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Unreinforced Masonry Seattle Earthquake Hazard

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Unreinforced Masonry Seattle Earthquake Hazard Seattle’s Earthquake Hazard Most earthquakes occur at tectonic plate boundaries YOU ARE HERE YOU ARE HERE JUAN DE FUCA PLATE PACIFIC PLATE ‘Subduction zone’ earthquakes are the BIG ONES, occurring where the downgoing plate is usually stuck. In a 50-year window, there’s a 10-14% chance of a M9 subduction zone earthquake. Fault dimensions M7.8 Great M9 Cascadia ‘San Francisco’ Earthquake Earthquake 1/26/1700 4/18/1906 M9.2 Sumatra Earthquake 12/26/04 20 meters (~60 feet) of motion in seconds! M7.8 Great M9 Cascadia ‘San Francisco’ Earthquake Earthquake 1/26/1700 4/18/1906 M9.2 Sumatra Earthquake 12/26/04 Mud on the sea-bottom keeps a record of many great earthquake s. ‘Intraplate’ earthquakes tend to be moderate in size & deep, occurring as the plate flexes on its way down. In a 50-year window, there’s an 84% chance of a M6.5 intraplate earthquake. ‘Crustal’ earthquakes occur because all the plate motion isn’t taken up along the subduction interface. In a 50-year window, the chances are 5% & 15% of an earthquake on the Seattle fault & of a crustal earthquake anywhere in the Puget Sound region, respectively. The Tibetan plateau is a buttress, pushing into Eurasia. The northward collision of India into Asia is occurring at ~4 cm/yr. The buttress of Tibet results in eastward displacement at ~4 mm/yr. The resulting stresses are released in crustal earthquakes. 300-500 yrs. yrs. 300-500 great earthquakes every every earthquakes great interface, until it breaks in in breaks it until interface, stuck part of the plate plate the of part stuck Stresses build along the the along build Stresses also collide at ~4 cm/yr. cm/yr. ~4 at collide also North American plates plates American North The Juan de Fuca & & Fuca de Juan The P E l a xp t e l Juan de de Juan o r e r Plate Fuca Fuca North American Plate et al al et 2007 from McCaffrey McCaffrey from earthquakes. mm/y, relieved in crustal crustal in relieved mm/y, results in motion ~4 ~4 motion in results North American continent continent American North The buttress of the ancient ancient the of buttress The along the plate interface. plate the along accumulated & released released & accumulated Not all the stresses are are stresses the all Not North American Plate Lots of faults! Individual faults may have long recurrence times, but the collective failure probability is higher, occasionally with dire consequences. The Wenchuan earthquake broke a fault ~225 km long, bounding a deep basin. http://quake.mit.edu/~changli/wenchuan.html Aftershocks are numerous, large, and will continue for years. http://quake.mit.edu/~changli/wenchuan.html The displacement on the Longmen Shan fault varied greatly. The largest fault slip (8-10 m) was 60 km NE of the epicenter. The larger the fault slip & closer it is to the surface, the greater the shaking. Fortunately the fault break propagated northeastward, focusing energy away from Chengdu. The epicenter tells us only where the break started. The mapped Seattle fault is a LOT shorter and the likely magnitude MUCH smaller (~M7). The South Whidbey Island fault may be significantly longer. Ground truth. Ground truth. Paleo-earthquakes. The Chinese landscape The Pacific Northwest landscape Seattle Seattle Courtesy R. Haugerud, USGS Local Geologic Structure is Important. Horizontal shaking from explosions recorded along a north-south line traversing the Seattle Basin, basin waves basin waves where sediments amplify and extend the shaking. SHIPS Experiment Seismograms High-resolution surficial/shallow geologic data; the GeoMap Proje http://geomapnw.ess.washington.edu 3D Simulation of the 2001 Nisqually Earthquake Shaking peak velocitie s lots of damage little damage 3D Simulation of the Lots of Simulations -> 2001 Nisqually Earthquakes south of Earthquake Shaking Seattle amplify more! peak velocitie amplification amplification s lots of damage little damage amplification amplification We ‘ground truth’ Lots of Simulations -> these with real Earthquakes south of measurements. Seattle amplify more! amplification amplification amplification amplification S W N E S 1 Hz Spectral Acceleration (%g) with 2% chance of being exceeded in 50 years one of 2002 national Using 3D simulations Using 3D simulations seismic hazard maps; with basin effects and nonlinear rock site condition and directivity ampl. for fill/alluvium GroundHazard Motion curves forHazard 5 Hz S.A.Curves These describe the likelihood of SHAKING, not a particular earthquake! GroundHazard curvesMotion for Hazard 1 Hz S.A. Curves 1/500-yr= .002/yr 10% PE in 50 YR (0.1/50-yr = .002/yr) GroundHazard curvesMotion for Hazard 1 Hz S.A. Curves 1/2500-yr= .0004/yr 2% PE in 50 YR (0.02/50-yr = .0004/yr) GroundHazard Motion curves forHazard 5 Hz S.A.Curves 1/2500-yr= .0004/yr 2% PE in 50 YR (0.02/50-yr = .0004/yr) Thank You! Questions? http://earthquake.usgs.gov/regional/pacnw http://www.pnsn.org.
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