CIE619-Lecture02-Seismology1

8/9/2019 CIE619-Lecture02-Seismology1

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Chapter 2

Elements of

Seismology and Seismicity

CIE 619

Chapter 2 Seismology and Seismicity1

CONTENT1. Introduction

2. Causes of Earthquakes

3. Theory of Plate Tectonics

4. Reids Elastic Rebound Theory

5. Fault Mechanisms

6. Definition of Seismic Waves

7. Location of an Earthquake

8. Modified Mercalli Intensity Scale

9. Richter Magnitude Scale

10. Evolution of Magnitude Scales

11. Relationships Between Magnitude Scales

12. Seismic Parameters Influencing Structural Response

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. ttenuat on e at ons ps

14. Elastic Seismic Wave Model

15. Eastern and Western North America Earthquakes

16. Directivity Pulse Phenomenon and Near-Field Ground Motions

17. References


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1. Introduction

Earthquake design of a structure always depends on degree of regionalseismic activity

Many seismological factors directly influence work of a structuralengineer: distribution of earthquake sources affecting the construction site

fault mechanisms of various sources

seismic activity of various sources in terms of recurrence of magnitudes

ground motion intensity;

attenuation of the ground motion with distance

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ap er prov es overv ew o un amen a proper es o var ousseismological aspects

Acquaint the structural engineer with language of seismologists.

2. Causes of Earthquakes Natural Earthquakes

crust measures between 60 and 100 km in depth

crust made of different segments that are continuously in motion

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Natural Earth uakes deformations occur in the rock, which cause a build up

of elastic energy

rupture or slip along fault line causes a sudden releaseof energy into a seismic shock (an earthquake) which,in turn, causes propagation of seismic waves andground shaking

Most earth uakes occur in two s ecific zones on the

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planet:

Circum Pacific Belt: South America, the California coast,Alaska, Japan, Formosa, Philippines, New Zealand;

Alpine Belt: the Mediterranean, North India, Indonesia.


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Earthquakes can damage the built

environment a number of ways, including:

Ground shaking

Fault rupture

Liquefaction or soil failure

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Flooding

Fire

2. Causes of Earthquakes Damage by Fault Rupture

1-dimensional

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Damage by Liquefaction or Soil Failure

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1995 Kobe Earthquake


Induced Earthquakes some human interventions influence amplitude and distribution of strains

in earths crust

interventions such as filling of a water reservoir, mining, excavation ofhuge quarries, high pressure injection of fluids to generate geothermicenergy, oil wells and underground nuclear explosions, may cause majorinduced earthquakes.

filling of water reservoirs usually causes most severe induced earthquakes may reach magnitude 6 on the Richter scale

More than 70 earthquakes of this type have been recorded world wide

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. . upta s an exce ent re erence on eart qua es n uce y t efilling of water reservoirs.

Course limited to naturally occurring earthquakes.

*Gupta, H.K. 1992. Reservoir-Induced Earthquake. Developments inGeotechnical Engineering, 64. Amsterdam, Elsevier.


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The Earth is characterized by a small number of lithospheric plates that

float on a viscous underlayer called the asthenosphere.

Geological evidence shows that plates undergo constant, gradualchange. Magma is continually upwelling at the mid-oceanic ridges and rises


.

In some areas, large sections of plates are forced to move beneath other

plates (surface layers of rocks are absorbed into the earths interior). These

areas are called subduction zones.

A plate being subducted beneath another

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Chapter 2 Seismology and Seismicity


Generally accepted as explanation for occurrence of most earthquakes

Proposed in the 1960's

Earths crust composed of several large plates that float on a viscousmedium

Continents and oceans are supported by those plates

Continents originally all linked together, but started drifting apart 200million years ago

Plates move from 1 to 15 cm every year.

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95% of earthquakes occur along the edges of the interacting plates3. Theory of Plate Tectonics

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Chapter 2 Seismology and Seismicity

Source: Earthquakes by Bruce A. Bolt


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5. Fault Mechanisms

Strike-slip fault

Normal fault

Underthrust fault

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5. Fault Mechanisms

-

Caused by transform motion (lateral motion) of plates

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5. Fault Mechanisms

Best-known strike-slip fault in the US:an n reas au t n a orn a.

Information on the fault and others in theUnited States is available at a number ofweb sites including: Southern California Earthquake Center

(SCEC) www.scec.org ,

California Geological Surveywww.consrv.ca.gov

United States Geological Survey(USGS) www.usgs.gov .

North American

Plate

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San Andreas Fault composed of manysegments or combinations of segments:14 by the count of USGS, with variousslip rates, maximum magnitudes and

return periods.

ac c

Plate

5. Fault Mechanisms Normal fault

Caused by diverge motion of plates

Top wall of fault plane slides downward

Fault plane generally inclined, slides on an inclination

angle

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5. Fault Mechanisms

Underthrust fault

Caused by subducted motion of plates

Top wall of fault plane is pushed upward

Particular type of underthrust fault is called thrust fault

characterized by a small inclination (< 45o) of fault

plane

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Focal oint focus or hypocentre where earthquake occurs

where fault line originates

located at depth varying from afew kilometres to 100 km

Epicentre

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point on ground surface

Focal depth Vertical distance between focal

point and epicentre.


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Two kinds of body waves Primary waves (P-waves) Secondary waves (S-waves)

compression waves, which travel

in direction of wave front

High frequency

First waves to reach a structure

,

perpendicularly to wave front

Lower frequency

Greater amplitude

Most destructive vibrations.

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6. Definition of Seismic Waves Two kinds of surface waves

Rayleigh waves Love waves

ground surface

the ground

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First arrival of seismic waves can be

ent e rom se smograp (x)

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7. Location of an Earthquake At least three geological stations

required to find location of epicentre

For each station i, apply differential

e uations between distance velocit ,

and time, to primary and secondary

waves

tstp = time difference, taken from the

v

1-

v

1

t-t=R

ps

ps

i

R1 R2

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ear qua e recor a e s a on,between the arrival of the P and S-

waves

vp = velocity of the primary waves

vs = velocity of the secondary waves


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7. Location of an Earthquake From theory of propagation of elastic

seismic waves (sect. 14), velocities of

-

v1-

v1

t-t=R

ps

ps

i

G=v

)2G+(=v

s

p

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me)(mass/voludensityrock=

ratiosPoisson'=modulussYoung'=E

modulusshear=G

constantsLame'

=

)2-(1)+(1=

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