Notes 1 Seimology Civl7119

CIVL7119/8119 -Earthquake Engineering

Shahram PezeshkThe University of Memphis

Define Seismic Environment•Regional Seismcity•Seismic Hazard•Site Dependent Effect

Define Static•Environment – Temp, Wind•Shrinkage, Creep•Material Densities•Service Loads

Functional Planning•Required Area•Service Loads•Restriction on Building Height (max),

Number and Story Height•Deflection Criteria

Select Structural System Configuration, Foundation System, Material Type, Non-Structural Elements, Connections, Etc.

Serviceability•Elastic Analysis•Design with Elastic

Response Spectra

Functionality

Ductility Elastic Analysis and Design with Inelastic Response Spectra

Preliminary Design

Serviceability

Functionality

DuctilityFinal

DesignEvaluate Preliminary Design

No Good

OK

Final Detailing

Basic Design – Analysis Process

Earthquake Notes by Dr. Shahram Pezeshk

Basic Seismology

n It is the science of earthquakes and related phenomena and

n It is through the science that seismic activity and thus the seismic design loads for a bridge/building may be quantified.


Seismology

n Find ways to reduce hazards of earthquakes by learning how to predict their consequences

n Determine ways the ground is likely to shake during the earthquake, how shaking will be and how long will last

n Knowledge of the ground motion that can be expected during an earthquake can make it possible to design structure economically and strong enough to survive being shaken

n It is essential to understand the characteristics of the earthquake source to predict both the occurrence and the ground motion that an earthquake will generate


Seismic Environment

n Historical record of earthquakes are important because:

n They tell us general locations where earthquakes frequently occur

n Approximately the recurrence intervaln Approximate size of the earthquake


Need from consultants:

n The design earthquake should be a magnitude M earthquake on the F fault with recurrence interval of Y years, or

n The earthquake hazard is represented by a magnitude M earthquake within 25 miles of the site, with probability P of occurrence during a 100 year interval.


Information Structural Engineers Require

n How energy is released

n How energy is transmitted over large areas

n The structure and nature of the earth's interior and assessing the likelihood of large earthquakes in certain regions

n Dimension of the original disturbance and the overall movement involved in it


Faulting

n It was not until the San Francisco earthquake 1906 that it was recognized that earthquakes were caused by slippage along a fault in earth's crust.

n Reid of John Hopkins University Discovered that:n for several hundred kilometers along the San Andreas fault fences and

roads crossing the fault had been displaced by as much as six meters. In addition, surveys conducted before and after the earthquake revealed that rocks parallels to the fault had been strained andsheared


Plate Tectonics

n The earth’s crust is divided into six continental-sized platesn Africann Americann Antarcticn Australia-Indiann Eurasiann Pacific


Plate Tectonics


Convection Currents in Mantle

n Near the bottom of the crust, horizontal component of convection currents impose shear stresses on bottom of crust, causing movement of plates on earth’s surface.

n The movement causes the plates to move apart in some places and converge in others.

Plate Tectonics: The crust in motion

Photo courtesy of the USGS


Spreading Ridge Boundaries

n Magma rises to surface and cools in gap formed by spreading plates.

n Magnetic anomalies are shown as stripes of normal and reversed magnetic polarity

San AndreasFault in SouthernCalifornia

Source: USGS


Elastic Reboud Theory of Earthquakes

n Rocks are elastic, and mechanical energy can be stored in them just as it is stored in a compressed spring. When the two blocks forming the opposite sides of the fault move by a small amount, the motion elastically strains the rocks near the fault. When the stress becomes larger than the frictional strength of the fault, the fictional bond fails at its weakest point. That point of initial rupture, called the hypocenter, may be near the surface or deep below it.


Elastic Reboud Theory of Earthquakes

Fault line goes through two buildings - no apparent damage to either building

Horizontal offset along a transform fault after the Loma Prieta earthquake in 1989 (California)

The Reelfoot Rift Beneath 3000 ft of Sediment

Source: USGS


Idealized Model of Earthquake Source

n Rupture begins at the hypocenter h kilometers below the surface,

n spreads across a fault plane at a velocity V and finally stops after growing into a region with an average length L and an average width w.

n The orientation of the fault plane is specified by it strike angle and dip angel. The slope between the two fault surfaces (large arrows) can have any orientation in the plane. On the average the slip requires t seconds to reach its final offset.


Epicenter & Hypocenter

n The elastic energy stored in the rocks is released as heat generated by friction and as seismic waves.

n The seismic waves radiate from the hypocenter in all directions, producing the earthquake.

n The point on the surface of the earth above the hypocenter is the epicenter of the earthquake


Faultsn Faults are formed when mutual slip of the rock

beds occurs on a certain plane. Depending upon direction, the slippages are classified as:

n Dip Slip:n Slippage takes place in a vertical direction

n Normal Fault: The upper rock bed slips downward n Reverse Fault: The upper rock bed slips upward


Faults

n Strike Slip:n Slippage takes place in a Horizontal direction

n Left Lateral Fault: As seen from one bedrock bed, the other bedrock bed slips toward the left

n Right Lateral Fault: As seen from one bedrock bed, the other bedrock bed slips toward the right


Definition of Fault Type


Subductionn One plate dips down and slides beneath the other in a

process known as SUBDUCTION.

n Generally, an oceanic plate slides, or subducts, beneath a continental plate (west coast of South America) or beneath another oceanic plate (Philippine)


Various Type of Faulting

Four different faulting that generate earthquake. Close to the fault,The type of faulting can have a significant influence on the ground Shaking, but at greater distances the influence is small. In actualEarthquakes there may also be a component of displacement Perpendicular to that shown in the diagram.


Well Know Faults

n San Andreas Faultsn 300-km-long, strike slip of 6.4m

n Caused San Francisco Earthquake of 1906 (M=8.3)n Imperial Valley Earthquake of 1940

n 60-km-long 5m right lateral shiftn Nobi Earthquake, Japan, 1981

n 80-km-long 6m vertical slip and a 2to 4 m horizontal slip, (M=8.4)

n Kansu Earthquake, China, 1920n 200-km-long left lateral fault (M=8.5)

n Kobe Earthquake, Japann Northridge, Californian Chelungpu, Taiwan, 1999


Earthquake Waves


Earthquake WavesWhen rupture along a fault occurs, the sudden release of energy sets off vibrations in the earth’s crust. These vibrations can travel both within the earth’s material (body waves) and on the earth’s surface (surface waves).

P-waves travel by compression and dilations in the direction of propagation, and have the fastest speed (several miles/sec). These waves travel through both solid and liquid.

The transverse waves travel by shear distortions normal to the direction of propagation. Although they are denoted S for Secondary waves, they transmit more energy than the P-waves. S-waves are plane polarized. Those that cause motion in the vertical plane containing the direction of propagation are called SV waves; horizontal waves are called SH waves.

Surface waves are so called because their motion is restricted to close to the ground surface. As the depth below the ground surface increases, the wave amplitudes become less and less. There are two types of surface waves during the earthquake. Love waves’ motion is similar to S-wave horizontally polarized, except that its effects die out as depth increases. Raleigh waves are similar to a rolling ocean wave. Material disturbed by Raleigh wave moves in elliptical path in the vertical plane containing the direction of propagation.

Surface waves travel more slowly than body waves, with Love waves being generally faster than Raleigh waves.


Earthquake Wavesn P-wave

2

(1 )

(1 2 )p

EV

νν ν ρ

−=

− −

n S-wave1

2(1 )s

G EV

ρ ρ ν= =

+

E = Young’s modulusG = Shear modulus

= Mass density= Poisson’s Ratio (0.25 for earth body)

ρν


Location of Earthquakesn P-waves travel faster than S-waves, they arrive

first at a given seismograph. The difference in arrival times will depend on the difference between the P- and S-waves. The distance between the seismograph and the focus of the earthquake is

1/ 1/p s

s p

td

V V−∆

=−

A B

C


Ground Motion Estimation

Shallow Soil Layers

Crustal RockSeismic Source:M0S(f)

Propagation PathG(R) D(R,f)

Near Site:Am(f) P(f,fm)


Seismic Survey Travel Time


Survey lines across San Andreas and Calaveras faults in California


Directivity Effect of Earthquakes Sites toward and Away from Direction of Fault

n Overlapping of pulses can lead to strong fling pulse at sites toward which the fault ruptures


Seismic Gap

n Cross section of the San Andreas fault from SF to Parkfield.n (a) seismicity in the 20 yrs prior to 1989 Loma Prieta

earthquaken (b) main shock and aftershocks of the Loma Prieta earthquake


Modified Mercalli Intensity Scale (MMI)

n Do not confuse MMI with magnituden An intensity scale is the intensity of the ground motion

intensity as determined by human feeling and by the effects ground motion on structures and living things

n MMI is graded based on intensity:n Goes from I to XII (from imperceptible to catastrophic

destruction n Subjective and descriptive scale that measure the

intensity of an earthquake by its effect on humann Is based on and is established on the basis of visible

damage and human feelings


MMI

n I. Not felt. Marginal and long-period effects of large earthquakes.

n II. Felt by persons at rest, on upper floors, or favorably placed.

n III. Felt indoors. Hanging objects swing Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake.

n IV. Hanging objects swing. Vibration like passing of heavy trucks; or sensation of a jolt like a heavy ball striking the walls. Standing cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes.

n IV, wooden walls and frame creak.n V. Felt outdoors; direction estimated. Sleepers awakened.

Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open, Shutters, picturesmove. Pendulum clocks stop, start, char, and change rate.


MMI

n Vl. 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 visibly, or heard to rustle.

n Vll. Difficult to stand. Noticed by drivers. 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.

n VIII. Steering of 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.

n IX. General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with complete collapse, masonry B seriously damaged. General damage to foundations. Frame structures if not bolted, shifted off foundations. Frames racked. Serious damage of reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluviated areas, sand and mud ejected, earthquake fountains, sand craters.

n 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 of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly.

n Xl. Rails bent greatly. Underground pipelines completely out of service.n Xll. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown

into the air.


Isoseismal Map

n Ideal isoseismal pattern shows a bell shapen Isoseismal pattern depends on

n Condition at epicentern The route of seismic wave from focus to the

observation pointn Geological conditions


Isoseismal Map


Isoseismal Map


Richter Magnitude

n The size of an earthquake is closely related to the amount of energy released. The magnitude M defined by Richter in 1935 is often used to express earthquake size.

n In 1935, Charles Richter used a Wood-Anderson seismometer to define a magnitude scale for shallow, local (epicentral distances less than about 600 km [375 miles]) earthquakes in southern California.

n Richter defined what is now known as the local magnitude as the logarithm (base 10) of the maximum trace amplitude (in micrometers) recorded on a Wood-Anderson seismometer located 100 km (62 miles) from the epicenter of the earthquake.

n The Richter local magnitude (ML) is the best known magnitude scale, but it is not always the most appropriate scale for description of earthquake size.


Richter Magnitude

n In 1935, Charles Richter used a Wood-Anderson seismometer to define a magnitude scale for shallow, local (epicentral distances less than about 600 km (375 miles)) earthquakes in southern California.

n The Richter Magnitude, M, is calculated from the maximum amplitude, A, of the seismometer trace (Wood-Anderson Seismometer, T0 = 0.8sec and >=0.80) at a distance of 100 km from the epicenter.

AM log=however, a standard seismometer is not always at 100 km from theepicenter, in which

0loglog AAM −=A0 = maximum recorded amplitude for a particular earthquake selected at a site, generally A0 = 0.001 mm for 100 km distance.


Example of the calculation of the Richter magnitude (ML) of a local Earthquake

Procedure:

Measure the distance to the focus using the time interval between the S and the P waves (S-P=24 seconds)Measure the height of the maximum wave motion on the seismogram (23 mm)Place a straight edge between points on the distance (left) and amplitude (right) scales to obtain magnitude ML = 5.0.


Surface Magnitude

n The Richter local magnitude does not distinguish between different types of waves. Other magnitude scales that base the magnitude on the amplitude of a particular wave have been developed.

n At large epicentral distances, body waves have usually been attenuated and scattered sufficiently that the resulting motion is dominated by surface waves.

n The surface wave magnitude is a worldwide magnitude scale based on the amplitude of Rayleigh waves with a period of about 20 sec. The surface wave magnitude is obtained from

Ms = log A + 1.66 log D + 2.0

where A is the maximum ground displacement in micrometers and D is the epicentral distance of the seismometer measured in degrees (360' corresponding to the circumference of the earth).


Surface Magnitude

n Note that the surface wave magnitude is based on the maximum ground displacement amplitude (rather than the maximum trace amplitude of a particular seismograph); therefore, it can be determined from any type of seismograph.

n The surface wave magnitude is most commonly used to describe the size of shallow (less than about 70 km (44 miles) focal depth), distant (farther than about 1000 km [622 miles]) moderate to large earthquakes.


Body Wave Magnitude

n For deep-focus earthquakes, surface waves are often too small to permit reliable evaluation of the surface wave magnitude.

n The body wave magnitude is a worldwide magnitude scale based on the amplitude of the first few cycles of p-waves which are not strongly influenced by the focal depth. The body wave magnitude can be expressed as

mb = logA - logT + 0.01D + 5.9

where A is the p-wave amplitude in micrometers and T is the period of the p-wave (usually about one sec). Body wave magnitude can also be estimated from the amplitude of one-second-period, higher-mode Rayleigh waves (Nuttli, 1973); the resulting magnitude, mbLg, is commonly used to describe intraplate earthquakes.


Moment Magnitude

n Moment magnitude is based on the total elastic energy released by the fault rupture and is related to the seismic moment Mo defined by

DGAM =0

Where G = Modulus of rigidity of the rock (dyne/cm2)A = Area of rupture surface of the fault (cm2)D = Average fault displacement (cm)

Moment Magnitude is Defined by Hank and Kanamori (1979) as

7.100

log32 −= MMw


Magnitude

n Chilean Earthquake, 1960n Fault Length = 600 milesn Mw = 9.5, Ms = 8.3

n San Francisco Earthquake, 1906n Fault Length = 200 milesn Mw = 7.9, Ms = 8.3


Comparison of Various Magnitudes

n Saturation of the instrumental scales is indicated by their flattening at higher magnitudes


Energy Release and Magnitude Correlation

aMEE +=0

loglog

Magnitude of an earthquake can be related to energy (Gutenberg and Richter, 1956):

ME 5.18.4log +=

E is the energy given in Joules.


Energy Release and Magnitude Correlation

What is the increase in energy if we increase magnitude by one:

13103080.665.18.46

log ×=→×+= EE

15109953.175.18.47

log ×=→×+= EE

32

6log

7log

=E

E


Peak Ground Acceleration Attenuation Equations

n Ground acceleration (in rock) will decrease as the distance from the epicenter increases. For this reason, equations of this type are called attenuation equations.

n Attenuation Equations are site dependent.n Typical attenuation equation:

log PGA = 0.55 +0.5mblg –0.83log10r-0.0019r


Relationship Between Hypocentral Distance, MMI, and Magnitude

n MMI = 8.16 + 1.45M-2.46ln(r)


Fault Length and Magnitude

n The length of an earthquake in (km) is related to the magnitude

n M = 0.98log(L) + 5.65


Correlation of Intensity, Magnitude, and Acceleration

n No exact correlation of the intensity (MMI), magnitude (M), and acceleration (PGA) are possible since many factors affects seismic behavior and structural performance

n Type of construction. Buildings in villages in underdeveloped countries perform much worse than high-rise buildings in developed countries. Therefore, these buildings will experience different damage levels.

n Within a geographical region with consistent design and construction methods, fairly good correlation can exist between structural performance and ground acceleration.


Peak Ground Acceleration

n The Peak Ground Acceleration (PGA) is one of the most important characteristics of an earthquake.

n PGA is given in units g's (i.e. as a fraction of gravitational acceleration):

2.32

)sec/( 2fta

386)sec/( 2ina

81.9)sec/( 2ma

Significant Earthquakes:

Pacoima Dam 1.25gParkfield, 1966 0.50g Loma Prieta, 1989 0.65g


Peak Ground Acceleration Attenuation

n Comparison of Isoseismal of the New Madrid and the San Francisco earthquakes (Nuttli, 1979).

7.5 - 8.2


Seismicity of the Central United States (1811-1987) (mb>3.0) (Mitchell, 1993).

Source: USGS, “Hazard maps help save lives and property” (FS-183-96)

Significant Earthquakes-Eastern North America

Central US Seismicity 1974-1994

Source: CERI

Source: CERI

Earthquakes occur in association with faults. This map shows seismicity “trends” which denote the active faults of the New Madrid seismic zone. There are at least five active faults in the NMSZ


Seismicity of the NMSZ (1974-1993)


Frequency of Occurrence

n The equation most commonly used to describe the occurrence of earthquakes is the well-known Gutenberg-Richter relationship:

bMaNc −=)log(

where Nc = is the number of events greater than or equal to magnitude M; a and b are constants.


bMaNc −=)log(


n The constant a , the activity parameter, provides a measure of the overall occurrence rate of earthquakes in the zone considered and is the zero magnitude intercept on a semi-log plot.

n The slope b, or b value, is controlled by the distribution of events between the higher- and lower-magnitude ranges.

If the equation is expanded to include an upper-bound as well as a lower-bound magnitude, the relationship becomes nonlinear at large magnitudes. The location and magnitude of a potential earthquake corresponding to certain recurrence interval, which would give the most severe ground shaking at studied area, could be found from the equation and seismotectonic study.



For the entire world, the approximate relationship (up to approximately M = 8.2) the approximate number of earthquakes, N, of a given magnitude M is:

MNc 9.07.7)log( −=

Approximate Expected Frequency of Occurrence of Earthquakes (per 100 years)

Magnitude Number4.75-5.25 2505.25-5.75 1405.75-6.25 786.25-6.75 406.75-7.25 197.25-7.75 7.67.75-8.25 2.18.25-8.75 0.6



log( ) 3.43 0.88( )c bN M= −

n The frequency of occurrence in the New Madrid seismic zone (NMSZ) according to Johnston and Nava (1985) can be estimated by mean recurrence rates for the NMSZ using the historical seismicity and the instrumental records (1974-1983).

n They used both linear regression and maximum likelihood techniques to determine the Gutenberg-Richter constants a and b for the best-fit line throughout the data.

n For the NMSZ (35o-37oN, 89o-90.5oW), they obtained



n The solid line indicates the exponential magnitude-recurrence model; the dashed line indicates the characteristic model (Toro et al. 1992).


Typical Seismogramsn Typical siesmograms recorded by

different instruments at the same site during the sameearthquake can be remarkably different.

n The top two sets of curves are the recordings of an accelerograph and a Carder displacement meter at El Centro, CA, from an earthquake at Borrego Mountain, some 60 km away. Both instruments were triggered by the initial P wave, or compression wave, from the earthquake; the first strong pulse on each recording is slower traveling S wave, or shear wave, which arrived seconds later.

n The prominent reverberations on the recording from Carder displacement meter are resonance of the seismic waves in the thick blanket of sediment in the Imperial Valley.


Typical Seismograms

n The Bottom part of curves is the the recording made at La Paz in Bolivia of the vertical component of the initial P wave from the same earthquake that was recorded by a short-period seismograph and a long-period seismograph in the World wide Network.


Earthquake Ground Motion

n Single-Shock Type. The focus is at a shallow depth and the bedrock is hard.

n A moderately long, extremely irregular motion. The depth of the focus is intermediate and the bedrock is hard as in the El Centro Earthquake of 1940.

n A long ground motion exhibiting pronounced prevailing periods. The wave is filtered by many soft layers, and the successive reflections occur at the boundaries, as in the Mexican earthquake of 1964.

n A ground motion involving large-scale permanent deformation of the ground. This occurred at Anchorage in Alaska earthquake of 1964

Newmark and Rosenblueth (1971) classified earthquake ground motions into four type:


Ground Motion Time HistoryNorth-South ground acceleration recorded

at Catech during ML6.4 San Francisco (Feb 9,1971)

The instrument was located at 20 Miles from the causative fault, and at this distance the duration of strong ground shaking was approximately 8 second, this being

the same as the duration of the slipping process onthe fault


Seismograms

n Displacement of the pendulum is proportional to ground motion Vg

n If T of pendulum > T of ground motion and if appropriate damping for the pendulum is chosen, this type of seismogram is called Displacement Seismograph or long periodseisomograph.

n If T of pendulum < T of ground motion and if appropriate damping for the pendulum is chosen, this type of seismograph is called Acceleration Seismograph or short period seismograph.

n If T of pendulum = T of ground motion and if appropriate damping for the pendulum is chosen, this type of seismograph is called velocity Seismograph

World Largest earthquakes

World Largest earthquakes


Basic Design Concepts

n The rarity of truly strong shaking at a site implies that such forces need not be resisted within elastic limit of the materials of construction

n It is not economical to design every structure to resist the strongest possible earthquake without damage, since most structures will never experience such shaking

n The philosophy implicit in modern building codes, which are design criteria, is to resist moderate shaking without damage, but to permit yielding and structural damage in the event of very strong shaking, provided the damage is not unduly hazardous to life and limb.


Analysis Procedure

n Minor Earthquaken Elastic Analysis

n Moderate Earthquake n Elastic/Inelastic - Probability Dependent

n Major Earthquaken Inelastic Analysis


Introduction to Seismic-Resistant Design

Limit State Ground Shaking

Conventional Building

Important Structures

Criteria

Serviceability Design Basis Earthquake

Minor Many times during service life 5-20 years

High probability of occurance 50-

100 years

No Damage to Structure or

non-structural elements

Functionality (Damagebility) Safe Shutdown

Moderate Several times 20-70 years

Low probability

70-250 years

No structural Damage, some non-structural

damage Ultimate

(Ductility) Maximum

Major Rare (worst Max expected)

50-200 years

Extremely Low prob 100-3000

years

Life Safety

Notes 1 Seimology Civl7119

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