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Earthquake Hazard Assessmentin the Pacific Northwest:Site ResponseThomas L. PrattU. S. Geological SurveySchool of OceanographyUniversity of WashingtonEarthquakes are inevitable:How do we deal with theireffects?Design buildings to an appropriate level ofsafetyPrepare emergency services to respondInform citizens how to prepare and respondEstimating the ground shakingfrom earthquakesProbabilistic Hazard MapsPeak Ground Acceleration likely in a giventime intervalExample: 2% chance of 0.25g acceleration being exceeded ina 50-year time spanNeed to know:How often are different types ofearthquakes likely to occur?How strong will the shaking be?How will the shaking vary across theregion?At what frequencies will the ground shake?Earthquake Hazard AnalysisPlate motionsGPSEQ monitoring (PNSN)Geophysics ard Maps
Washington State EQsProbabilistichazard mapBuildings and Resonant Frequency1-story building (house) 0.1 sec period 10 Hz10-story building 1 sec period 1 Hz [e.g., library]35-story building 3.5 sec 0.3 Hz [e.g., Smith Tower]75-story building 5 sec 0.13 Hz [e.g., Columbia Tower]ProbabilisticHazard MapsGroundMotionModelingSite ResponseEstimatesBuildingDesign
Simple Spectral Ratio – site/bedrock siteH/V ratio (horizontal/vertical recording)Horizontal siteHorizontal bedrockHorizontal siteVertical sitesite response curveamplification .Spectral ratios(to eliminate source signature)1086420G7009eq 18eq 28averagenoise01020frequency (Hz)resonance,impedancechangesFocusingFr 1/4TA Ir/Is !rvr/!svseq 6Resonance modelsFocusing in Seattle Basin
Surface WavesPuget Lowlandmovie1999 “Dry” SHIPSGeophysical dataChi-Chi earthquakeLocal earthquake, M2.8Local earthquake, coda magnitude 2.8UTM easting 21038time(sec) - X/6.86620000
Seattle SHIPS 2002Low freqs:large overSeattle BasinHigh freqs:Small overSeattle BasinSeattleSHIPS200148.5Seattle SHIPS 200248.090 seismometers (3-component) recordingcontinuously from Jan 27 to May 24, 2002Identical instrumentation with 2 Hz sensorsLocal: magnitude 2.0 to 2.1 or greatervisible across array ( every 3 days?)Teleseisms: magnitude 7.0 to 7.3 or greatervisible across array ( 22.50Longitude122.00121.50LatitudeSeattle SHIPS (site response)
Chi-Chi earthquake(surface waves long durations)Kingdome SHIPSKingdome demolitionMovieNisqually EarthquakeEffects of surface wavesRelatively modest ground motionsMaximum PGA of 0.27gNon-linear soil responseAmplifications much smaller for strong groundmotions (mainshock versus aftershock)
0.5 Hz Amplification Factor 15-55 sec after OT 0.5 Hz Amplification Factor 40-65 sec after OTnisquallyampvariationwith time5 km3.3 Hz Amplification for MainshockNisquallyamp vsafteramp5 km5 km3.3 Hz Amplification Factor5210.5 Hz Amplification Factor521Results from SHIPS/NisquallySeattle Basin characterized by amplifications of 8to 12 at low frequencies(0.3 to 0.8 Hz (3.33 sec to 1.25 sec periods)At higher frequencies (5 to 10 Hz) the basincauses decreased shakingAmplification is time variant(direct arrivals versus surface waves?)Amplifications of 5 could come from resonance inthe shallow deposits ( 500 m)Amplifications from focusing likely 2Surface waves within the basin causeamplifications of as much as 16Non-linear effects come into play at moderateground accelerationsEND3.3 Hz Amplification for M3.4 Aftershock
(surface waves long durations) Kingdome SHIPS Kingdome demolition Movie Nisqually Earthquake Effects of surface waves Relatively modest ground motions Maximum PGA of 0.27g Non-linear soil response Amplifications much smaller for strong ground motions (mainshock versus aftershock)
