Introduction to Technical Seismology

Internal Structure of Earth

- Earth is not solid inside

- Magma is rising from the inner part to earth surface

Plate Tectonics and Continental drift

- Earth crust is divided into six (or sevenaccording to other scientists) large plates andseveral smaller

- The plates tend to move from each other

Plate Tectonics and Continental drift

Wegeners Theory: One continent (Pangaea) existed 270million years ago

Plate Tectonics and Continental drift

Pangaea separation and plate movement in millions of years period

Plate movement and Earthquake generation

Plate movement can cause:

- Volcanoes

- Mountain creation

- Earthquake generation

Plate movement and Earthquake generation

Earthquake generation:

- Right after the last earthquake small forces (stresses) exist at the interface between the plates

- After several years, because of the continuous movement of the plates, the forces between them increase

- The forces reach a point that exceeds the plate strength. The sudden breaking of the interface gives birth to earthquake

Plate movement and Earthquake generation

NORMAL REVERSE

STRIKE SLIP(lateral throw)

DIP SLIP(vertical throw)

- Faults are the earth crust regions that failure takes place and earthquakes initiate

- There are many types of faults regarding the type of movement of the adjacent plates

Plate movement and Earthquake generation

- Some times faults may reach Earth surface

- Faults are easily identified by discontinuity at the rock formations

Fault surface in limestone (Arkitsa, Greece)Discontinuity of rock formation

People staring at the fault

Plate movement and Earthquake generation

- Some times faults may reach Earth surface

- Faults are easily identified by discontinuity at the rock formations

San Andreas fault in California, USASecondary fault appears at the surface

Earthquake travel from source to site

Parameters that affect the seismic wave propagation from source to site

- Source characteristics (fault type, rock formations etc)

- Path characteristics (route from source to site, rock formations, attenuation-decrease of motion with distance)

- Local soil conditions (soil type, topographic effects, site effects)

Hypocenter

Epicenter

D

B

A

C

E

Source

Path

Local soil conditions

Rock

Seismic waves

Earthquake force measurements

Earthquake motion recordings of:

- Displacements

- Velocities

- Accelerations

Magnitude measurement: Richter magnitude scale

For example the magnitude of El Centro earthquake (15/10/1979) was 6.9 of the Richter scale.

Intensity measurement: Mercalli intensity scale

Measurements regarding the extend of earthquake effects at a particular location (damage extend of buildings, road system, natural environment etc)

1 Richter difference means 32 times difference in the earthquake energy. Thus a magnitude 7 earthquake is 32 times more powerful than a magnitude 6 earthquake and almost 1000 than a magnitude 5.

Major Earthquake Recordings

- Seismographs to record displacements

Instruments to measure earthquake results:

- Accelerographs to record accelerations

(Used for small earthquakes)

(Used for strong earthquakes)

Recordings in 3 directions

E W

N

S

Up

EW (East-West)

NS (North-South)

(Horizontal)

(Horizontal)

Up (vertical-Z)

Major Earthquake Recordings

Acceleration Time-History

EW (East-West)

NS (North-South)

UP (Vertical)

(Horizontal)

(Horizontal)

PGA=0.5m/sec

Time (sec)

Acc (

m/s

ec)

PGA (Peak Ground Acceleration)

Major Earthquake Recordings

Velocity Time-History

EW (East-West)

NS (North-South)

UP (Vertical)

(Horizontal)

(Horizontal)

PGV=0.069m/sec

Time (sec)

Velo

city (

m/s

ec)

PGV (Peak Ground Velocity)

Major Earthquake Recordings

Displacement Time-History

EW (East-West)

NS (North-South)

UP (Vertical)

(Horizontal)

(Horizontal)

PGD=0.044m

Time (sec)

Dis

pla

cem

ent

(m)

PGD (Peak Ground Displacement)

Connection to Structural Dynamics

Why are Accelerations important for Engineers ?

- The earthquake does not create directly Forces to structures but

only Accelerations and base Displacements

m

..

u

..

u

..

F m u

m F

[M]

- If we know the Acceleration that a structure receives then we can

calculate the earthquake Force

T h

M=Fh

Connection to Structural Dynamics

Is the PGA (Peak Ground=soil Acceleration) what a structure receives ?

NO. The PGA value refers to the soil

surface

The Acceleration that the structure mass

receives is different

m

T

..

u

soil

Creation of Response Spectrum

Response spectrum shows what is the acceleration that each structure

receives according to the structures period T.

..

gu PGA

Acceleration Response Spectrum

T1

..

gu

T2 T3 T4 T5 T6T=0

PGA

Acc (

m/s

ec)

period T (sec)

a1a2

a3

a4

a5

a6

a1

a2

a3 a4

a5

a6

More flexible structures

(larger period T)

PGA

soil

1) Earthquake input motion

2) Measure Sdof response at each period T

3) Create Response Spectrum using the maximum value of

Sdof response at each T

All Sdof5% damping

Acceleration Response Spectra

Acceleration Time-History and Response Spectrum

PGA=0.5m/sec

Acc (

m/

sec)

Acc (

m/

sec)

PGA

period T (sec)time t (sec)

m=2t

T=1.0sec

..

u

soil

..

2gu PGA 0.5m / s

Response spectrum

..2u 1.182m / s

..

F m u 2.36 KN M=F h=7.08KNm

h=3m

EXAMPLE

Acceleration Response Spectra

Acceleration Time-History and Response Spectrum

PGA=0.5m/sec

Acc (

m/

sec)

Acc (

m/

sec)

period T (sec)time t (sec)

EXAMPLE

T=0.25secStructure A:

A B C

T=0.55secStructure B:

T=1.90secStructure C:

Which Structure will receive the greater acceleration from this earthquake and why?

Acceleration Response Spectrum

Acc (

m/

sec)

period T (sec)

T=0.1 n

n: number of stories

Approximately for a structure T=0.1 2=0.2sec

T=0.1 7=0.7sec

T=0.1 15=1.5sec

2 stories:

7 stories:

15 stories:

2 stories 7 stories 15 stories

Acceleration Response Spectra from several Earthquakes

Response Spectra (5% damping)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60T (sec)

Acc (

m/

se

c)

Earthq. 1

Earthq. 2

Earthq. 3

Which earthquake is worse for:

1 story structures

3 story structures

6 story structures

Resonance

When a structure has the same period with the earthquake

RESONANCE

Maximization of damages at the structure

Response Spectra (5% damping)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60T (sec)

Acc (

m/

se

c)

A B

Resonance: Structure A

Structure B will suffer less damages than structure A

Response Spectrum (5% damping)

Resonance

RESONANCE may also be cause by wind

Tacoma Bridge failure

Response Spectrum (5% damping Y-Y)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00T (sec)

Acc

(m

/se

c)

Response Spectrum (5% damping Y-Y)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00T (sec)

Acc

(m

/se

c)

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00t (sec)

Acc

(m/

sec

)

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

0.00 5.00 10.00 15.00 20.00 25.00t (sec)

Acc

(m

/se

c)

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00t (sec)

Acc

(m

/se

c)

Acceleration Response Spectra from several Earthquakes

PGA=1.57m/sec

Umbria 29/4/84

PGA=1.81m/sec

Bagnoli-Irpino23/11/80

PGA=3.5m/sec

Friuli 6/5/76Response Spectrum (5% damping X-X)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00T (sec)

Acc

(m

/se

c)

Response Spectrum (5% damping X-X)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00T (sec)

Acc

(m

/se

c)

Response Spectrum (5% damping Y-Y)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00T (sec)

Acc

(m

/se

c)

Response Spectrum (5% damping Y-Y)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00T (sec)

Acc

(m

/se

c)

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

0.00 5.00 10.00 15.00 20.00 25.00t (sec)

Acc

(m

/se

c)

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

0.00 5.00 10.00 15.00 20.00 25.00t (sec)

Acc

(m/

sec)

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

0.00 5.00 10.00 15.00 20.00 25.00t (sec)

Acc

(m/

sec

)

Acceleration Response Spectra from several Earthquakes

PGA=1.57m/sec

Umbria 29/4/84

PGA=1.81m/sec

Bagnoli-Irpino23/11/80

PGA=3.5m/sec

Friuli 6/5/76

All Response Spectra in same scale

Response Spectra (5% damping Y-Y)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00T (sec)

Acc (

m/

se

c)

Acceleration Response Spectra from several Earthquakes

All Response Spectra in same diagram

Bagnoli-Irpino

Umbria

Friuli

period