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January 29, 2008 1 Earthquake Engineering GE / CEE - 479/679 Topic 3. Faulting and Paleoseismology 1 January 29, 2008 John G. Anderson Professor of Geophysics

John G. Anderson Professor of Geophysics

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Earthquake Engineering GE / CEE - 479/679 Topic 3. Faulting and Paleoseismology 1 January 29, 2008. John G. Anderson Professor of Geophysics. Brittle behavior Lower temperatures Lower pressures Lithified materials High strain rates. Ductile behavior Higher temperatures Higher pressures - PowerPoint PPT Presentation

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Page 1: John G. Anderson Professor of Geophysics

January 29, 2008 1

Earthquake EngineeringGE / CEE - 479/679

Topic 3. Faulting and Paleoseismology 1January 29, 2008

John G. Anderson

Professor of Geophysics

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Factors favoring

• Brittle behavior– Lower temperatures– Lower pressures– Lithified materials– High strain rates

• Ductile behavior– 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 faultAnticlinal fold forms above a fault

Examples:1983 Coalinga, California earthquake1987 Whittier, California earthquake1994 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=7.3Modified 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=7.3Modified 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

• Whereμ = 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

Length of surface rupture, L = 33 km

Average surface displacementD = 100 cm

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W

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W Z = 16 km

W=Z/sin(dip) = 22.6 km

dip = 45o

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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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Estimation of slip rate (SR)

SR=D/RI

If we observe the second earthquake, the challenge is to date the first one.

After the first earthquake

Tens to thousands of years later (RI)

After the second earthquake

Ready for the second earthquake

D

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It may be that this is all that we observe:

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.

Time before present

Slip

?

Slope of this line gives the slip rate

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Estimation of slip rate (SR)SR=D/RI

Recurrence Interval (RI): Time between earthquakes.

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.

Time before present

Slip

?

Slope of this line gives the slip rate

Slip per Event (D): Slip in an individual earthquake.

D

RI

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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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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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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 )

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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 )

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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 )

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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.