02-SeismicHazardPP201104

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Seismic Hazard Analysis

Dam Safety Risk Analysis Best Practices

April 4, 2011



Lower Van Norman Dam February 9, 1971

M 6.6 earthquake at about 14 km = 0.5 g



Van Norman Dam pre and post

earthquake cross sections











Seismic Failure Modes of Dams

• Sliding and cracking – concrete dams

• Liquefaction of foundation

• Embankment deformation and loss of

freeboard• Cracking of embankment leading to piping

• Fault displacement through the dam• Overtopping from landslides into reservoir



M 7.6 Chi-Chi Earthquake

September 21, 1999 Chelongpu Fault - Taiwan



Sheffield Dam – June 29, 1925Constructed in 1917

Acceleration 0.15g

M 6.3 at 6 miles

Shaking lasted 15 to 18 seconds



Purpose of Seismic Hazard Studies• Develop probabilistic earthquake

loadings for dam stability analyses

– Identify earthquake sources

– Characterize activity rates and magnitudes – Estimate ground motion exceedance rates

– Develop probabilistic time-histories



Components

• Seismic source characterization

• Development of hazard curves

• Development of uniform hazard spectra

(UHS)• Development of target response spectra

• Development of scenario ground motiontime histories



Seismic Source Characterization

• Faults with surface expression

• Subduction zone interfaces

• Subduction slab• Background seismicity not associated

with known faults• Zones of distributed deformation



Faults

• Idealized model of complex behavior

• Need rate of earthquake activity (event rate) –inferred from slip rate and simple fault model

• Fault model properties

– Geometry (location, length, dip, down-dip extent)

– Sense of slip (strike slip, normal, reverse)

– Segmentation – rupture scenarios – Maximum magnitude

– Recurrence model – distribution of magnitude



Fault Types

• Normal

• Reverse (thrust)

• Strike-slip



Normal fault surface rupture

Borah Peak, Idaho – October 28,1983 - M 6.9



Strike Slip Fault

Surface Expression

San Andreas Fault



Lauro Dam

Cascadia Subduction Zone



Cascadia Subduction Zone

(CSZ)



Subduction Zone









Probabilistic Seismic Hazard Analysis

“PSHA in a Nutshell”

• Goal – Determine rates at which specific peak ground

motions are exceeded (PHA, PHV)• Includes the contributions from all potentialearthquake sources

• Incorporates the rate and magnitude of theseearthquake sources

• Input – Paleoseismic and historical seismic data

– Empirical relationships between ground motion andearthquake magnitude and distance (attenuationrelationships)

– Site Response



11-m-high

scarp

Fault Trench

Reconstruct History of

Earthquakes



Geologic Record of Earthquakes





Background Seismicity

• Accounts for earthquakes on unidentified

faults• Maximum magnitude - western U.S,

usually assume M ~ 6 ½ • Rate of activity from historical seismicity





Site Response

• Ground motion prediction equations

depend on site conditions – Soil column response - characterized by

shear wave velocity (VS30) – shear wave

velocity in upper 30 m

– Shallow basin response – characterized by

depth to 1.0 or 2.5 km/s layer depth• Determined by geophysical exploration



PSHA Fundamental Premise

• Loading rate ≤ Earthquake rate

• Loading return period ≥ Earthquake return

period• Earthquake rate ≈ loading rate, for loadings ≈ 0.

This means the y-intercept of hazard curve

provides a good estimate of the total earthquake

rate.

( ) ( ) ( )Pr | Rate loading Rate EQ loading EQ= ×



PSHA Principles

• For multiple independent earthquake

sources, the total loading rate is just thesum of the rates of the individual sources

• Rates are additive

• Loadings are not additive – Loading considering all sources is determined

from the total loading rate (total hazard curve) – Loading for all sources ≠ sum of individualloadings









UNIFORM HAZARD SPECTRUM

• Provides the response spectral

acceleration at a specified return period asfunction of spectral response period

• Same hazard, e.g. 1 in 10,000, for allresponse periods (uniform)

• Determined by reading values from a

hazard curve for a given response period







Target Spectra and



Target Spectra and

Conditional Mean Spectra (CMS)• For risk analyses, realistic scenario earthquakes must be

generated at a range of specified return periods, and thisrequires generating target response spectra

• UHS is not a realistic target response spectrum – Not the response spectrum of any actual earthquake

– Actual earthquakes often have a peak at one spectralperiod, but the level tends to diminish away from the

peak period – Unlikely for an actual earthquake to have peaks at all

spectral response periods

– UHS sets the envelope of the set of target spectra for a specified return period

• Conditional Mean Spectra (CMS) are a type of targetresponse spectra developed to produce realistic scenario

earthquakes



CMS

• Response spectrum obtained from actualearthquakes conditioned on the peak response

occurring at a specified target spectral period,typically chosen to be the critical period of astructure

• However, we rarely know what the critical periodfor a particular structure because manystructures behave nonlinearly

• As a result many potential critical periods needto be considered









TIME HISTORIES

• For dynamic analyses, ground motion timehistories (velocity or acceleration) are developedfor a set of return periods (typically 1,000 to50,000 years)

• Suites of time histories are developed for eachreturn period to represent the intrinsic variabilityin potential ground motions of futureearthquakes.

• Time histories are used for dynamic analysesusing programs such FLAC , SHAKE, or LS- DYNA



Time History Development

• A PSHA does not necessarily provide allimportant earthquake characteristics for engineering analyses, for example -

– Duration of shaking – Timing and phasing of peak ground motions

• This limitation is partly addressed by selectingrecords of historical earthquakes at similar magnitudes and distances, and then modifyingthose records to be consistent with the hazardcalculated from the PSHA

• In order to do this, the total hazard must bedisaggregated to find the magnitude and thedistance of the primary contributing earthquake

scenarios

S i i S Di i



Seismic Source Disaggregation

Historic earthquake record



90

Spectral

matching to

CMS target

spectrum

Historic earthquake record(Loma Prieta, Gilroy station)

10,000-yr scenario crustalsource earthquake

velocity

displacement

acceleration

acceleration

shaking duration



Dashed lines = target spectra

Solid lines = spectra of matchedsynthetic records for scenario EQ

10,000-yrscenarioearthquake

Spectral matching methodprovides a good fit to target

spectra

Vertical

Horizontals



Conclusions

• Probabilistic ground motions are required for quantitative risk analysis

• First key factor in determining the hazard is theearthquake rate of the controlling source

– Geologically determined slip rate

– Historical seismicity

– Geodesy and GPS

• Second key factor is determining the range of possibleground motions, given a likely set of scenario

earthquakes

( ) ( ) ( )Pr | Rate loading Rate EQ loading EQ= ×

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