Earthquake Strong Motions

8/3/2019 Earthquake Strong Motions

1/45

Strong ground motion

(Engineering Seismology)

Earthquake shaking capable ofcausing damage to structures


2/45

The release of the accumulated elastic strain energy

by the sudden rupture of the fault is the cause of the

earthquake shaking


3/45

Horizontal motions are of mostHorizontal motions are of most

importance for earthquake engineeringimportance for earthquake engineering

Shaking often strongest on horizontal component:

Earthquakes radiate larger S waves than P waves

Decreasing seismic velocities near Earths surface produce

refraction of the incoming waves toward the vertical, so that the

ground motion for S waves is primarily in the horizontal direction

Buildings generally are weakest for horizontal shaking


4/45

Questions

What are the most useful measures of

ground motion?

What factors control the level of groundmotion?


5/45

Measures of groundMeasures of ground--motion formotion for

engineering purposesengineering purposes

PGA (peak ground acceleration)

PGV (peak ground velocity)

Response spectral acceleration(elastic, inelastic) at periods ofengineering interest

Intensity (Can be related to PGA andPGV.)


6/45

Peak ground acceleration (PGA) easy to measure because the response of most instruments is

proportional to ground acceleration liked by many engineers because it can be related to the force

on a short-period building

convenient single number to enable rough evaluation ofimportance of records

BUT it is not a measure of the force on most buildings and it is controlled by the high frequency content in the ground

motion (i.e., it is not associated with a narrow range offrequencies); records can show isolated short-duration, high-amplitude spikes with little engineering significance


7/45

0 5 10 15

-500

0

500

Accelerati

on

(cm/s

2)

1994 Northridge Earthquake, Sylmar Hospital Free-field site

NS Component

0 5 10 15

-500

0

500

Acceleration

(cm/s

2)

Vertical Component

0 5 10 15

-500

0

500

Time (sec)

Acceleration(cm/s

2)

EW Component

File:C:\rose\ch09\sylm

_3comp

_acc.d

raw;

Date:2003-0

9-1

5;

Time:19:06:50

P wave arrives before S

wave. S-Trigger time = 3.2

sec, hypocentral distancebetween approx. 5*3.2=

16 km and 8*3.2= 26 km

P-motion much higher

frequency than S, andpredominately on vertical

component.

Is the horizontal S-wave

motion polarized?


8/45

Peak ground velocity (PGV)

Many think it is better correlated with damage

than other measures

It is sensitive to longer periods than PGA

(making it potentially more predictable using

deterministic models)

BUT it requires digital processing (no longer an

important issue)


9/45


10/45

Large Recorded Ground VelocitiesLarge Recorded Ground Velocities


11/45

Peak ground displacement (PGD)

The best parameter for displacement-based design?

BUT highly sensitive to the low-cut (high-pass) filter that

needs to be applied to most records (in which case the

derived PGD might not represent the true PGD, unlikePGA, for which the Earth imposes a natural limit to the

frequency content). For this reason I (Dave Boore)

recommend against the use of PGD.


12/45

Acceleration Response Spectra

at Periods (or frequencies) of

Engineering Interest


13/45

ug

Elastic response spectra (manyElastic response spectra (many

structures can be idealized asstructures can be idealized as

SDOF oscillators)SDOF oscillators)


14/45


15/45


16/45


17/45

10 20 30 40 50 60

-100

10

20

Time (sec)

-50

5

-0

.001

0

0.001

-1

0

1

-10

0

10

-5

0

5

0.1 1 10 100

10-4

0.001

0.01

0.1

1

10

100

Period (sec)

RelativeDisplacemen

t(cm)

1999 HectorMine Earthquake (M 7.1)

station 596 (r= 172 km), transverse component

10 20 30 40 50 60

-2*10-40

2*10-4

Time (sec)

-5

0

5

Tosc = 0.025 sec

Tosc = 0.050 sec

Tosc = 1.0 sec

Tosc = 10 sec

Tosc = 40 sec

Tosc = 80 sec

Ground acceleration (cm/sec2)

Ground displacement (cm)

At long periods, oscillator

response proportional to base

displacement


18/45

0.1 1 10 100

0.01

0.1

1

10

100

Period (sec)

Acceleratio

n

(cm/s2)

0.1 1 10 100

10-4

0.001

0.01

0.1

1

10

100

Period (sec)

Relative

Displacement(cm)

1999 HectorMine Earthquake (M 7.1)

station 596 (r= 172 km), transverse component

convert displacement spectrum into acceleration

spectrum (multiply by (2T/T)2). For velocity

spectrum, multiply by 2/T.

Acceleration or velocity spectra usually used in

engineering


19/45

Frequencies of groundFrequencies of ground--motion formotion for

engineering purposesengineering purposes 10 Hz --- 10 sec (usually below

about 3 sec)

Resonant period of typical N storystructure ~ N/10 sec

Corner periods forM5, 6, and 7 ~

1, 3, and 9 sec


20/45

Frequency Responseof Structures


21/45

I Barely feltII Felt by only few people

III Felt noticeably, standing autos rock slightlyIV Felt by many, windows and walls creakV Felt by nearly everyone, some dished and windows brokenVI Felt by all, damaged plaster and chimneysVII Damage to poorly constructed buildings

VIII Collapse of poorly constructed buildings,slight damage to well built structures

IX Considerable damage to well constructed buildings,buildings shifted off foundations

X Damage to well built wooden structures, some masonry

buildings destroyed, train rails bent, landslidesXI Few masonry structure remain standing, bridges

destroyed, ground fissuresXII Damage total

Modified Mercalli Intensity


22/45

What Controls the Level of Shaking? Magnitude

Directivity Larger fault, more energy released and over a larger area

Distance from fault Shaking decays with distance

Local site response (rock or soil) amplify the shaking Strongest shaking in rupture direction

Pockets of higher shaking (lens effect)


23/45

Earthquake Magnitude

Earthquake magnitude scales originated

because of

the desire for an objective measure of

earthquake size

Technological advances -> seismometers


24/45

Modern Seismic Magnitudes

Today seismologists use different seismic wavesto compute magnitudes

These waves generally have lower frequencies

than those used by Richter These waves are generally recorded at

distances of1000s of kilometers instead of the100s of kilometers for the Richter scale


25/45

Teleseismic MS and mb

Two commonly used modern magnitudescales are:

MS, Surface-wave magnitude (Rayleigh Wave)

mb

, Body-wave magnitude (P-wave)


26/45


27/45

Why use moment magnitude?

It is the best single measure of overallearthquake size

It does not saturate

It can be estimated from geologicalobservations

It can be estimated from paleoseismologystudies

It can be tied to plate motions and recurrencerelations


28/45

(From J. Anderson)


29/45

(From J. Anderson)


30/45


31/45

Directivity

Directivity is a consequence of a moving source

Waves from far-end of fault will pile up withwaves arriving from near-end of fault, if you are

forward of the rupture This causes increased amplitudes in direction of

rupture propagation, and decreased duration.

Directivity is useful in distinguishing earthquake

fault plane from its auxiliary plane because itdestroys the symmetry of the radiation pattern.


32/45

Rupture Directivity

Hypocenter


33/45

Example of observed

directivity effects in the

M7.3 Landers

earthquake groundmotions near the fault.

Directivity played a key

role in the recent SanSimeon, CA, earthquake


34/45

2003 San Simeon2003 San Simeon

M6.5 EarthquakeM6.5 Earthquake


35/45

Rupture DirectivityRupture Directivity


36/45

Damage in Oceano

2003 San Simeon Earthquake

Cracking in river levee

Failed foundation


37/45

Effect of Distance

Ground motion generally

decreases with increasingepicentral distance


38/45

2003 San Simeon Earthquake

Distance and directivity


39/45

Amplitude and Intensity

M7.6 Pakistan earthquake 2005

Seismic waves loseamplitude with distancetraveled - attenuation

So the amplitude of the waves

depends on distance from theearthquake. Therefore unlikemagnitude, intensity is not asingle number.


40/45

Site Amplification

Ground shaking is amplified at soft

soil (low velocity) sites

Shear-wave velocity is commonly usedto predict amplification

VS30 ( time it takes for a shear wave to

travel from a 30 m depth to the land

surface, i.e., time-averaged 30-m velocity)


41/45

Ground Motion Deconvolution

(Steidl)


42/45

Amplification of PGA

as a function of VS30

0

1

2

3

4

5

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Mean Shear-Wave Velocity to 30 m (100 ft) (v , m/s)

Short-PeriodAmplificationF

aw

rtSC-Ib

I=0.1g; ma = 0.35

I=0.2g; ma = 0.25

I=0.3g; ma = 0.10

I=0.4g; ma = -0.05

Fa (0.1g) for Site Class Intervals

Fa for Site Classes

Soft soils

Gravelly soils and

Soft rocks

Firm to Hard rocks

Fa = (v SC-Ib / v )m a = (1050 m/s / v)

m a

SC-IV

SC-II

SC-Ib

SC-III

Stiff clays and

Sandy soils

(a)


43/45

Velocities of Holocene and

Pleistocene Units Oakland, CA

Velocity, m/s

0 100 200 300 400

Depth,m

0

5

10

15

20

25

30

X

X

X

X

X

X

X

X

X

X

XMerritt SandX

Pleistocene alluvial fan

Holocene alluvial fan

Younger bay mud

Holocene Pleistocene

D di t ib ti d i th


44/45

Damage distribution during the

1989M6.9 Loma Prieta

earthquake correlated quite wellwith Vs30.


45/45

Summary of Strong Ground

Motion from Earthquakes

Measured using PGA, PGV, pseudo-spectralacceleration or velocity PSA or PSV, and

intensity. Increases with magnitude.

Enhanced in direction of rupture propagation(directivity).

Generally decreases with epicentral distance. Low-velocity soil site gives much higher ground

motion than rock site. Vs30 is a good predictorof site response.

Earthquake Strong Motions

Documents

Transcript of Earthquake Strong Motions