introduction to technical seismology -...
TRANSCRIPT
-
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