Brief summary of Seattle URM Policy Committee actions

Recent inventory of Seattle URM’s by Nancy Devine; about 1160 URM’s Debate about cost-benefit studies Bolts-plus, life safety retrofit Final meeting of URM Policy Committee was April 4 (Policy Committee started in 2012; Technical Committee 2008) Staff is writing final report of Committee to be reviewed by Committee and then go to Mayor and City Council (initial report was in 2013) Recommends city ordinance for retrofit; provides different timelines for retrofit depending on factors such as purpose of building, number of stories, soil condition, number of occupants (7-13 year timeline for completion) Various funding options and incentives considered; list provided in report

Derived from 3D Ground-Motion Simulations Broadband Synthetic Seismograms for Magnitude 9 Earthquakes on the Cascadia Megathrust Derived from 3D Ground-Motion Simulations A. Frankel1, E. Wirth2, J. Vidale2, W. Stephenson1, N. Marafi2 1U.S. Geological Survey 2University of Washington PNSN RAC meeting May 23, 2017

The M9 Project: University of Washington funded for 4 years by NSF USGS is collaborating with UW in producing broadband (0-10 Hz) synthetic seismograms for M9 Cascadia earthquakes; considering a range of rupture scenarios Assessment of Hazard, including uncertainties Assessment of Impact Evaluation of tall building response and damage from long-duration, long-period ground shaking (Berman, Eberhard, Marafi) Evaluation of landslides and liquefaction from ground shaking (Duvall, Wartman, Kramer, Greenfield, Grant) Evaluation of tsunami effects on structures near coast (Motley, LeVeque, Gonzalez) Development of Shakemaps and tsunami- Inundation maps for emergency management, improving community resilience (Bostrum, Abramson) Testing of Earthquake Early Warning (Vidale, Bodin) Synthetic seismograms produced from 3D simulations of M9 Cascadia earthquakes (Frankel, Wirth, Marafi) Tsunami simulations for M9 Cascadia earthquakes (Gonzalez, LeVeque) Supercomputer time provided by Pacific Northwest National Laboratory and Texas Advanced Computing Center, University of Texas

see SSA poster by Thompson et al. Bill Stephenson developed the 3D velocity model for Cascadia. Used seismic refraction/refraction data and tomography for Seattle basin, Moschetti et al. (2010) crustal tomography, McCrory et al., plate interface ; Seattle basin model was validated by modeling waveforms and basin amplification of M6.8 Nisqually earthquake and other local earthquakes (Frankel et al., 2009); also see SSA poster by Thompson et al. 100 m grid spacing in top 5 km 300 m horizontal spacing 5-60 km depth N S 1100 km 550 km Minimum Vs= 600 m/s, Similar to surficial glacial sediments Figure from Delorey et al. (2014)

Background slip Ave slip velocity= 0.65 m/s Max rise time 25 s Deeper, high stress drop M8.0 sub-events (asperities) Ave slip velocity = 5.4 m/s Max rise time 1.5 s Mw 9.0 compound rupture model based on modeling strong-motion records of Mw 9.0 Tohoku, Japan and Mw 8.8 Maule, Chile earthquakes (Frankel, 2013, 2016) and Mw 8.3 Tokachi-Oki earthquake (Wirth et al.) For most Cascadia cases, sub-events dominate ground motions for periods less than 8 s Sources are additive; rupture proceeds from same hypocenter for the background and sub-events

M9 CSZ Simulation Results Movie by Nasser Marafi, UW Seattle Snoqual mie Portland Medford 300 second simulation

Northern hypo; easternmost downdip edge 200 bar sub-events Broadband (0-10 Hz) synthetic seismograms from one of the M9 simulations Combines 3D simulations (≤ 1 Hz) with stochastic synthetics ( ≥1 Hz) using matched filters 0.2 g Black: NS Red: EW Green UD 0.6g 0.4g 091316r10 run Northern hypo; easternmost downdip edge 200 bar sub-events

m Map of 3 second spectral acceleration (g) From 110416 run Hypo at 43.0N 125W Middle choice for downdip Edge Synthetics from simulations are saved on a grid with spacing of 1 km m

“Logic Tree” for M9 simulations we will do 50 simulations to sample logic tree and assess sensitivity to rupture parameters Down-dip edge of rupture

Hypo near northern sub-event Easternmost choice of downdip edge Sites in Seattle and Tacoma basins 3 second response Spectral accelerations From 2 M9 simulations (blue and red crosses) With BC Hydro prediction And observations from Maule earthquake for Close-in stations 11,000 points, 4 km grid spacing Sites in Seattle and Tacoma basins Hypo at trench; lat. of Seattle Mid choice for downdip edge

Amplification of response spectra at Seattle Basin sites relative to site outside of basin Observations from 2001 Satsop earthquake and results from M9 simulations Uses geometrical average of SA from 2 horizontal components; similar vs30’s inside and outside basin. Basin amplifications are higher than found from crustal earthquakes. Approx. basin amp term from Campbell and Bozorgnia (2013) Crustal earthquakes, Z2.5= 6.5 km

ground motions < 8 s dominated by sub-events Relatively bad and good scenarios for Seattle. Both scenarios use same background slip ground motions < 8 s dominated by sub-events Forward rupture directivity affecting Seattle region Rupture not directed toward Seattle region Same background slip used for both cases M8.0 sub-events M8.0 sub-events “

in simulations done to date Response spectra for Seattle basin site from better and worst-case scenarios in simulations done to date compared to design spectrum (MCEr) used in new 40 story building in Seattle “worst case” Better case

Take Home Points Basin amplification: Finding Seattle basin amplification larger than that found in empirical study of crustal earthquakes in other regions (NGA West 2). Factor 2-4 compared to factor of 1.5 (relative to rock site just outside basin); Basin-edge generated Rayleigh wave causes much of basin amplification at 1-3 s. Basin amplification depends on azimuth of incoming energy and rupture directivity. Inter-event variability and uncertainty: Synthetic response spectra (1-10 s) are sensitive to hypocenter, slip distribution, location of sub-events (asperities) and down-dip edge of rupture. Factor of 4 variation in response spectra (1-10 s) observed for different sub-event slip and hypocenters. Stay Tuned for the results from all 50 simulations; synthetics will be available via Design Safe at University of Texas (TACC)

Guess the M9 synthetics (SP2) and actual records from Maule and Tohoku at comparable distances Velocity waveforms (0.25-1.0 Hz) m/s Time (s)

Guess the M9 synthetics (SP2) and actual records from Maule and Tohoku at comparable distances Velocity waveforms (0.25-1.0 Hz) M9 M9 M9 M9 m/s Maule EQ, CRMA near Santiago Tohoku EQ, MYGH12 EW Time (s)