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L2-1 EARTHQUAKES

CIVE 430 ENGINEERING GEOLOGY

SUMMER 2013

What causes an earthquake? An earthquake is the vibration of the

Earth produced by the rapid release of accumulated energy in elastically strained rocksEnergy released radiates in all directions

from its source, the Focus Energy propagates in the form of seismic

waves Sensitive instruments around the world

record the event

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What causes an earthquake? Movement of Tectonic PlatesEarth is divided into sections called

Tectonic plates that float on the viscous-fluid-like interior of the Earth. Earthquakes are usually caused by sudden movement of earth plates

Rupture of rocks along a faultFaults are localized areas of weakness in

the surface of the Earth, sometimes the plate boundary itself

PLATE BOUNDARIES

FAULTS

Where Do EQs Occur?

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Release of Accumulated Energy

The point within Earth where faulting begins is the FOCUS, or HYPOCENTER

The point directly above the focus on the surface is the EPICENTER

The Focus and Epicenter of an Earthquake

Definitions related to EQ Location

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Rocks bend under stress while storing elastic energy. When the strain in the rocks exceeds their strength, breaking will occur along the fault. Stored elastic energy is released as the earthquake. Rockssnap back, or rebound to their original condition.

ELASTIC REBOUND THEORY

ELASTIC REBOUND THEORY

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SAN ANDREAS

SAN ANDREAS

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Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks

Small earthquakes, called foreshocks, sometimes precede a major earthquake by days (in some cases, by as much as several years)

Foreshocks and Aftershocks

The study of earthquake waves, Seismology, dates back almost 2000 years to the Chinese Seismographs(instruments that record seismic waves).

The first seismograph called Di-Dong-Di was invented by Cheng Heng (132 A.D.).

Study of Earthquakes

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Seismograph?

Seismograph?

The ancient Chinese seismographconsist of a special vase that had eightsculpted dragons mounted around thevase in eight primary directions. Eachdragon held in its mouth a metal ball.When the ground shook, some of theballs would fall from the mouths of thedragons into the waiting mouths of thesculpted frogs to show how the groundhad moved.

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The energy released during the earthquake travels as waves

Modern Seismograph can measure the intensity and duration of these waves in different directions.

A Seismogram is visual record of arrival time and magnitude of shaking associated with seismic wave, generated by a seismograph.

Earthquake Waves

Seismograph

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Modern Seismograph (Horizontal)

Modern Seismograph (Horizontal)

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Seismogram

BODY WAVES

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BODY WAVES

BODY WAVES

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BODY WAVES

SURFACE WAVESRayleigh

Love

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WAVES

Compression (P) wave is fastest wave.

Shear (S) wave is second fastest wave.

Rayleigh (R) or surface wave is very close to S-wave velocity (90 to 94%).

Love (L) wave: interface boundary effect (approximately same velocity as R-waves)

EARTHQUAKES

CHARACTERISTICS

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Location and Intensity of Earthquake

Seismographic stations around the World work together to

record earthquake location determine earthquake Magnitude

Locating the Source of an Earthquake (Method 1)

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Locating the Source of an Earthquake (Method 1)

Locating the Source of an Earthquake (Method 1)

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Locating the Source of an Earthquake (Method 2)

Distance from Epicenter to Location of Seismograph =

(215 km)

Locating the Source of an Earthquake (Method 2)

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Locating the Source of an Earthquake (Method 2)

Earthquakes usually occur at some depth below the ground Surface. The depth can also be calculated from seismograph records

Earthquake foci are described as:

Shallow: less than 70 km depth

Intermediate/Deep: 70 - 300 km depth

90% of earthquake foci are less than 100 km deep

Large earthquakes are mostly at < 60 km depth

No earthquakes occur deeper than 300 km

Earthquake Depth

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EARTHQUAKE INTENSITY

Modified Mercalli Intensity Scalefrom FEMAI. People do not feel any Earth movement.

II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.

III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.

IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.

V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.

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Modified Mercalli Intensity Scalefrom FEMAVI. Everyone feels movement. People have trouble walking. Objects fall

from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.

VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.

VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.

IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.

Modified Mercalli Intensity Scalefrom FEMA

X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.

XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.

XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

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Richter Local Magnitude, MLFunction of amplitude of

recorded seismic wave.

Moment Magnitude, MWFunction of (1) Amount of slip

(meters) on the fault, (2) Area of fault plane, and (3) Rigidity of the rocks.

EARTHQUAKE MAGNITUDE

Earthquake -Richter Magnitude

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Richter Local Magnitude

ML = log10A(mm) + (Distance correction factor)

Here A is the amplitude, in millimeters, measured directly from the photographic paper record of the Wood-Andersonseismometer, a special type of instrument.

The distance factor comes from a table that can be found in Richter's (1958) book Elementary Seismology.

ENERGY

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Strange Animal Behaviorstress in the rocks causes hairline fractures to form, the cracking of the rocks emits high pitched sounds and minute vibrations imperceptible to humans but noticeable by many animals.

Foreshocks unusual increase in the frequency of small earthquakes before the main shock

Changes in water level porosity increases or decreases with changes in strain

Seismic Gapsbased of the chronological distribution of major earthquakes

Predicting Earthquake?

Predicting Earthquake?

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Prepare a Seismic Risk Map for the globe which identifies rock types, liquefaction potential, landslide potential.

Extensive geologic surveying has to be done to identify all active faults, including hidden faults.

Earthquake Resistant Design of StructuresEnact building codes to design and build earthquake-resistant structures in high seismic risk areas.

Critical facilities such as nuclear power plants and dams should be built on stable ground and as far as possible from active faults.

PROTECTING AGAINST EARTHQUAKE DAMAGE

Bigger Faults Make Bigger EarthquakesBigger Faults Make Bigger Earthquakes

1

10

100

1000

5.5 6 6.5 7 7.5Magnitude

Kilometers

8

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Bigger Earthquakes Last a Longer TimeBigger Earthquakes Last a Longer Time

1

10

100

5.5 6 6.5 7 7.5 8

Magnitude

Sec

onds

What Controls the Level of Shaking?What Controls the Level of Shaking?

MagnitudeMore energy released

DistanceShaking decays with

distanceLocal soilsamplify the shaking