Earthquake Engineering
GE / CEE - 479/679
Topic 3. Faulting and Paleoseismology 1
January 29, 2008
John G. Anderson
Professor of Geophysics
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Factors favoring
• Brittle behavior
–
–
–
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Lower temperatures
Lower pressures
Lithified materials
High strain rates
• Ductile behavior
–
–
–
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Higher temperatures
Higher pressures
Loose materials
Low strain rates
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.
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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
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Strike slip faulting can be challenging to recognize because the slip is
horizontal, and the dominant markers also are primarily horizontal.
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Blind thrust fault
Anticlinal fold forms above a fault
Examples:
1983 Coalinga, California earthquake
1987 Whittier, California earthquake
1994 Northridge, California earthquake (?)
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Faulting in a Recent Earthquake
• Normal faulting - “typical” Basin and
Range
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Borah Peak
Earthquake
Oct 28, 1983
Ms=7.3
Modified Mercalli
Intensity Map
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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.
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• 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.
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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.
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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.
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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.
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Borah Peak
Earthquake
Oct 28, 1983
Ms=7.3
Modified Mercalli
Intensity Map
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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.
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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.
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Photo courtesy of Craig dePolo
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Photo courtesy of Craig dePolo
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Photo courtesy of Craig dePolo
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Photo courtesy of Craig dePolo
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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.
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Displacements during the 1983 Borah Peak, Idaho, earthquake
surface rupture (frome Crone and others, 1987)
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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.
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Wells and Coppersmith, 1994
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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)
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Focal Mechanisms show that
aftershocks had a
predominantly normal
faulting mechanism.
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Seismic Moment
• Definition:
M0=μAD
• Where
μ = shear modulus (often G in engineering)
A = fault area
D = average slip on the fault
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Example, determine the seismic moment of the 1983 Borah Peak,
Idaho, earthquake from the surface rupture
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Example, determine the seismic moment of the 1983 Borah Peak,
Idaho, earthquake from the surface rupture
Length of surface rupture, L = 33 km
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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
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W
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dip = 45o
W
Z = 16 km
W=Z/sin(dip)
= 22.6 km
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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
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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
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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.
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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
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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.
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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.
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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
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Wells and Coppersmith, 1994
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Magnitude prediction equations
• Best known: Wells and Coppersmith
(1994).
– Distributed to the class.
– Recommendation: read the article, and learn
how to use these relations.
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Wells and Coppersmith, 1994
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Wells and Coppersmith, 1994
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Wells and Coppersmith, 1994
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Wells and Coppersmith, 1994
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Wells and Coppersmith, 1994
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Scatter in Wells & Coppersmith (1994)
• One of the sources of scatter in the Wells &
Coppersmith relations comes from fault
physics.
• The observation is that, when normalized
for rupture length, faults with slower slip
rates (longer recurrence intervals) tend to
have larger earthquakes.
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Kanamori and Allen
(1982) suggested that
faults with longer repeat
times generally have
more slip, when
normalized to a
common rupture length.
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Noting the inverse
relationship between
repeat time and slip
rate, Anderson et al
(1998) proposed a
relationship between
rupture length, slip
rate, and moment
magnitude.
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Slip rate & repeat time
• Next question – how often do these
earthquakes happen?
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After the first earthquake
Estimation of
slip rate (SR)
SR=D/RI
Tens to thousands of years later (RI)
Ready for the
second earthquake
D
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After the second
earthquake
If we observe
the second
earthquake,
the challenge
is to date the
first one.
It may be that this is all
that we observe:
Slip
Slope of this
line gives the
slip rate
?
In this case, we need to seek older, more
subtle features (like the yellow line).
We can estimate the slip rate if we can
determine the times of the yellow and the
green earthquakes.
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Time before present
Estimation of
slip rate (SR)
SR=D/RI
Recurrence Interval (RI): Time between
earthquakes.
Slip per Event (D): Slip in an individual
earthquake.
The slip rate cannot be determined from
the slip and time of only the most recent
earthquake, since only an incomplete
cycle has taken place.
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Slip
Slope of this
line gives the
slip rate
?
D
RI
Time before present
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Based on Wells and
Coppersmith (1994):
logAD=-4.8+0.69M
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Appendix. Complete Definitions
of MMI scales.
• This starts with a listing of the MMI scale
as originally defined.
• It follows with a relatively modern
reinterpretation of the upper end of the
scale.
• This also includes two commonly-used
abbreviations of the MMI scales.
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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.
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Modified Mercalli Intensity Scale of 1931
• I. Not felt -- or, except under especially favorable
circumstances.
• Under certain conditions, at and outside the boundary of
the area in which a great shock is felt:
sometimes birds, animals, reported uneasy and disturbed;
sometimes dizziness or nausea experienced;
sometimes trees, structures, liquids, bodies of water, may
sway; doors may swing, very slowly.
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Modified Mercalli Intensity Scale of 1931
• II. Felt indoors by few, especially on upper floors, or by
sensitive or nervous persons.
• Also, as in grade I, but often more noticeably:
sometimes hanging objects may swing, especially when
delicately suspended;
sometimes trees, structures, liquids, bodies of water, may
sway, doors may swing, very slowly;
sometimes birds, animals, reported uneasy and disturbed;
sometimes dizziness or nausea experienced.
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Modified Mercalli Intensity Scale of 1931
• III. Felt indoors by several, motion usually rapid
vibration.
• Sometimes not recognized to be an earthquake at first.
Duration estimated in some cases.
Vibration like that due to the passing of light or lightly
loaded trucks or heavy trucks some distance away.
Hanging objects may swing slightly.
Movements may be appreciable on upper levels of tall
structures.
Rocked standing motor cars slightly.
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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.
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Modified Mercalli Intensity Scale of 1931
• V. Felt indoors by practically all, outdoors by many or most:
outdoors direction estimated.
• Awakened many, or most.
Frightened few -- slight excitement, a few ran outdoors.
Buildings trembled throughout.
Broke dishes, glassware, to some extent.
Cracked windows -- in some cases, but not generally.
Overturned vases, small or unstable objects, in many instances, with
occasional fall.
Hanging objects, doors, swing generally or considerably.
Knocked pictures against walls, or swung them out of place.
Opened, or closed, doors, shutters, abruptly. Pendulum clocks
stopped, started, or ran fast, or slow.
Moved small objects, furnishings, the latter to slight extent.
Spilled liquids in small amounts from well-filled open containers.
Trees, bushes, shaken slightly.
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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.
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Modified Mercalli Intensity Scale of 1931
•
•
VII. Frightened all -- general alarm, all ran outdoors.
Some,or many, found it difficult to stand.
Noticed by persons driving motor cars.
Trees and bushes shaken moderately to strongly.
Waves on ponds, lakes, and running water.
Water turbid from mud stirred up.
Incaving to some extent of sand or gravel stream banks.
Rang large church bells, etc.
Suspended objects made to quiver.
Damage negligible in buildings of good design and construction, slight to
moderate in well-built ordinary buildings, considerable in poorly built or
badly designed buildings, adobe houses, old walls (especially where laid up
without mortar), spires, etc.
Cracked chimneys to considerable extent, walls to some extent.
Fall of plaster in considerable to large amount, also some stucco.
Broke numerous windows, furniture to some extent.
Shook down loosened brickwork and tiles.
Broke weak chimneys at the roof-line (sometimes damaging roofs).
Fall of cornices from towers and high buildings.
Dislodged bricks and stones.
Overturned heavy furniture, with damage from breaking.
Damage considerable to concrete irrigation ditches.
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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.
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Modified Mercalli Intensity Scale of 1931
• IX. Panic general.
• Cracked ground conspicuously.
Damage considerable in (masonry) structures built
especially to withstand earthquakes:
threw out of plumb some wood-frame houses built
especially to withstand earthquakes;
great in substantial (masonry) buildings, some collapse in
large part; or wholly shifted frame buildings off
foundations, racked frames;
serious to reservoirs; underground pipes sometimes
broken.
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Modified Mercalli Intensity Scale of 1931
•
•
X. Cracked ground, especially when loose and wet, up to widths of several
inches; fissures up to a yard in width ran parallel to canal and stream banks.
Landslides considerable from river banks and steep coasts.
Shifted sand and mud horizontally on beaches and flat land.
Changed level of water in wells.
Threw water on banks of canals, lakes, rivers, etc.
Damage serious to dams, dikes, embankments.
Severe to well-built wooden structures and bridges, some destroyed.
Developed dangerous cracks in excellent brick walls.
Destroyed most masonry and frame structures, also their foundations.
Bent railroad rails slightly.
Tore apart, or crushed endwise, pipe lines buried in earth.
Open cracks and broad wavy folds in cement pavements and asphalt road
surfaces.
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Modified Mercalli Intensity Scale of 1931
•
•
XI. Disturbances in ground many and widespread, varying with ground
material.
Broad fissures, earth slumps, and land slips in soft, wet ground.
Ejected water in large amount charged with sand and mud.
Caused sea-waves ("tidal" waves) of significant magnitude.
Damage severe to wood-frame structures, especially near shock centers.
Great to dams, dikes, embankments, often for long distances.
Few, if any (masonry), structures remained standing.
Destroyed large well-built bridges by the wrecking of supporting piers, or
pillars.
Affected yielding wooden bridges less.
Bent railroad rails greatly, and thrust them endwise.
Put pipe lines buried in earth completely out of service.
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Modified Mercalli Intensity Scale of 1931
•
•
XII. Damage total -- practically all works of construction damaged greatly or
destroyed.
Disturbances in ground great and varied, numerous shearing cracks.
Landslides, falls of rock of significant character, slumping of river banks, etc.,
numerous and extensive.
Wrenched loose, tore off, large rock masses.
Fault slips in firm rock, with notable horizontal and vertical offset
displacements.
Water channels, surface and underground, disturbed and modified greatly.
Dammed lakes, produced waterfalls, deflected rivers, etc.
Waves seen on ground surfaces (actually seen, probably, in some cases).
Distorted lines of sight and level.
Threw objects upward into the air.
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Modified Mercalli Intensity Scale (Dewey et al, 1994)
• VI. Worst effects include some windows broken out; a
few instances of fallen plaster or damaged old masonry
chimneys on single-family houses; large cracks in
interior walls; many small objects overturned and
fallen; many items thrown from store shelves; many
glassware items or dishes broken; light furniture
overturned and moderately heavy furniture displaced.
Effects on people not used to define intensities of VI or
above.
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Modified Mercalli Intensity Scale (Dewey et al, 1994)
• VII. Worst effects include significant damage to
unreinforced masonry buildings, including cracks in
bearing walls and 'out-of-plane' movement or fall of
upper walls and parapets; many old masonry chimneys
fallen or broken at the roofline on single-family homes;
some masonry fences fallen or destroyed; heavy
furniture overturned.
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Modified Mercalli Intensity Scale (Dewey et al, 1994)
• VIII. Worst effects include considerable damage to old,
unreinforced masonry buildings, with partial collapse; many cases
where wood-frame houses are moved on their foundation if not
anchored and braced; damage to wood-frame apartment buildings
having open first-stories, with some cases of apartments being
destroyed; significant damage to reinforced, lined, masonry
chimneys on single-family homes, and widespread damage to old
masonry chimneys; structural damage to some reinforced-concrete
structures built when a seismic code was in effect; very heavy
furniture moved conspicuously or overturned.
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Modified Mercalli Intensity Scale (Dewey et al, 1994)
• IX. Worst effects include multiple cases of structural
damage to reinforced-concrete buildings and parking
structures built when a seismic code was in effect, with
some cases of partial or complete collapse; collapse of
elevated freeway sections; widespread damage to
unreinforced masonry buildings (e.g., old brick
buildings), with total collapse; widespread incidence of
wood-frame houses shifted off foundations where not
securely anchored and braced; widespread destruction
of wood-frame apartment buildings having large open
areas in their first stories; widespread collapse of
masonry (brick, block or stone) chimneys, whether
reinforced or not, on single-family homes; furniture
and building contents generally overturned and thrown
across room.
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Modified Mercalli Intensity Scale (Dewey et al, 1994)
• X-XII. As originally defined and as modified in 1931, these
intensity levels described earthquake effects that involve
permanent changes in the shape of the ground (fault rupture,
landsliding, liquefaction, etc.). Nowadays, however, Intensities X,
XI and XII are increasingly regarded as approximately the same
level of shaking as Intensity IX. The many phenomena originally
associated with intensities X and above are apparently related less
to the level of ground shaking than to the presence of ground
conditions susceptible to spectacular failure, or to the ease with
which seismic faulting of different style and depth can propagate
to the ground surface.
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Comment on estimating MMI
• Recommended procedure: identify a phenomenon
of the shaking, compare with this description, and
select the smallest intensity consistent with the
phenomenon.
• For example, suppose a report says “rare pieces of
art fell off the mantle”. This phenomenon is
absent at MMI=IV, but present for MMI=V, so the
observation constrains MMI to be at least V.
• Be sure to consider also what apparently did not
happen, as a check on the temptation to assign an
intensity that is too high.
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Human response
to shaking, from
Trifunac and
Brady (1975)
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Modified Mercalli Intensity Scale (Richter, 1956)
• I. Not felt. Marginal and long-period effects of large earthquakes.
• II. Felt by persons at rest, on upper floors, or favorably placed.
• III. Felt indoors. Hanging objects swing. Vibration like passing of
light trucks. Duration estimated. May not be recognized as an
earthquake.
• IV. Hanging objects swing. Vibration like passing of heavy trucks;
or sensation of a jolt like a ball striking the walls. Standing motor
cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery
clashes. In the upper range of IV wooden walls and frames creak.
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Modified Mercalli Intensity Scale (Richter, 1956)
• V. Felt outdoors; direction estimated. Sleepers wakened. Liquids
disturbed, some spilled. Small unstable objects displaced or upset.
Doors swing, close, open. Shutters, pictures move. Pendulum
clocks stop, start, change rate.
• VI. Felt by all. Many frightened and run outdoors. Persons walk
unsteadily. Windows, dishes, glassware broken, knickknacks,
books, etc., off shelves. Pictures off walls. Furniture moved or
overturned. Weak plaster and masonry D cracked. Small bells ring
(church, school). Trees, bushes shaken (visible, or heard to rustle).
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Modified Mercalli Intensity Scale (Richter, 1956)
• VII. Difficult to stand. Noticed by drivers of motor cars.
Hanging objects quiver. Furniture broken. Damage to
masonry D, including cracks. Weak chimneys broken
at roof line. Fall of plaster, loose bricks, stones, tiles,
cornices (also unbraced parapets and architectural
ornaments). Some cracks in masonry C. Waves on
ponds; water turbid with mud. Small slides and caving
in along sand or gravel banks. Large bells ring.
Concrete irrigation ditches damaged.
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Modified Mercalli Intensity Scale (Richter, 1956)
• VIII. Steering of motor cars affected. Damage to
masonry C; partial collapse. Some damage to masonry
B; none to masonry A. Fall of stucco and some masonry
walls. Twisting, fall of chimneys, factory stacks,
monuments, towers, elevated tanks. Frame houses
moved on foundations if not bolted down; loose panel
walls thrown out. Decayed piling broken off. Branches
broken from trees. Changes in flow or temperature of
springs and wells. Cracks in wet ground and on steep
slopes.
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Modified Mercalli Intensity Scale (Richter, 1956)
•
•
•
•
IX. General panic. Masonry D destroyed; masonry B seriously damaged.
(General damage to foundations.) Frame structures, if not bolted, shifted
off foundations. Frames racked. Serious damage to reservoirs.
Underground pipes broken. Conspicuous cracks in ground. In alluviated
areas sand and mud ejected, earthquake fountains, sand craters.
X. Most masonry and frame structures destroyed with their foundations.
Some well-built wooden structures and bridges destroyed. Serious damage
to dams, dikes, embankments. Large landslides. Water thrown on banks
to canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches
and flat land. Rails bent slightly.
XI. Rails bent greatly. Underground pipelines completely out of service.
XII. Damage nearly total. Large rock masses displaced. Lines of sight and
level distorted. Objects thrown into the air.
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Modified Mercalli Intensity Scale (Richter, 1956)
•
•
•
•
•
Note: To avoid ambiguity, the quality of masonry, brick, or other material is
specified by the following lettering system. (This has no connection with the
conventional classes A, B, and C construction.):
Masonry A. Good workmanship, mortar, and design; reinforced, especially
laterally, and bound together by using steel, concrete, etc.; designed to resist
lateral forces.
Masonry B. Good workmanship and mortar; reinforced, but not designed to
resist lateral forces.
Masonry C. Ordinary workmanship and mortar; no extreme weaknesses, like
failing to tie in at corners, but neither reinforced nor designed to resist
horizontal forces.
Masonry D. Weak materials, such as adobe; poor mortar; low standards of
workmanship; weak horizontally.
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93
Modified Mercalli Intensity Scale (Bolt, 1993)
• I. Not felt except by a very few under especially favorable
circumstances.
(I Rossi-Forel scale)
• II. Felt only by a few persons at rest, especially on upper floors of
buildings. Delicately suspended objects may swing.
(I to II Rossi-Forel scale)
• III. Felt quite noticeably indoors, especially on upper floors of
buildings, but many people do not recognize it as an earthquake.
Standing automobiles may rock slightly. Vibration like passing of
truck. Duration estimated.
(III Rossi-Forel scale)
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94
Modified Mercalli Intensity Scale (Bolt, 1993)
• IV. During the day felt indoors by many, outdoors by few. At night
some awakened. Dishes, windows, doors disturbed; walls make
creaking sound. Sensation like heavy truck striking building. Standing
automobiles rocked noticeably.
(IV to V Rossi-Forel scale) (1-2 cm/s) (0.015g-0.02g)
• V. Felt by nearly everyone, many awakened. Some dishes, windows,
and so on broken; cracked plaster in a few places; unstable objects
overturned. Disturbances of trees, poles, and other tall objects
sometimes noticed. Pendulum clocks may stop.
(V to VI Rossi-Forel scale) (2-5 cm/s) (0.03g-0.04g )
January 29, 2008
95
Modified Mercalli Intensity Scale (Bolt, 1993)
• VI. Felt by all, many frightened and run outdoors. Some heavy
furniture moved; a few instances of fallen plaster and damaged
chimneys. Damage slight.
(VI to VII Rossi-Forel scale) (5-8 cm/s) (0.06g-0.07g )
• VII. Everybody runs outdoors. Damage negligible in buildings of good
design and construction; slight to moderate in well-built ordinary
structures; considerable in poorly built or badly designed structures;
some chimneys broken. Noticed by persons driving cars.
(VIII Rossi-Forel scale) (8-12 cm/s) (0.10g-0.15g )
January 29, 2008
96
Modified Mercalli Intensity Scale (Bolt, 1993)
• VIII. Damage slight in specially designed structures;
considerable in ordinary substantial buildings with partial
collapse; great in poorly built structures. Panel walls thrown
out of frame structures. Fall of chimneys, factory stack,
columns, monuments, walls. Heavy furniture overturned.
Sand and mud ejected in small amounts. Changes in well
water. Persons driving cars disturbed.
(VIII + to IX Rossi-Forel scale) (20-30 cm/s) (0.25g-0.30g )
• IX. Damage considerable in specially designed structures;
well-designed frame structures thrown out of plumb; great in
substantial buildings, with partial collapse. Buildings shifted
off foundations. Ground cracked conspicuously. Underground
pipes broken.
(IX + Rossi-Forel scale) (45-55 cm/s) (0.50g-0.55g )
January 29, 2008
97
Modified Mercalli Intensity Scale (Bolt, 1993)
• X. Some well-built wooden structures destroyed; most
masonry and frame structures destroyed with foundations;
ground badly cracked. Rails bent. Landslides considerable
from river banks and steep slopes. Shifted sand and mud.
Water splashed, slopped over banks.
(X Rossi-Forel scale) (More than 60 cm/s) (More than
0.60g )
• XI. Few, if any, (masonry) structures remain standing.
Bridges destroyed. Broad fissures in ground. Underground
pipelines completely out of service. Earth slumps and land
slips in soft ground. Rails bent greatly.
• XII. Damage total. Waves seen on ground surface. Lines
of sight and level distorted. Objects thrown into the air.
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