Upload
r-mega-mahmudia
View
223
Download
0
Embed Size (px)
Citation preview
8/19/2019 Fundamental of Engineering Seismology
1/62
FUNDAMENTALS ofENGINEERING SEISMOLOGY
MEASURING GROUND
MOTION
8/19/2019 Fundamental of Engineering Seismology
2/62
The first known instrument for earthquakes measurement is the Chang
seismoscope built in China in 132 B.C.
Balls were held in the dragons’ mouths by leer deices connected to an internal
pendulum. The direction of the epicenter was reputed to be indicated by the first
ball released.
!"#$%&'() "#&T*+%#,"$
8/19/2019 Fundamental of Engineering Seismology
3/62
Jargon
seismoscope – an instrument that documents the occurrenceof ground motion (but does not record it over time)
seismometer – an instrument that senses ground motion and
converts the motion into some form of signal
accelerometer – a seismometer that records acceleration, also
known as strong ground motion
geophone – another name for a seismometer, commonly used
in active source seismology
8/19/2019 Fundamental of Engineering Seismology
4/62
More Jargon
seismograph – a system of instruments that detects and
records ground motion as a function of time
seismogram – the actual record of ground motion produce by
a seismograph
seismometry – the design and development of seismic
recording systems
data logger – device that converts analog to digital signal and
stores the signal
8/19/2019 Fundamental of Engineering Seismology
5/62
Chronology of Instrumentation
132 – first seismoscope (eng, China)
1751 – seismoscope which etched in sand (!ina, Italy)
1784 – first attempt to record ground motion as a function of
time using a series of seismoscopes (Cavalli, Italy)
1875 – first true seismograph (Cecchi, Italy)
8/19/2019 Fundamental of Engineering Seismology
6/62
Chronology of Instrumentation
1889 – first known seismogram from a distant earth"uake isgenerated (#ebeur$%aschwit&, 'ermany)
1914 – first seismometer to use electromagnetic transducer tosense ground motion ('alit&in, #ussia)
1969 – first digital seismograph (data recorded in discrete
samples on a magnetic tape) (* researchers)
1990s – broadcast of real time seismic data via internet
8/19/2019 Fundamental of Engineering Seismology
7/62
ow *eismometers +ork
Fundamental Idea: o record ground motion
a seismometer must be decoupled from theground If the seismometer moves with the
ground then no motion will be recorded
*ince the measurements are done in a moving reference frame(the earth-s surface), almost all seismic sensors are based on
the inertia of a suspended mass, which will tend to remain
stationary in response to e.ternal motion he relative motion
between the suspended mass and the ground will then be afunction of the ground-s motion avskov and /lguacil
8/19/2019 Fundamental of Engineering Seismology
8/62
-rinciples of seismographs
0oors in C/# College (swing on tilted a.is)
8/19/2019 Fundamental of Engineering Seismology
9/62
he current is proportional
to the mass velocity
1lectro$magnetic
sensor
2elocity transducer3
moving coil withina magnetic field
avskov and /lguacil
8/19/2019 Fundamental of Engineering Seismology
10/62
8/19/2019 Fundamental of Engineering Seismology
11/62
#nalog $trong!otion
#ccelerographs
11USGS - DAVID BOORE
8/19/2019 Fundamental of Engineering Seismology
12/62
Analog accelerographs
Three important disadvantages of analog aelerographs!
1" Al#a$s triggered %$ a speified threshold of aeleration #hih
means the first motions are often not reorded
&" The limitation of nat'ral fre('en$ of analog instr'ments" The$
are generall$ limited to a%o't &) *+"
," It is neessar$ to digiti+e the traes of analog instr'ments asthe$ reord on film or paper most important disadvantage as it
is the prime so're of noise.
These instr'ments prod'e traes of the gro'nd
aeleration against time on film or paper" /ost #idel$'sed analog instr'ment is the 0inemeteris S/A-1
Dr" Sinan Aar Strong Ground Motion Parameters – Data Processing 1&
8/19/2019 Fundamental of Engineering Seismology
13/62
!odern seismic
monitoring
8/19/2019 Fundamental of Engineering Seismology
14/62
Modern *eismometers
• / conductive (metallic) mass is decoupled fromsurrounding magnets inside a protective casing
• 'round motion causes the mass to move relative to the surrounding magnetic field
• his creates an electric current with anamplitude that is proportional to the velocity ofthe mass
8/19/2019 Fundamental of Engineering Seismology
15/62
Modern *eismometers
• his electric current is transmitted to a digiti&erwhich converts the analog (continuous) signal toa digital (discrete) signal
• 1ach discrete observation of the current iswritten to a computer disk along with the
corresponding time
• hese times series- are downloaded to computersand processed4analy&ed
8/19/2019 Fundamental of Engineering Seismology
16/62
Digital accelerographs
Digital aelerographs ame into operation almost )2 $ears after
the first analog strong motion reorders" Digital instr'ments
provide a sol'tion to the three disadvantages assoiated #ith the
earlier aelerographs!
1" The$ operate ontin'o'sl$ and %$ 'se of pre-event memor$ are
a%le to retain the first #ave arrivals"
&" Their d$nami range is m'h #ider3 the transd'ers having
nat'ral fre('enies of )2 to 122 *+ or even higher
," Analog-to-digital onversion is performed #ithin the instr'ment3th's o%viating the need to digiti+e the reords"
Dr" Sinan Aar Strong Ground Motion Parameters – Data Processing 14USGS - DAVID BOORE
8/19/2019 Fundamental of Engineering Seismology
17/62
*ensitivity
• he sensitivity of seismometers to ground
motion depends on the freuency of the
motion
• he variation of sensitivity with fre"uency
is known as the instrument response of aseismometer
8/19/2019 Fundamental of Engineering Seismology
18/62
he amplitude and fre"uency range of seismic signals is very large
he smallest motion of interest is limited by the ground noise he
smallest motion might be as small as or smaller than 56 nm +hat
is the largest motion7 Considering that a fault can have a
displacement of 65 m during an earth"uake, this value could be
considered the largest motion his represents a dynamic range of(65465$65) 8 6566 his is a very large range and it will probably
never be possible to make one sensor covering it *imilarly, the
fre"uency band starts as low as 555556 & (earth tides) and could
go to 6555 & hese values are of course the e.tremes, but a good
"uality all round seismic station for local and global studies should
at least cover the fre"uency band 556 to 655 & and earth motions
from 6 nm to 65 m
/mplitude and fre"uency range
avskov and /lguacil
8/19/2019 Fundamental of Engineering Seismology
19/62
avskov and /lguacil
It is not possible to make one single instrument covering this range of values and instruments withdifferent gain and fre"uency response are used for different ranges of fre"uency and amplitude *ensors
are labeled eg short period (*%), long period (9%) or strong motion oday, it is possible to make
instruments with a relatively large dynamic and fre"uency range (so called broad band instruments
(!!) or very broad band (2!!)) and the tendency is to go in the direction of increasing both the
dynamic and fre"uency range
avskov and /lguacil
8/19/2019 Fundamental of Engineering Seismology
20/62
:rom I/*%1I$;M*
8/19/2019 Fundamental of Engineering Seismology
21/62
Instrument #esponse
• *eismometers that are sensitive to ground motions with
high fre"uencies are called s!ort"period seismometers
hey are useful for recording nearby (within =555 km)
earth"uakes and are also used in active source seismice.periments
• *eismometers that are sensitive to ground motions with
long fre"uencies are called lon#"period seismometershey are useful for recording teleseismic earth"uakes,
normal modes, and earth tides
8/19/2019 Fundamental of Engineering Seismology
22/62
Instrument #esponse
• he most advanced seismometers are called$road$and seismometers and can record
both high and low fre"uencies – they recordover a broad band of fre"uencies
•
!roadband seismometers are much moree.pensive, and more easily damaged, thanshort period seismometers
M h i l
8/19/2019 Fundamental of Engineering Seismology
23/62
&(t)8 y(t)$.(t) relative displacement
*pring force
0amping force
0amping oscillator
constants3
Mechanical sensor
%& d& m& m'− − = +& &&
ym
5=m(d ! =
m
) ( =5
=
5 5= & ! & & 'ω ω + + = −&& &
8/19/2019 Fundamental of Engineering Seismology
24/62
M h i l
8/19/2019 Fundamental of Engineering Seismology
25/62
Input harmonic motion
(fre"uency domain)
Mechanical sensor
=
5 5
=
=
=
( ) ( )
( ) ( )
( )
( )
( )
* t
* t
* t
* t
* t
& ! & & '
' t + e
& t , e
' - e
& * , e
& , e
ω
ω
ω
ω
ω
ω ω
ω
ω
ω ω
ω ω
ω ω
+ + = −
=
=
= −
=
= −
&& &
&&
&
&&
( )
( )
( )
=
= =
5 5
=
== = = = =
5 5
6 6 5
= =
5
( )( )
( ) =
( ) ( )
>
Im ( ) =( ) tan tan
#e ( )
d
d d
d
d
d
, .
+ !*
/ .
!
. !
.
ω ω ω
ω ω ω ωω
ω ω ω
ω ω ω ω
ω ωω ω
ω ω ω
− −
= =
− +
= =− +
−Φ = = ÷ ÷ ÷ −
8/19/2019 Fundamental of Engineering Seismology
26/62
( )
= = =
5 5
== = = = =
5 5
( ) 6( )
( ) =
6( ) ( )
>
a
a a
, .
+ !*
, / . /
!
ω ω
ω ω ω ω ωω
ω ω
ω ω ω ω
−= =
− − +
= = =
− +
8/19/2019 Fundamental of Engineering Seismology
27/62
a v s k o v a n d / l g u a c i l
accelerometer
:rom displacement to velocity and to
acceleration3 divide by the fre"uency
(remove a &ero from the origin)
:rom mechanical seismometer to velocity
transducer and to accelerometer, multiply
by the fre"uency
(add a &ero in the origin)
:lat response in acceleration9ow sensitivity in displacement
8/19/2019 Fundamental of Engineering Seismology
28/62
0isplacement at very low fre"uencies produce very low
accelerations
( , where . is the ground displacement and f the fre"uency)
It is therefore understandable why it is so difficult to produce
seismometers that are sensitive to low fre"uency motion
oday, purely mechanical sensors are only constructed to have resonance
fre"uencies down to about 65 & (short period sensors), while sensors
that can measure lower fre"uencies are based on the :orce !alance%rinciple (:!/) of measuring acceleration directly
' f ' =∝
8/19/2019 Fundamental of Engineering Seismology
29/62
:orce$balance (*ervo) *ensors
he force$balance accelerometer is shown below where a
pendulous, high$magnetic permeability mass is hung from a
hinge he ?down? or ?null position? is detected by the null
detector and the counterbalancing force is provided by a
magnetic coil
8/19/2019 Fundamental of Engineering Seismology
30/62
“Broadband” seismometers (velocity sensors, usingelectronics to extend the frequency to low values) arestarting to be used in engineering seismology: theboundary between traditional strong-motion and wea-motion seismology is becoming blurred (indistinct,fu!!y)"
8/19/2019 Fundamental of Engineering Seismology
31/62
0igital strong$motion recording
• !roadband3 nominally flat response from dc to atleast >5 & – !ut noise4 baseline problems can limit low$fre"uency
information
– igh$fre"uency limit generally not a problem becausethese fre"uencies are generally filtered out of themotion by natural processes (e.ception3 very hard rocksites)
•
igh dynamic range (/0C 6@ bits or higher)• %re$event data usually available
8/19/2019 Fundamental of Engineering Seismology
32/62
AD5 Analog-digital onversion.
• 6'anta least digital o'nt.
6 7 &89&:
;here
8/19/2019 Fundamental of Engineering Seismology
33/62
E=amples
• 8 7 &g 7 &>?@1 m9s9s
• : 7 1& %its
6 7 "?4 m9s&
DR 7 44 d%
• : 7 & %its
6 7 2"222&, m9s&
DR 7 1,@ d%
8/19/2019 Fundamental of Engineering Seismology
34/62
/agnifiation 'rves
8/19/2019 Fundamental of Engineering Seismology
35/62
/agnifiation 'rves
:ot sho#n! %road%and 2"2&D5 se.
:ote noth3 d'e to Earth
noiseC this noise an %eseen in reordings from
modern %road%and
instr'ments"
,)
8/19/2019 Fundamental of Engineering Seismology
36/62
$eismic $ensors and $eismometry/ -rof. ". 0ielandt/ r. C. !ilkereit
:rom ;ew Manual of *eismological
8/19/2019 Fundamental of Engineering Seismology
37/62
:rom ;ew Manual of *eismological
8/19/2019 Fundamental of Engineering Seismology
38/62
Analogue and Digital Records of small earthquake from
Adacent Instruments at !rocisa Nuo"a #Ital$%
%$arrival lost in analog recording
8/19/2019 Fundamental of Engineering Seismology
39/62
*ummary
• he first legitimate seismometer was built in 6AB
• he first seismogram of a distant earth"uake was recordedin 6AAD
• he first digital seismometers were deployed in the early6DB5s
• he first broadband seismometers were deployed in the6DA5s
8/19/2019 Fundamental of Engineering Seismology
40/62
*ummary
• *eismometers record motions as small as 65$D m,
at fre"uencies of about 5556 & to 655 &
• here are over 65,555 seismometers around the
world that are continually recording ground
motion
8/19/2019 Fundamental of Engineering Seismology
41/62
*eismograms• *eismograms are records of 1arth-s motion as a function of
time
8/19/2019 Fundamental of Engineering Seismology
42/62
*eismograms
•
*eismograms record ground motion in terms of – displacement
– velocity
– acceleration
• ;ormally a seismometer samples ground motion about
=5 times per second (=5 &), but this number can be as
high as 55 & Modern accelerometers sample at =55
sps
8/19/2019 Fundamental of Engineering Seismology
43/62
8/19/2019 Fundamental of Engineering Seismology
44/62
Seismograms are com&osed of
'&hases(
8/19/2019 Fundamental of Engineering Seismology
45/62
*eismograms
•'round motion is a vector (whether it isdisplacement, velocity or acceleration), so it takes
E numbers to describe it hus, seismometers
generally have three components3
– 2ertical (up is positive)
– ;orth$*outh (north is positive)
–
1ast$west (east is positive)
Fhori&ontals
8/19/2019 Fundamental of Engineering Seismology
46/62
Components of Motionhere are simple mathematical operations that allow
seismologists to rotate (abstractly) the hori&ontal components3
;
1+
*
eart!ua)e
seismometer
8/19/2019 Fundamental of Engineering Seismology
47/62
Components of Motionhere are simple mathematical operations that allow
seismologists to rotate (abstractly) the hori&ontal components3
;
1+
*
eart!ua)e
seismometer
Modified
Coordinate *ystem
he new
components are
called3(6) #adial, #
(=) ransverse, #adial
ransverse
8/19/2019 Fundamental of Engineering Seismology
48/62
8/19/2019 Fundamental of Engineering Seismology
49/62
;etworks and /rrays
8/19/2019 Fundamental of Engineering Seismology
50/62
)road*+and Seismogra&h Net,orks
8/19/2019 Fundamental of Engineering Seismology
51/62
Many networks of instruments, both
traditional Gstrong$motionH and, morerecently, very broad$band, high dynamic$
range sensors and dataloggers
,yoshin (et
8/19/2019 Fundamental of Engineering Seismology
52/62
,yoshin (et,("T
4apanesestrong motion
networkhttp566www.knet.bosai.go.7p
• 1888 digital instruments
installed after the ,obe
earthquake of 199:
• free field stations with anaerage spacing of 2: km
• elocity profile of each station
up to 28 m by downhole
measurement
• data are transmitted to the
Control Center and released
on 'nternet in 3; hours after
the eent
• more than 2888
accelerograms recorded in ;
years
8/19/2019 Fundamental of Engineering Seismology
53/62
#eminder3 %lay Chuettsu and ottori
movies
8/19/2019 Fundamental of Engineering Seismology
54/62
Chuetsu
8/19/2019 Fundamental of Engineering Seismology
55/62
ottori
8/19/2019 Fundamental of Engineering Seismology
56/62
/ number of web sites provide data from
instrument networks• !ut no single web site containing data from
all over the world
• /n effort is still need to add broad$banddata into the more traditional data sets
8/19/2019 Fundamental of Engineering Seismology
57/62
)USGS - DAVID BOORE
8/19/2019 Fundamental of Engineering Seismology
58/62
)@USGS - DAVID BOORE
8/19/2019 Fundamental of Engineering Seismology
59/62
)?USGS - DAVID BOORE
8/19/2019 Fundamental of Engineering Seismology
60/62
42USGS - DAVID BOORE
NGA h 44 b k l d 4 4+1! *I1* – 0//!/*1*
8/19/2019 Fundamental of Engineering Seismology
61/62
NGA $ http344peerberkeleyedu4nga4
8/19/2019 Fundamental of Engineering Seismology
62/62
1;0