheother;(causedby
tensionforces!)
Normal

8. -whentwoplatescollideandonesideofthe
fracturemovesontopofanother;(causedbycompression
forces!!)
-whentwoplatesslidepasteachother.(caused
byshearforces!)
Reverse
Strike-Slip

9. Faults caused by blocks of crust pulling apart under the forces of tension are called normal
faults.Entire mountain ranges can form through these processes and are known as fault block
mountains (examples: Basin and Range Province, Tetons).
In a normal fault, the hanging-wall block moves down relative to the foot-wall block.
The footwall is the underlyingsurface of an inclined fault plane.
The hanging wall is the overlyingsurface of an inclined fault plane.
Faults: Normal Faults
Relative movement of two blocks indicating a
normal fault.(Credit: Modified after U.S. Geological
Survey Department of the Interior/USGS)
Diagrammatic sketch of the two types of
blocks used in identifying normal faults.
Hanging
Wall
Foot Wall
Footwall
block
Hanging
wall block

10. Faults caused by blocks of crust colliding under the forces of compression are called reverse
faults.
Reverse faults are a prevalent feature in continent-continent collisions. Usually, there is also
accompanying folding of rocks.
During reverse faulting, the hanging wall block moves upward (and over) relative to the
footwall block.
Faults: Reverse Faults
Hanging
Wall
Foot Wall
Footwall
block
Hanging
wall block
Relative movement of two blocks indicating a
reverse fault. (Credit: U.S. Geological Survey Department of
the Interior/USGS)
Diagrammatic sketch of the two
types of blocks used in identifying
reverse faults.

11. Intheexampleabove,thefencehasbeenoffset
totheright,thereforeitiscalledarightlateral
strike-slipfault.(Credit:U.S.GeologicalSurveyDepartmentof
theInterior/USGS)
Strike-slip faults occur when two blocks move in
horizontal but opposite directions of each other.
Depending on the direction of offset, it can be a
“right-lateral offset” or a “left-lateral offset.”
The photograph above displays a light-
colored pegmatite vein offset to the right
in a schistose matrix.Photo courtesy of K.
McCarney-Castle.
Faults: Strike-slip faults
Right-lateral offset

12. How is direction of fault motion determined?

13. “First motion” studies can determine the type
of fault that produced an earthquake

14. Where Do Earthquakes Occur?

15. Seismicity and Plate Boundaries

16. Convergent Plate Boundaries
•Deep earthquakes
typically are found only
at convergent plate
boundaries
•Shallow earthquakes
also occur at these
boundaries

17. Intraplate Earthquakes
•Earthquakes also occur distant from plate boundaries, typically with shallow-
foci
•Some of the most destructive earthquakes (e.g. New Madrid) have been
intraplate, but may be associated with ancient plate boundaries. Their cause is a
mystery.

18. Potential Earthquake Hazards, USA

19. Severityofdamageiscontrolledbymorethanjustearthquake
magnitude-GroundmaterialpropertiesalsoaffectsShaking
Amplitude:bedrockisbetterthanloosesoil

20. Folding
During mountain building processes rocks can undergo folding as well as faulting.
Sometimes rocks deform ductilely, particularly if they are subjected to heat and pressure. At
elevated temperature and pressure within the crust, folds can form from compressional forces.
Entire mountain rages, like the Appalachians, have extensive fold systems.
The conditions of whether a rock faults or folds vary with temperature, pressure, rock
composition, and strain rate. In the same location, some rocks can fold while others fault. Often
folding is just a precursor to faulting.
Z-fold in schist with white felsic dike (hammer for
scale). Near Lake Murray, S.C.Photo courtesy of K.
McCarney-Castle
Large fold in outcrop (geologists for scale). Near
Oakridge, Tennessee, Appalachian Mtns.Photo courtesy
of K. McCarney-Castle.

21. Measuring earthquakes
•Seismometers:
instruments that detect
seismic waves
•Seismographs
Record intensity, height
and amplitude of seismic
waves

22. Seismographs
•Seismic waves
are recorded by
seismographs –
now mostly digital
recordings
•Record both
horizontal and
vertical earth
movements

23. Measuring Earthquakes
•Themovementofmaterialsintheoutercore(whichisaliquid)ofthe
EarthisinferredtobethecauseofEarth’smagneticfield.
•AcompassneedlewillalignwiththelinesofforceofEarth’smagnetic
field.IronandNickelaremetalsthateasilymagnetize,andare
inferredtobethemetalsinEarth’score.
•Theenergyspreadsoutwardinalldirectionsasvibrationscalled
SeismicWaves.Seismicwavescanbemeasuredandrecordedbya
seismograph.
•Seismographsareinstrumentsoradevicethatdetectsandrecords
seismicorearthquakewaves.Itmeasurestheverticalgroundmotion
andthehorizontalgroundmotions(N-S/E-W).Italsotraceswave
shapesontopaperandtranslateswavesintoanelectronicsignal.
•Thevibrationrecord,calledaseismogram,lookslikejaggedlineson
paper.Seismogramsaretracesofamplified,electronicallyrecorded
groundmotionmadebyseismographs.
•MeasuringthetimebetweenthearrivalofthePandSwaves
determinesthedistancebetweentherecordingseismographandthe
earthquakeepicenter.

24. Earthquake Waves
Measuring Earthquakes
Seismographsare instruments that
record earthquake waves.
Seismogramsaretraces of amplified,
electronically recorded ground motion
made by seismographs.

25. Seismograph

26. Types of Seismographs

27. Seismogram Printout

28. Seismic waves: properties
•Velocity: function of the physical properties of the
rock the wave is traveling through
–Velocity increaseswith rock density
–Velocity changeswhen passing from one material
to another (increases/decreases)
–Liquids: S-waves do not get transmitted through
liquid; P-waves slow down
•Why is this important?
–If we know the velocity of the wave, we can infer
the type of rock it traveled through-that’s how we map
the interior of the Earth!!!

29. Seismic waves are generated by the release of energy during an earthquake. They
travel through the earth like waves travel through water.
The location within the Earth where the rock actually breaks is called the focusof
the earthquake. Most foci are located within 65 km of the Earth’s surface; however,
some have been recorded at depths of 700 km. The location on the Earth’s surface
directly above the focus is called the epicenter.
The study of seismic waves and earthquakes is called seismology, which is a branch
of geophysics.
Seismic Waves
Two types of seismic waves are generated at the earthquake focus:
1.Body wavesspread outward from the focus in all directions.
2.Surface wavesspread outward from the epicenter to the Earth’s surface, similar to
ripples on a pond. These waves can move rock particles in a rolling motion that
very few structures can withstand. These waves move slower than body waves.

30. There are two types of Body Waves:
1.Primary Wave (P wave):
•Compressional wave (travels in the same direction the waves move).
•A type of seismic wave that compresses and expands the ground.
•Very fast (4-7 km/second)
•Can pass through a fluid (gas or liquid)
•Arrives at recording station first
Example: A slinky.
Seismic Waves, continued

31. 2.Secondary Wave (S wave):
•Transverse wave (travels perpendicular to the wave movement).
•A type of seismic wave that moves the ground up and down or side
to side.
•Slower moving (2-5 km/second)
•Caused by a shearing motion
•Cannot pass through a fluid (gas or liquid)
Example: Shaking a rope.
Seismic Waves, continued

33. Primary or
“P”Wave
Secondary
or “S”
Wave

34. SurfaceswavesareproducedwhenearthquakeenergyreachestheEarth’s
surface.Surfacewavesmovesrockparticlesinarollingandswaying
motion,sothattheearthmovesindifferentdirections.
Thesearetheslowestmovingwaves,butarethemostdestructivefor
structuresonearth.
Diagramrepresentingthedamagingback-
and-forthmotionofasurfacewave.
(Credit:McGrawHill/Glencoe,1
st
ed.,pg.163)
Seismic Waves, continued

35. Types of Seismic Waves
•Seismographs detect three main wave types:
–P Waves(primary waves) the fastest of the two body types (~6-
8 km/s); they are the first to arrive
–S Waves(secondary or shear waves) body waves travel
typically half as fast as P-waves
–Surface Wavestravel at and near the air-earth interface, are
the slowest and last to arrive, travel at speeds lower than shear
waves.

36. Seismic wave forms
P wave
S wave
L wave
(Rayleigh wave)
L wave
(Love wave)

37. P waves(compressional) 6–8 km/s. Parallel to direction of movement
(slinky), also called primary waves. Similar to sound waves.

38. S waves(shear) 4–5 km/s. Perpendicular to direction of movement
(rope), also called secondary waves. Result from the shear strength of
materials. Do not pass through liquids.

39. Surface Waves
•Travel at and near the surface-air interface.
•Amplitude of motion decreases exponentially with depth. (scaled
by wavelength)
•Usually largest amplitude, longest period, and most destructive.

40. Propagation of Earthquake Waves

43. Locating Earthquakes
IftheP-andS-velocitiesare
known,thendifferencesin
traveltimesofPandSwaves
maybeusedtoestimate
distancefromthestationto
theepicenter.

44. The deepest well ever drilled was 12 kilometers in Russia. The Earth’s radius is 6,370 km, so
drilling can barely “scratch the surface.”
A branch of geology called seismology lets us obtain information about the interior of the Earth
without directly observing it. Seismologists apply principles and laws of physics to study the
internal structure of the earth.
Geophysics includes the study of seismic waves and the Earth’s magnetic and gravity fields and
heat flow. Because we cannot directly observe the Earth's interior, geophysical methods allow us to
investigate the interior of the Earth by making measurements at the surface. Without studying
these things, we would know nothing of the Earth’s internal structure.
Keeping in mind that seismic waves have similar properties to other types of waves (e.g. light
waves), the wave properties of reflection and refraction are useful for learning about the Earth’s
interior. Results of early studies showed that the internal structure of the Earth consists of distinct
layers:
Mapping the earth: Seismic waves
1.Seismic reflection:Seismic waves bounce (reflect) off rock boundaries of different rock type,
and their travel times are recorded on a seismogram. The seismogram records the time it took
for the waves to travel to the boundary, reflect off of it and return to the surface. Seismologists
can measure the time this takes and calculate the depth to the boundary.
Seismograph
Station
Layer B
Reflecting
Boundary
Layer A
Seismic waves reflect off of a rock
boundary in the earth and return to a
seismograph station on the surface.

45. 2. Seismic refraction:Waves change velocity and direction (refract) when they enter a medium of
different density than the one they just passed through.
Low-velocity layer: Seismic wave travels slow.
Example: Granite
High-velocity layer: Seismic wave travels fast.
Example: Gabbro
Research from seismic reflection and refraction has led to many important discoveries such as:
1.There are three main layers of the Earth: The crust, mantle, and core.
2.The continental crust is thicker than oceanic crust and seismic waves travel slower in the
continental crust meaning that they are made up of different kinds of rock (granite/basalt).
3.There is a distinct boundary between the crust and the mantle called the Mohorovicic
discontinuity, or, simply, the Moho. At this boundary, seismic waves are refracted.
4.There is a layer within the mantle up to 70 km thick beneath the ocean and up to 250 km thick
beneath the continents where waves travel slower than in more shallow layers. This layer is
called the low-velocity zone, and scientists have concluded that this zone is at least partially
liquid. In plate-tectonic theory, it is called the asthenosphere, which is the semi-molten region
of the earths’ interior just below the earth’s rigid crust that allows for tectonic plate movement.
5.P-waves can pass through the outer core but S-waves cannot. The outer core is a molten liquid.
Layer A
Layer B

46. In the early 1900’s scientists discovered that parts of the Earth’s surface did not receive direct
earthquake waves.
A cross-sectional figure of the layers of the earth with P-
and S-wave propagation.McGraw Hill/Glencoe, 1
st
ed., pg. 172
Scientists found that direct P-waves “disappear” from seismograms in a region between 104 and
140 degrees away from an epicenter. The seismic waves are bent, or refracted, upon encountering
the core-mantle boundary, casting a shadow called the P-wave shadow zone.
Direct S-waves are not recorded in the entire region more than 104 degrees away from an
epicenter, and this is referred to as the S-wave shadow zone. The S-wave shadow zone, together
with the knowledge that liquids do not transmit S-waves, is evidence that the outer core is liquid
(or behaves as liquid).
S-waves cannot travel through liquids because they are
shear waves, which attempt to change the shape of what
they pass through. Simply put, a liquid “doesn’t care”
what shape it’s in—for example, you can empty a bottle
of water into an empty box, and it will change shape with
the shape of container. Liquids cannot support shear
stresses, so shearing has no effect on them. Therefore, a
liquid will not propagate shears waves.

47. BothP-andS-wavesslowdownwhentheyreachtheasthenosphere.Becauseofthis,scientists
knowthattheasthenosphereispartiallyliquid.
Theasthenosphere(Greek:weak)istheuppermostpartofthemantle,whichispartially
molten.Lithospheric,ortectonic,platesareableto“slide”indifferentdirectionsontopofthe
asthenosphere.Becauseseismicwavestravelslowlyhere,itisalsocalledthe“lowvelocity”zone.
Theboundarybetweenthecrustandthemantleisdistinguishedbyjumpinseismicwave
velocity.ThisfeatureisknownastheMohorovicicDiscontinuity.
Seismic Waves: Mapping the earth
Changesinvelocity(km/s)ofP-and
S-wavesallowseismologiststoidentify
thelocationsofboundarieswithinthe
earthsuchastheMohorovicicboundary
neartheearth’ssurfaceandthe
boundarybetweenthemantleandouter
core.
Mohorivicic
Discontinuity
Modified after McGraw Hill/Glencoe, 1st ed., pg. 173
Low
Velocity
Zone
MOLTEN

48. Seismic Waves: Epicenter location
Although S-waves, P-waves and surface waves all start out at the same time, they travel at
different speeds. The speed of a traveling seismic wave can be used to determine the location of an
earthquake epicenter.
A seismograph records the arrival time and the magnitude of horizontal and vertical
movements caused by an earthquake. The arrival time between different seismic waves is used to
calculate the travel time and the distance from the epicenter.
The difference in arrival time between primary waves and secondary waves is used to calculate
the distance from the seismograph station to the epicenter.
It is crucial that seismic waves are recorded by three different seismograph stations in order to
estimate the location of the epicenter (see next slide.)
Thisexampleshowsseismicwaves
arrivingatdifferenttimesattwo
seismographstations.StationBis
fartherawayfromStationAsothe
wavestakelongertoreachStation
B.Primarywavesarrivefirst,
followedbysecondarywaves,and
thensurfacewaves.Credit:Modified
afterPlummer/McGeary,7
th
ed.,pg.
Station A
Station B
1st
2nd
3rd
1st
2nd
3rd
R1
R2

49. Earthquake scientists, or seismologists, can locate the epicenter of an earthquake as long as
the vibrations are felt at three different seismograph stations.
1. Locate at least 3 stations on a map that recorded the seismic waves
2. Calculate the time difference between arrival of P-waves and arrival of S-waves from a
seismogram. The time difference is proportional to the distance from the epicenter.
Because the direction to the epicenter is unknown, the distance defines a circle around the
receiving station. The radius of each circle equals that station’s distance from the
earthquake epicenter.
3. The epicenter is where the circles intersect.
Seismic Waves: Epicenter location
Epicenter location using three stations.
McGraw Hill/ Glencoe, 1
st
ed., pg 170
Earthquake nomenclature based on
proximity to the epicenter:
-Local event:Epicenter of felt
earthquake < 100km away
-Regional event:Epicenter of felt
earthquake is 100 to 1400 km away
-Teleseismic event:Epicenter of felt
earthquake > than 1400 km away

50. The Earth’s magnetic field is used along with seismic waves to help understand the interior of
the Earth. The geomagnetic field surrounds the Earth in a south to north direction and has been
studied for many years due to its importance in navigation.
The current hypothesis for the origin of geomagnetic field is that it is created by electrical
currents generated within the liquid outer core. The heat of the outer core (5000-6000°C) drives
convection of the molten material. The convection of a metallic liquid creates electric currents,
which, in turn, creates a magnetic field. This is a dynamo model, where mechanical energy of
convection generates an electrical energy and a magnetic field.
Magnetism has two important components: intensity and direction. Most rocks differ in their
magnetism, depending upon their content of iron-bearing minerals, i.e. magnetic susceptibility.
For example, a body of magnetite or gabbro is strongly magnetic, and their magnetic signature
makes them stand out from other rocks. Additionally, when rocks form, they acquire the direction
(polarity) of the magnetic field existing at that time.
Magnetic anomalies are deviations from the normal magnetic direction (i.e. today’s), or they are
exceptionally large differences (positive or negative) in magnetic intensity relative to average
values of surrounding rocks. Anomalies occur all over the earth and are sometimes indicative of
different rock types under the earth’s surface.
Magnetic intensity anomalies can be measured by magnetometers, which are often towed
behind ships or flown over land surfaces to aid in mapping deposits in the earth.
Mapping the earth: Magnetic anomalies

51. The Big Ones: Earthquakes from around the world
Charleston Earthquake: The Charleston Earthquake struck this eastern South Carolina city on August 31,
1886. East Coast earthquakes are felt over a much larger area than earthquakes occurring on the West Coast,
because the eastern half of the country is mainly composed of older rock that has not been fractured and
cracked by frequent earthquake activity in the recent geologic past. Rock that is highly fractured and
crushed absorbs more seismic energy than rock that is less fractured. The Charleston earthquake, with an
estimated magnitude of about 7.0, was felt as far away as Chicago, more than 1,300 km to the northwest,
whereas the 7.1-magnitude Loma Prieta earthquakes was felt no farther than Los Angeles, about 500 km
south. Approximately 60 people were killed as a result of this historic earthquake.
New Madrid Earthquake: Three quakes over 8.0 occurred from Dec 16, 1811 to February 7, 1812. This was an
unusual place for an earthquake, and seemed to represent a weak point in the North American tectonic plate.
Survivors reported that they saw the ground rolling in waves. Scientists estimate that the possibility of
another earthquake at the New Madrid fault zone in the next 50 years is higher than 90 percent. The most
widely felt earthquakes ever to strike the United States were centered near the town of New Madrid,
Missouri, in 1811 and 1812. Three earthquakes, felt as far away as Washington D.C., were each estimated to
be above 8.0 in magnitude.
Damage to Stanford University. Source USGS:
http://earthquake.usgs.gov/regional/nca/1906/18april/casualties.php
San Francisco Earthquake(April 18, 1906, 5:12am): Occurred
along the San Andreas Fault. The total population at the time
was 400,000, the death toll was 3,000 and 225,000 were left
homeless. 28,000 buildings were destroyed and damages were
estimated at $400 million from the earthquake plus fire, but only
$80 million from the earthquake alone. Horizontal displacement
was 15 feet with a visible scar 280 miles long. Fires caused by
broken gas lines raged for 3 days until buildings were destroyed
to create a firebreak. The vast percentage of damage was done
by fires.

52. TheLomaPrietaEarthquake(5:04pm
October17,1989)occurredalongtheSan
AndreasFaultZonewiththeepicenterlocated
intheSantaCruzMountains,70milessouth
oftheSanFrancisoearthquake.Itregistered
6.9ontheRichterScalewithafinaldeathtoll
of63peopleanddamagesof$6billion.
Resultingfireswerehardtomanagedueto
brokenwaterlines.Thetotaltremortime
lastedapproximately15seconds.
TheGoodFridayEarthquake(March27,1964at5:36pm):At9.2ontheRichterScale,thiswas
thelargestearthquaketooccurintheU.S.inrecordedhistory.Thedeathtollwasonly115dueto
thescarcityofpeopleinthisarea.Damageswereapproximately$300million.A30milex125mile
blockoflandwasraised40ft,andasimilarblockdropped3-6feet.Thetremor,whichlasted3
minutes,createdatsunamithatdrowned100peopleinAlaska,OregonandCalifornia.Landslides
destroyedpartsofAnchorage,90milesaway.
Lisbon,Portugal:November1,1755.9:40amonAllSoulsDay.Thequaketriggeredatsunami
withawave50-fthigh,whichcrashedthroughthecity.Buildingscollapsedofkillingmanypeople,
andwavessweptthousandsmoreaway.Firesranuncheckedfor3days,completingthe
destructionofthecapitol.Over60,000peoplediedinthecityaloneandthousandsmorein
surroundingareas.
Photo left: Cypress Freeway collapsed-42 people killed at this
section. Right: Bay bridge section collapse.
Source:http://www.olympus.net/personal/gofamily/quake/famous/pri
eta.html

53. Earthquake Magnitude:
Scales based on Seismograms
•M
L=local(e.g.Richterscale)-basedonamplitudeofwaveswith
1speriodwithin600kmofepicenter.
•M
b=body-wave(similartoabove)
•M
s=surfacewave(waveperiodsof20smeasuredanywhereon
globe
•M
o=seismicmoment
•M
w=momentmagnitude

54. The Richter scale
Steps:
1.Measuretheinterval(inseconds)betweenthearrivalofthefirstP
andSwaves.
2.MeasuretheamplitudeofthelargestSwaves.
3.Usenomogramtoestimatedistancefromearthquake(S-Pinterval)
andmagnitude(joinpointsonS-PintervalscaleandSamplitude
scale).
4.Useseismogramsfromatleastthreegeographiclocationstolocate
epicenterbytriangulation.

56. Determining the location of an earthquake
First, distance to earthquake is determined.
1. Seismographsrecord seismic waves
2. From seismograph record called the seismogram, measure time delay between P
& S wave arrival
3. Use travel time curve to determine distance to earthquake as function of P-S
time delay
Now we know distance waves traveled, but we don'tknow the direction from
which they came.
We must repeat the activity for each of at least three (3)stations to triangulate a
point (epicenter of quake).
Plot a circle around seismograph location; radius of circle is the distanceto the
quake.
Quake occurred somewhere along that circle.
Do the same thing for at least3 seismograph stations; circles intersect at epicenter.
Thus, point is triangulated and epicenter is located.

57. Locating an Earthquake

58. How is an Earthquake’s Epicenter Located?
Seismic wave behavior
–P waves arrive first, then S waves, then L and R
–Average speeds for all these waves is known
–After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance
from the seismograph to the epicenter.

59. How is an Earthquake’s Epicenter Located?
Time-distancegraphshowingthe
averagetraveltimesforP-andS-
waves.Thefartherawaya
seismographisfromthefocusof
anearthquake,thelongerthe
intervalbetweenthearrivalsofthe
P-andS-waves.

60. How is an Earthquake’s
Epicenter Located?
•Threeseismographstations
areneededtolocatethe
epicenterofanearthquake
•Acirclewheretheradius
equalsthedistancetothe
epicenterisdrawn
•Theintersectionofthe
circleslocatestheepicenter

62. •Withmultiplestations,
thelocationofthe
epicentercanbe
estimated.
•Focus(depth)canbe
inferredfromtravel
timesand‘depth’phases
duetofree-surface
reflections.
Determining Epicenter Location and
Focal Depth by Triangulation

63. Earthquake magnitude:
scales based on rupture dimensions (equivalent to
energy released )
•M
o= seismic moment.
= m* A * d, where mis the shear modulus
of rock; A is the rupture area, and d is
displacement
•M
w= moment magnitude.
= 2/3 * log M
o-10.7
N.B. moment scales do not saturate

64. Saturation of
non-moment
scales

65. How are the Size and Strength of an Earthquake Measured?
•Modified Mercalli Intensity Map
–1994 Northridge, CA earthquake,
magnitude 6.7
•Intensity
–subjectivemeasure
of the kind of
damage done and
people’s reactions
to it
–isoseismal lines
identify areas of
equal intensity

66. How are the Size and Strength of an Earthquake Measured?
•Magnitude
–Richter scale
measures total amount
of energyreleased by
an earthquake;
independent of
intensity
–Amplitude of the
largest wave produced
by an event is
corrected for distance
and assigned a value
on an open-ended
logarithmic scale

67. Earthquakes can be very destructive at the Earth’s surface. The magnitude of an earthquake is a
measure of how destructive it is. Basically the magnitude corresponds to how much energy is
released.
The Richter Scale is used to express earthquake magnitude on the basis of the height (amplitude)
of the largest line (seismic wave, P or S) on a seismogram. The Richter scale was originally
developed for earthquakes in Southern California. The utility of this scale was its ability to account
for decreased wave amplitude with increased distance from the epicenter. Richter’s scale is also a
logarithmic scale.
Today, a standard magnitude scale is used, Seismic Moment, which more accurately represents
the energy released in an earthquake, especially large magnitude events.
The majority of earthquakes are minor and have magnitudes of 3-4.9 on the Richter scale. These
can be felt, but cause little or no damage, and there are about 55,000 of these earthquakes each year.
Thousands of earthquakes are recorded every day with magnitudes < 3.0 but are almost never
felt.
The Mercalli scale is different from the Richter scale because it measures the intensity of how
people and structures are affected by the seismic event. In essence, it measures damage. It is much
more subjective and uses numbers ranging from 1 (no damage) to 12 (total destruction).
Earthquake classification scales

68. Earthquake Magnitude
•Earthquake strengths range from imperceptible to
catastrophic. Several scales are used:
–Richter Magnitude–a logarithmic measure of how much the ground
moved at the seismograph as seismic waves pass by
–Moment magnitude–a logarithmic measure proportional to total
area of fault rupture and seismic energy released
–Modified Mercalli scale–Not a magnitude, but a “measure” of the
perception of the earthquake –what people felt, and how much
damage there was, useful for historical events preceding the invention
of the seismograph. This scale is useful for studying historic
earthquakes that occurred prior to modern seismographs.

69. Earthquake size: two ways to
measure
1)Magnitude:Richter Scale
•Measures the energy released by fault
movement
•related to the maximum amplitude of the S
wave measured from the seismogram
•Logarithmic-scale; quantitativemeasure
•For each whole number there is a 31.5 times
increase in energy
•eg. an increase from 5 to 7 on the Richter scale =
an increase in energy of 992 times!!

71. Richter
Magnitudes
Description Earthquake Effects
Frequency of
Occurrence
Less than 2.0Micro Micro-earthquakes,notfelt. About 8,000 per day
2.0-2.9 Minor Generallynotfelt,butrecorded. About 1,000 per day
3.0-3.9 Minor Oftenfelt,butrarelycausesdamage. 49,000 per year (est.)
4.0-4.9 Light
Noticeableshakingofindooritems,rattlingnoises.
Significantdamageunlikely.
6,200 per year (est.)
5.0-5.9 Moderate
Cancausemajordamagetopoorlyconstructed
buildingsoversmallregions.Atmostslightdamage
towell-designedbuildings.
800 per year
6.0-6.9 Strong
Canbedestructiveinareasuptoabout100miles
acrossinpopulatedareas.
120 per year
7.0-7.9 Major Cancauseseriousdamageoverlargerareas. 18 per year
8.0-8.9 Great
Cancauseseriousdamageinareasseveralhundred
milesacross.
1 per year
9.0-9.9 Great Devastatinginareasseveralthousandmilesacross.1 per 20 years
10.0+ Great
Neverrecorded;seebelowforequivalentseismic
energyyield.
Extremely rare
(Unknown)
The Richter Scale
Complications of the Richter scale include:
1.The Richter scale originally only applied to shallow-focus earthquakes in southern
California so now must be modified.
2.Magnitudes calculated from seismograms above 7 tend to be inaccurate.

72. 2) Intensity:Mercalli Scale:
–Assigns an intensity or rating to measure an earthquake at a
particular location (qualitative)
–I (not felt) to XII (buildings nearly destroyed)
–Measures the destructive effect
•Intensity is a function of:
•Energy released by fault
•Geology of the location
•Surface substrate: can magnify shock waves e.g.
Mexico City (1985) and San Francisco (1989)

73. I.Not felt except by a very few under especially favorable conditions.
II.Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended
objects may swing.
III.Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do
not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the
passing of a truck. Duration estimated.
IV.Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes,
windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking
building. Standing motor cars rocked noticeably.
V.Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects
overturned. Pendulum clocks may stop.
VI.Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster.
Damage slight.
VII.Damage negligible in buildings of good design and construction; slight to moderate in well-built
ordinary structures; considerable damage in poorly built or badly designed structures; some
chimneys broken.
VIII.Damage slight in specially designed structures; considerable damage in ordinary substantial
buildings with partial collapse. Damage great in poorly built structures. Fall of chmineys, factory
stacks, columns, monuments, walls. Heavy furniture overturned.
IX.Damage considerable in specially designed structures; well-designed frame structures thrown out
of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off
foundations.
X.Some well-built wooden structures destroyed; most masonry and frame structures destroyed with
foundations. Rail bent.
XI.Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
XII.Damage total. Lines of sight and level are distorted. Objects thrown into the air.
The Mercalli scale of earthquake intensityCredit: USGS

74. Frequency of Occurrence of Earthquakes
Descriptor Magnitude Average Annually
Great 8 and higher 1 ¹
Major 7 -7.9 17 ²
Strong 6 -6.9 134 ²
Moderate 5 -5.9 1319 ²
Light 4 -4.9
13,000
(estimated)
Minor 3 -3.9
130,000
(estimated)
Very Minor 2 -2.9
1,300,000
(estimated)
¹ Based on observations since 1900.
² Based on observations since 1990.

75. Large earthquakes occur much less frequently
than smaller ones -longer recurrence interval

76. Seismic hazards
1.Locating faults
2.Estimating recurrence: history and
geology
3.Measuring relative motions and crustal
deformation
4.Learning from analogies
5.Assessing probabilities

77. 1. Locating faults:
Seattle Fault (LIDAR image)

78. The Hayward
fault runs
through UC
Berkeley
campus
(US $1 billion
seismic upgrade
program)
Lawrence
Livermore
UC Berkeley

79. San Francisco
City Hall, 1906
2. Recurrence -historical records

81. 3. Prediction:
current crustal
deformation

82. 3. Prediction: crustal velocity (mm/yr)
from repeated GPS measurements at permanent stations
Why are all stations
moving to NW?

83. 4. Learning from analogues
(Turkey -California)

84. The Bay Area:
earthquake
probabilities
(AD2000-2030)
N.B. A probability of
70% over 30 years is
equivalent to a daily
probability of
1 : 15 000
5. Assessing probabilities

85. Probabilities, yes!
but prediction, no!
•1996 -Earthquake prediction group of Japanese
Seismological Survey voluntarily disbands (after Kobe)
•2000 -British researcher argues that prediction of main
shock impossible at present; immediate goal should be
prediction of aftershock location and magnitude

86. Individual seismic hazards
•Shaking = accelerated ground motion
•Liquefaction = failure of waterlogged sandy
substrates
•Landslides, dam failures, etc.
•Tsunamis = seismic sea waves
•Fire, etc.

87. Predictions of shaking intensity on
San Andreas fault (long segment) in the Bay Area

88. Ground motion, structural damage and basin
morphology: Mexico City, 1985
periodic periodicrandom
body\surface surface/body
Damage
heavy light heavy
ridge
basin basin

89. Cascadia: megaearthquakes at the
plate boundary
M
w = 9.2?

90. Parkfield, CA
“Earthquake Capital of the World”
Earthquake Hazard Potential Map

91. What causes
earthquakes?
Elastic Rebound
Sources of stress
Plate Tectonics
Other stresses

92. Seismic (Earthquake) Waves
•Travel outward from focus
•Focus: site of initial rupture
•Epicenter: point on surface above the focus

93. Elastic Rebound and Fault Rupture
•Forces applied, often due to
plate tectonics
•Rocks accumulate strain
(deformation)
•Elastic rebound when fault
ruptures
•Seismic waves radiated
•Slip along fault apparent if
near the surface
•Frequency of earthquakes
depends both on earthquake
size and rate of strain
accumulation

94. What Happens During an Earthquake?

95. What Happens During an Earthquake

96. What Happens During an Earthquake

97. What Happens During an Earthquake

99. Damage: Causes
Ground motion
Duration of Shaking
Surface Rupture
Poor building design

100. Effects
Rupture
Death
Bldg collapse

101. Effects
Fires
Liquifaction
Landslides

102. Damage:
Key Factors
Amount & duration of shaking
Water content of soil
Population concentration
Building construction
Distance from Epicenter
Depth of focus
Direction of rupture
Material amplification

103. Tsunamis
•Tsunamisarewavesgeneratedbyfaultmotionorslumpsontheseafloor.
•Anunderwaterearthquakewithamagnitudeof8orhigherontheRichter
scalecanaffecttheearth’soceansbycausingatsunami.Lesscommonly,
tsunamisarealsocausedbysubmarinelandslidesorvolcanicexplosions.
•Tsunamisaresometimescalledseismicseawaves.Theycandestroy
everythinginthecoastalzone,buttheyoccurassmallunnoticeablewaves
outatsea.
•Tsunamiscantravelatspeedsupto800km/hrandformwavesover20m
highastheybreak.

104. Theearthquakegeneratesarollingwaveoutintheopenwater;
however,asthewavesapproachshore,theystartto“feel”thebottom
oftheseafloor.Thewavesslowdownnearthebottom,causinga
hugewavetobuildupontopasthetopisstillmovingatitsoriginal
speed.Thisisthesameasthewayregularwavesforminthesurf
zone.Thetsunami,however,canbeaone-hundredfoothighwallof
watermovingat450milesperhour.
Sketch showing the generation of a Tsunami
from an underwater earthquake.
Credit: McGraw Hill/Glencoe, 1
st
ed., pg. 178
Earthquakes: Tsunamis
Dislocation of seafloor

106. Can Earthquakes be Predicted?
Earthquake Precursors
–changes in elevation or tilting of land surface,
fluctuations in groundwater levels, magnetic field,
electrical resistance of the ground
–seismic gaps

107. Can Earthquakes be Predicted?
Earthquake Prediction Programs
–include laboratory and field studies of rocks before, during,
and after earthquakes
–monitor activity along major faults
–produce risk assessments

108. Earthquake Prediction -Still Science Fiction
•Large earthquakes do tend to follow a cycle of rupture, followed by declining
aftershocks and a period of quiescence
•During the quiet period strain is building toward another rupture
•Recurrence intervals vary from 10’s of years to 100’s
•Great interest in seismicity prior to written history
•-Helps us predict where, but whenis another matter

109. Earthquake risk and prediction
•Long-term methods
1)seismic hazard maps
2) probability analysis based
on:
-historical EQ records
-geologic EQ records
-slip-rate on active faults
-frequency and magnitude
of recent EQ's
Real-time 24 Hour
Forecast

110. Short-term predictions
Precursor phenomena (<1 year to days)
1. Foreshocks:usually increase in magnitude
2. Ground deformation
3. Fluctuations in water well levels
4. Changes in local radio wave characteristics
5. Anomalous animal behavior???

111. Impacts of Earthquake Prediction

118. …And that was
just a 7.2 on
the Richter
scale!