fundamental of engineering seismology

8/19/2019 Fundamental of Engineering Seismology

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FUNDAMENTALS ofENGINEERING SEISMOLOGY

MEASURING GROUND

MOTION


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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.

!"#$%&'() "#&T*+%#,"$


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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


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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


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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)


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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


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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


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-rinciples of seismographs

0oors in C/# College (swing on tilted a.is)


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he current is proportional

to the mass velocity

1lectro$magnetic

sensor

2elocity transducer3

moving coil withina magnetic field

avskov and /lguacil


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#nalog $trong!otion

#ccelerographs

11USGS - DAVID BOORE


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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&


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!odern seismic

monitoring


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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


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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


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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


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*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


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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


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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


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:rom I/*%1I$;M*


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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


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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


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&(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 + + = −&& &


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M h i l


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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 = = ÷ ÷ ÷ −


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( )

= = =

5 5

== = = = =

5 5

( ) 6( )

( ) =

6( ) ( )

>

a

a a

, .

+ !*

, / . /

!

ω ω

ω ω ω ω ωω

ω ω

ω ω ω ω

−= =

− − +

= = =

− +


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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


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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 ' =∝


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: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


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“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)"


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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


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AD5 Analog-digital onversion.

• 6'anta least digital o'nt.

6 7 &89&:

;here


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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%


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/agnifiation 'rves


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/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"

,)


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$eismic $ensors and $eismometry/ -rof. ". 0ielandt/ r. C. !ilkereit

:rom ;ew Manual of *eismological


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:rom ;ew Manual of *eismological


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Analogue and Digital Records of small earthquake from

Adacent Instruments at !rocisa Nuo"a #Ital$%

%$arrival lost in analog recording


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*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


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*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


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*eismograms• *eismograms are records of 1arth-s motion as a function of

time


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*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


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Seismograms are com&osed of

'&hases(


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*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


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Components of Motionhere are simple mathematical operations that allow

seismologists to rotate (abstractly) the hori&ontal components3

;

1+

*

eart!ua)e

seismometer


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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


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;etworks and /rrays


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)road*+and Seismogra&h Net,orks


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Many networks of instruments, both

traditional Gstrong$motionH and, morerecently, very broad$band, high dynamic$

range sensors and dataloggers

,yoshin (et


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,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


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#eminder3 %lay Chuettsu and ottori

movies


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Chuetsu


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ottori


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/ 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


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)USGS - DAVID BOORE


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)@USGS - DAVID BOORE


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)?USGS - DAVID BOORE


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42USGS - DAVID BOORE

NGA h 44 b k l d 4 4+1! *I1* – 0//!/*1*


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NGA $ http344peerberkeleyedu4nga4


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