Lect4Eqks03

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CEE 227 - Earthquake Engineering

U.C. Berkeley Spring 2003 ©UC Regents 4-1

Part 2 - EngineeringCharacterization of Earthquakes

and Seismic Hazard

Ultimately what we want is a seismic “intensitymeasure” that will allow us to quantify effect of anearthquake on a structure.

Sa

PeriodΤ

Sd

Τ

Sd

Probability of exceedencePeriod

T given



Earthquake EnvironmentSources of Earthquakes• Meteor Impact• Volcanoes• Reservoir Induced• Well Injection• Tectonic

➔ Associated withcontinental drift

■ at edges of continentalplates

■ mid-continent

■ mid-ocean ridges

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Earthquake Occurrence



Earthquake Fatalities

Total: 22711

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Earthquakes in the US



US Earthquakes

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Large Earthquakes in 1900s



Largest Earthquakes in US

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Finding Historic Earthquakes and FaultsMany www sites have databases to locate pastearthquakes in a region.

➔ See www.usgs.gov➔ http://www.wsspc.org/links/li

nks.html#maps➔ /www.consrv.ca.gov

1819 event

2001 Event



Finding Faults➔ United States Geologic

Survey (quadrangles)➔ California Division of Mines

& Geology (Alquist PrioloSpecial Study Zones)

➔ Geologic reconnaisance

South San Francisco.San Andreas Fault

Berkeley, Hayward Fault

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Elastic Rebound Theory

A CB D

Initial Time

Fault

StraightFence




A CB D

Initial Time

Fault

StraightFence

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A CB D

Initial Time

Fault

StraightFence

Fault Offset13 ft




A CB D

Initial Time

Fault

StraightFence

d

time

d

timePermanent

Displacement

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FaultingFault length and offset can be significant

Event Length Offset1906 San Francisco 250 miles 21 feet1959 Chile 600 miles --1971 San Fernando 15 miles 3 feet1857 Fort Tejon -- 49 feet

Epicentral distance Vs. distance to fault trace

Epicenter

EpicentralDistance

SiteDistanceto fault

Fault



Basic Types of Faulting• Surface Faulting

➔ Strike-slip➔ Dip slip

■ Normal

■ Reverse

➔ Combinations• Mid-Plate events - No

apparent fault rupture due todepth or cover by alluvialdeposit

• Subduction zones - Commonin Japan, Mexico, PudgetSound, South America

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Subduction Zone Faulting



Major Faults inCalifornia

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Faulting in Northern and Southern CA



Earthquake Shaking in Bay Area1906 San Francisco 1989 Loma Prieta

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Terminology

Hypocenter (start of rupture)

Epicenter (ground surface)Surface trace

Site

A

B

A

OB

OA - Epicentral DistanceOB - Hypocentral DistanceOC - Distance to surface traceOD - Distance to rupture

D

O

Section EE

E E



Seismic HazardHazard depends on size,location and frequency ofoccurrence of earthquakes nearbuilding site, and characteristicsof ground shaking produced.

Ground shaking due to aparticular event depends on:

➔ Source Mechanism➔ Travel Path➔ Geological/Local Soil Effects➔ Soil-Structure Interaction

BuildingSite

Travel PathMechanism

Site

Region

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Regional SeismicityEstimated based on:• Tectonic Features (Potential

estimated from local faults -Length, annual movement,etc.)

• Historic Events (frequency ofoccurrence, gaps)

• Statistical analysis of similarregions

Site

Region

Magnitude

Frequency

x% probabilityin “y” years

Not agood site

Map



Generic Seismic Source Regions

San Andreas

CascadiaSubduction

WasatchBasin & RangeTectonic Province

New Madrid Region Charleston, SC

New England

Quebec

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Motion Depends on Source MechanismSource Effects• Type of fault: Strike slip

or thrust (relativehorizontal and verticalmovement)

• Rupture process(continuous and regularvs. multiple and irregular)

• Directivity effects(Doppler effects)

• Near-sourcephenomenon -- “fling”

Higher frequency

Direction of Rupture

Lower frequency wavesRUPTURE SLOWER THAN SOUND

D

B

C

AFaultOffset

NEAR FAULT MOTION DIFFERENT



Near Source Effects

D

B

C

AFaultOffset

Following Rupture

d

t

d

t

Rupture

Displ.

Velocity

AccelerationFault paralleldisplacementoffset oftencorrespondsto velocityandaccelerationpulses withdurationtrupture

trupture

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Fault Perpendicular Motions

High ShearStresses at Tip ofRupture

Displ.

Velocity

trupture

Acceleration



Travel Path EffectsWave Propagation

➔ P Waves

➔ S-Waves➔ Surface Waves

P S Surface

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Travel Path Effects - 2• Reflection• Dispersion• Attenuation• Focusing

➔ Intensity reduces withdistance

➔ Longer predominantperiod of motion atdistance

➔ Longer apparentduration at distance

Site 3

Travel PathsFault

Site 4Site 1

Site 2

Ground Acceleration, g

Distance, km

Time

ag

Time Time



Local Soil ConditionsSoil acts like dynamic oscillator

Rock

Surface

Soil

Sa

TSa

T

Softer, deeper or weaker soilwill have longer predominantfrequency content

Rock

Firm

Soft

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Travel Paths: WUS vs CEUS

Flat anduniform geology

Mountains andIrregular GeologyMountains andcomplex geology

MMI VII: Considerabledamage to poorly

built structures



US Seismic Hazard

Firm Soil

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Soil-Structure InteractionCombination of:• Inertial Effects - Mass of

structure influences overallresponse of soil structuresystem (Stiff and heavybuildings on soft soil)

• Kinematic Effects -Flexibilityof soil influences dynamicresponse of structure (tall,light buildings on soft soil)

• Effective damping (yieldingof soil, radiation, uplift, etc.)

See Section 5.8 FEMA 368

Soil

Structure

Structure



Qualitative Comments on Ground MotionsFour General Types of Motions:• Single shock - small magnitude,

close event• Moderately long, irregular motion

- moderate to large magnitudeevent recorded on firm ground atmoderate distance from fault.

• Long motion with pronouncedpredominant period - soft,deepsoil

• Large pulse (often most easilyseen in velocity or displacement) -moderate to large event recordedat close distance

time

accel.

time

accel.

time

accel.

time

accel.

Ts

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Some common records

1940Imperial Valley

Earthquake

Short effective duration

32%g

Vertical motion:Less intenseHigher frequency



1985 Chile EarthquakeRock Site Soil Site

60 sec 120 sec

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Near-Fault CA Records

Vertical > horizontal

80%g



Comments• Motions in different regions can differ substantially.• Motions can vary regionally (fault mechanism and

travel path) and by soil condition for the same event.➔ Motions recorded at sites in very close proximity can be

quite different.• Ground motions have three (six) components, not one.• Components in different directions will differ (vertical

has much higher frequency content, two horizontalcomponents differ).

�How can we characterize earthquakes for design?�Do we design differently for different types of ground

motions?

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Lect4Eqks03