Future Earthquake Shaking in the Los Angeles Region

Thomas Heaton (Caltech)Anna Olsen (Univ. of Colorado)Masumi Yamada (Kyoto Univ.)

Key Issues• I will concentrate on ground shaking and its impact on

buildings. Stiff buildings vs. flexible buildings• Modern high-rise buildings and base-isolated buildings

have not yet experienced large long-period ground motions (pgd > 1 m).

• But they will• Is statistical prediction of long period ground motions

technically feasible?• Maybe … but it will look very different from psha for short

periods• Will the design of long-period buildings change dramatically

in the next 100 years?• “Excuse me senator … could you repeat that question?”

How do buildings resist earthquake forces?

Front View

Top View

Image: Courtesy EERI

Tall Building AnatomyTall Building Anatomy

Different Types of Buildings Have very Different Characteristics

• Wood frame houses are light, stiff, strong, and ductile (very good in earthquakes).

• Un-reinforced brick buildings (pre 1935) are heavy, stiff, weak, and brittle (very poor in earthquakes).

• Concrete shear-wall buildings are heavy, stiff, strong and fairly ductile. Can be very good, but must be strong and ductile enough to handle high accelerations.

• Moment-resisting frame buildings (most high-rises) are light, flexible, and weak. Pre 1975 concrete frames and pre 1995 steel frames are far less ductile than intended.

• Pre 1975 concrete frame buildings are potentially very dangerous in moderately strong shaking.

• Modern high-rise construction performs well for high-frequency motions, but is vulnerable when ground displacements are large, or when the ground “resonates” at the natural frequency of the building.

One of the great disappointments is that there has been little progress in the retrofitting of “nonductile” concrete frame buildings. Most people wholive or work in them are not aware of the serious risk involved.

Flexible, but brittle, pre 1975 concrete frame

Flexible Steel Frame

Plastic Hinge of Steel Beamspecial post 1994 connection

Cucapah-El Major M 7.2 earthquake Easter 2010 in Mexicali

John Hall’s design of a 20-story steel MRF building

•Building U20 1994 UBC zone4 Stiff soil, 3.5 sec. period

•Building J20 1992 Japan code 3.05 sec period Similar to current IBC with

highest near-source factor •Both designs consider Perfect welds Brittle welds

Pushover Analysis

•Special attention to P-delta instability•Story mechanism collapse•Frame 2-D fiber-element code of Hall (1997)•2 m roof displacement is near the capacity of any of these designs

0 50 100 150 200 250 3000

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40Pushover Curves

Bas

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Lateral Roof Displacement cm

U20BU20PU6BU6PJ20BJ20PJ6BJ6P

20-story steel-frame building subjected to a 2-meter

near-source displacement pulse (from Hall) • triangles on the frame indicate the failures of welded column-beam

connections (loss of stiffness).

Ph.D. Thesis of Anna Olsen, 2008• collected state-of-the-art simulations of crustal

earthquakes• 37 earthquakes, over 70,000 ground motions

– 1989 Loma Prieta (Aagaard et al., 2008)– 1906 San Francisco, with alternate hypocenters (Aagaard

et other al., 2008)– 10 faults in the Los Angeles basin (Day et al., 2005)– Puente Hills fault (Porter et al., 2007)– TeraShake 1 and 2 (K. Olsen et al., 2006, 2007)– ShakeOut, from Chen Ji

• Moment magnitudes between 6.3 and 7.8• Long-period (T > 2 s) and broadband (T > 1 s)• PGD and PGV calculated from vector of north-

south and east-west components

From Anna Olsen

• Synthesized ground motions

• 30% probability of collapse contours

• pgv > 1 m/s gets the building to yield

• and pgd > 1 m collapses it

•Severe damage or collapse in many areas•Stronger, stiffer building (J20) performs better than more flexible building (U20)•Brittle weld buildings 5 times more likely to collapse than perfect-weld buildings•Results summarized in Olsen and others (BSSA, 2008)

Large displacements can overwhelm base isolation systems

• 2-meter displacement pulse as input for a simulation of the deformation of a 3-story base-isolated building (Hall, Heaton, Wald, and Halling

• The Sylmar record from the 1994 Northridge earthquake also causes the building to collide with the stops

3-sec spectral displacement• Typical base isolator is

3 sec with a maximum allowed displacement of 40 cm

• Nonlinear isolator displacements exceed linear by 20% to 40% (Ryan and Chopra)

• Described in Olsen and others (BSSA, 2008)

• Anything in yellow or red would exceed current typical base isolation system

meters

Maps of Building Responses

• M 7.15, Puente Hills fault• 6-story, more flexible design

with sound welds• Colors follow FEMA 356:

– Blue: peak IDR < 0.007– Green: peak IDR > 0.007– Yellow: peak IDR > 0.025– Red: peak IDR > 0.05– Pink: simulated collapse

Shakeout Simulation (Aagaard and Graves)

Response of three different 20-story buildings with and withoutBrittle welds to the Shakeout motions(Swaminathan Krishnan

• PEER Tall Building Initiative to conduct performance based analysis of three 40-story buildings in downtown LA (5 ½ to 6 s fundamental periods).

• “Working with engineering consultants and experts experts at SCEC, we selected records to represent frequent (25-yr) and extremely rare (4975-yr) shaking. The latter is well beyond the shaking level commonly considered.”

• “the code-designed cases (2006 IBC, 2008 LATBDC) have acceptable performance under the 475-yr motions, and survive the 2475-yr motions. Under the 4975-yr event, however, some elements may fail.”

• Similar study of One Rincon Hill in downtown San Francisco for similar building design.

Spec

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• 15 realizations of spectrum compatible motions used by PEER Tall Building Initiative for 40-story (6 second) building analysis in downtown LA

• These are the Maximum Considered Earthquake Spectra (MCE) for a 2,476-year repeat (life safety level

• This project is considered as a PEER/SCEC collaboration

1.0 m/s

-1.0 m/s

PEER Spectrum Compatible 2,500-yr Ground Velocities for 40-story 6-second Building in Los Angeles

PEER Spectrum Compatible 2,500-yr Ground Displacements for 40-story 6-second Building in Los Angeles

1.0 m

-1.0 m

PEER Spectrum Compatible 2,500-yr Ground Displacements for 40-story 6-second Building in Los Angeles

1.0 m

-1.0 m

1 meter pgd is considered as extreme ground motion

1906 San Francisco Ground Motions

•Magnitude 7.8•Same slip distribution, three hypocenter locations•Long-period PGD exceeds 2 m near the fault•Long-period PGV exceeds 1.5 m•Simulations by Aagaard and others (BSSA, 2008)

All strong motions recorded at less than 10 km from rupture from M>6

From Masumi Yamada

•Near-source pga’s are log-normal•Same distribution will apply 100 years from now

Short periods are Gaussian statistics

• Can reliably determine the mean and standard deviation

• How many people will die in auto accidents?• How many people will suffer a heart attack?• How many buildings will experience some

level of pga?

•Long-period ground motions are not log normal•A few large earthquakes can completely change the distribution•Cannot predict what the shape of this distribution will look like 100 years from now•Area(M)~10M10-bM=constant, if b=1•i.e., given that a fault slips, all values of slip are equally likely•The small pgd’s will come in a few at a time as smaller but numerous eq’s occur•The large pgd’s will arrive in a large clump when infrequent large eq’s occur

Long Periods are power law statistics

• Probabilities are difficult to estimate for power law.

• How many people will die in• A war?• A pandemic?• What will your 401k look like in 20 years?

PEER LA 5,000 yr

Graves near-source

PEER LA 2,500 yr

Existing Near-sourcerecords

• 40 years of strong motion recording

• What will this distribution look like in AD 4,500?

• Have SCEC scientists really meant to say that the 5,000-yr pgd is 1.2m in LA?

-30 -20 -10 0 10 20 300.2

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Distance from the fault (km)

Slip

(m

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Y=0.7/(1+(0.13X)1.6)0.5modeldip=45dip=60dip=75dip=90

PGD per unit fault slip

(Aagaard et al. 2001)

Near-Source PGD’s are roughly 2/3 of the fault slip in nearby segments.

But what will the fault slip be?

For strike-slip fault,Dis

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Predicting Near-Source pgd’s

• Must predict fault slip amplitude on known and unknown faults

• Use of magnitude desensitizes the analysis to fault slip … even smaller events may have large slips

Concluding Remarks • 2,500 yr probalistic prediction based on 40-years of collection of strong

motion data• Implies that we know the rules of the game and that we will not change

our minds• Can save a bunch of money by stopping funding of PSHA research … the

problem is basically already solved• Votes from a committee of experts … If we took a vote of a such a

committee in 1980, would it reach the same conclusion as a committee today? How about in 2040? What about 3040?

• Scenarios are criticized because no one knows how to assign a probability … somehow with PSHA this problem magically goes away

• If you believe that, then I’ve got some “highly rated” mutual funds that I’d like to sell you … the probability that they’ll crash is negligible

• The honest answer is … “WE DON’T KNOW” … there should not be any long-period Natl. PSHA maps, they are only misleading

What Government Policy Actions are Needed?

• Government should ask three professional communities (earth science, academic engineers, practicing engineers) to jointly provide a frank assessment of the impact of plausible future earthquakes (e.g. Puente Hills Thrust M 7.0).

• Occupants of buildings should be provided with a mechanism for recognizing deficient buildings.

• Non-ductile concrete buildings are a widely recognized major hazard, and there should be a program to deal with them.

• There should also be a program to deal with brittle welds in pre-1995 steel frame buildings.