Seismically Induced Landsliding in Seattle - Kate Allstadt
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2013 EERI Annual Meeting Session: Earthquake-Induced Ground Failures
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Seismically Induced Landsliding in Seattle - Kate Allstadt
- 1. Seismically Induced Landsliding in Seattle A Mw 7 Seattle Fault EQ Scenario Kate Allstadt with Art Frankel and John Vidale
- 2. 'Worst-case scenario' study for Seattle • Combination of methods from seismology, eng. geology, geotechnical eq. engineering • New: generated broadband synthetic seismograms including site amplification, to simulate landslide triggering • Mined rich geologic/geotechnical datasets that already exist for Seattle • Just one of many possible scenarios for one city...
- 3. Bigger picture • Seismically induced landslide hazard warrants more attention in Seattle and many other prone areas • Details of ground motion are important • Future? – library of plausible scenarios – probabilistic hazard maps – real-time triggered landslide hazard assessment
- 4. Earthquake triggered landslides = trouble worldwide
- 6. Seattle Seattle Fault Earthquake hazard Zone
- 7. Seattle Seattle Basin Seattle Fault Zone
- 8. Seattle Seattle Basin Steep unconsolidated slopes! Steep, landslide prone slopes Seattle Fault Zone Tubbs 1974
- 9. Seattle Dense population >600,000 people Seattle Fault Zone
- 10. Past seismically induced landslides in the area • Worst quakes haven’t happened yet since western settlement • Landslides triggered by at least 13 quakes since mid-1800s • 2001 Nisqually quake - $34 million in landslide damage Salmon Bay/ Olympia/Cedar River after Nisqually quake (from Highland, 2003)
- 11. Past seismically induced landslides in the area Karlin et al., 2004
- 12. Past seismically induced landslides in the area Karlin et al., 2004
- 13. What if Seattle Fault Earthquake Happened Today? Karlin et al., 2004
- 14. Approach • Simulate shallow landsliding triggered by a Mw7 Seattle fault earthquake • 5m grid, City limits of Seattle • Newmark method
- 15. Newmark Method Ground Motions q ac = critical acceleration a = ground acceleration ac = (FS -1)gsinq Factor of safety slope (from Harp et al., 2006) from Lidar Newmark (1965)
- 16. Fault rupture • 45km segment • Blakely et al., 2002 surface projection of frontal fault • Dips 45o toward south • Seismogenic from 3 to 15km depth (17km rupture width)
- 17. Fault Rupture Model 3150 subevents, 20 sec rupture time
- 18. Generate Broadband Synthetics Methods from Frankel (2009) Long Periods (<1Hz): Short Periods (>1Hz), Stochastic Finite Difference method using Andy method for each subevent Delorey’s 3D basin velocity model popping off at rupture velocity Combine with crossover at 1Hz
- 19. Realistic broadband seismograms (Available if you want to use them for other purposes) N Distance from fault (surface projection) M7 Cape Mendocino, Rrup 7km 2 2 S M7.6 Chi-chi, Rrup 10km Accelerograms PEER Ground Motion Database
- 20. Validation of base synthetics against 2008 NGA attenuation relations 3 km (± 1km) 2 Synthetic seismograms ±1std 1.8 Campbell and Borzognia 1.6 Abrahamson and Silva 1.4 Chiou and Youngs Mean of NGA 1.2 SA (g) 1 0.8 0.6 0.4 0.2 Directivity 0 pulse added to NGA relations 0.01 0.1 1 10 (Shahi and Vs30 = 600 m/s Period (T) Baker, 2011)
- 21. Basin effects become more apparent further into basin 15km Synthetic seismograms ±1std 0.8 3D velocity Campbell and Borzognia 0.7 model Abrahamson and Silva Chiou and Youngs 0.6 Mean of NGA 0.5 Basin effects SA (g) 0.4 0.3 0.2 0.1 0 0.01 0.1 1 10 Period (T) Vs30 = 600 m/s
- 22. 1D site amplification 600 m/s base layer
- 23. Built representative Vs profiles for entire city Vs (m/s) 0 500 1000 0 Vashon Till 10 20 Advance 30 Outwash Depth (m) 40 50 60 70 Lawton 80 Clay 90 Pre-Vashon Example Transfer Function from ProShake Equivalent linear methods to approximate transfer functions
- 24. Validation of transfer functions Compared with spectral ratios of Nisqually earthquake ground motions
- 25. Max PGA after applying transfer functions Up to 2g’s in some localized areas (thin low velocity layers over high velocity layers) Non-linear effects moderate gm in fill areas Before After
- 26. Relating Newmark Displacement to probability of failure • One curve exists: Relative Hazard Zones Jibson et al., 2000 • Based on Northridge Very landslide inventory High High Moderate • This project: Used Low Jibson curve to define hazard zones and designate slope This topic needs work! failures for scenario Incorporate new post-quake landslide inventories
- 27. Validation: Run simulation using Nisqually Ground 26 landslides Motions, Dry soils triggered Dearth of landsliding triggered – only in a few VERY steep areas
- 28. If soils had been saturated = different story 7,500 All soils completely landslide saturated sources to surface triggered (Thankfully, not a likely scenario)
- 29. How about the Seattle Fault scenario quake? • Dry soils • 5,000 landslides triggered • Hanging wall and coastal bluffs most affected
- 30. How about the Seattle Fault scenario quake? • Dry soils • 5,000 landslides triggered • Hanging wall and coastal bluffs most affected % of land that fails
- 31. And if soils are completely saturated? • 30,000 landslides triggered • Hanging wall and coastal bluffs worst • but inland and North Seattle affected too % of land that fails
- 32. Risk – Potential Infrastructure Impacts
- 33. Buildings at Risk Saturated soils Dry soils
- 34. Buildings at Risk Including runout (60-meter zone downslope, or till 2o) Saturated soils Dry soils
- 35. Linear infrastructure Within High to Very High Hazard Zones Dry soils Saturated soils ~1 km of each ~10 km of each
- 36. Linear infrastructure Within 60-meter runout zone (or to 2o) Dry soils Saturated soils ~10 km of each ~100 km of each
- 37. How do these results compare to static landslide hazard? – 36% of failed cells are outside City of Seattle's 'Potential sliding areas' – 28% of failed cells from this study are in low and medium static landslide hazard zones from Harp et al. 2006
- 38. Is the detail worth the trouble?
- 39. Is the detail worth the trouble?
- 40. Is the detail worth the trouble? To 50,000 ->
- 41. More research needed: • Relation between Newmark displacement and probability of failure • Frequency dependence of landslide triggering • How to include deep seated slides?
- 42. • Summary fault quake Landsliding triggered by a Seattle will be extensive and potentially devastating • Prone areas don't necessarily overlap with static landslide hazard zones • Details of ground motion make a huge difference • Methods can be expanded to more scenarios, probabilistic, and real-time applications
Editor's Notes
- -Talk about study I worked on with Art and John-look at seismically induced landslide haz in this city-This was a focused study of one scenario – a M7 quake on the Seattle fault
- -Considered a worst-case scenario-What makes this study different from others is that we generated broadband seismograms for the scenario earthquake and used those to trigger LS-To do this, we integrated methods-Rich datasets already available-BUT just one scenario earthquake for one city and you may be wondering why do I care? I don't live in Seattle (unless you do)
- -And I hope to demonstrate that this proejct we worked on serves to raise awareness, first of all, that seismically induced landslide hazard has the potential to be devastating to Seattle, but has received very little attention relative to other earthquake hazards, and I believe this is the case in other areas as well-I hope to show you that the details of the ground motion, spatial variability and site amplification, make a huge difference in the outcome, so using a single simplified ground motion parameter can severely underpredcit-and finally, that the methods we developed for this project would be fairly easy to expand to more scenarios for various earthquake types and various levels of ground saturation-If we do enoough scenarios and quantify the uncertainties, they can even be expanded to developing a probabilistic seisimcally induced landslide hazard map-and could also be set up to run automatically when an earthquake occurs using ground motions from our ever densifying strong motion networks to identify potential problem areas during rescue and recovery efforts
- LS triggered by EQs have been a big problem in many historical earthquakes-direct casualties-block access-disrupt infrastructure and slow down recovery after quake-but not every place – need landslide prone steep slopes, earthquake nearby (shallow crustal), and dense populations
- ...like SEattle Earthquake sources, subduction, in subducted plate, and in shallow crust above
- Southern edge of that basin is the Seattle fault zoneS dipping thrust fault Fault that most directly threatens the cityRecurrence interval not well known, every couple thousand years, last one 900 A.D., past recurrence intervals have been 200 to 12,000 years
- Southern edge of that basin is the Seattle fault zoneS dipping thrust fault Fault that most directly threatens the cityRecurrence interval not well known, every couple thousand years, last one 900 A.D., past recurrence intervals have been 200 to 12,000 years
- Make things even better, the city is made up almost entirely of unconsolidated soilsSteep in many places, cut by Puget sound, lake washington, rivers – steep landslide prone slopes shown in redParticular troublesome layers = permeable sand over impermeable clay, get water seeping out of hillsides at this contact
- And then of course we have our dense population of >600,000 people, important commericial and industrial areas on top of all this
- -There are signs of seismically induced landsliding here in the distant and not so distant past–Seds in Lake WA contain slides from at least 7 coherent events in last 3500 yrs, some sunken forests, lakewide turbidity currents-Nisqually eq caused $34 million in damages, dammed rivers, mostly shallow landslides (<3m), not much in Seattle proper-Other previous earthquakes also caused landslidinglandslide deposits from previous large earthquakes are preserved in Lake Washington->M6.8 Nisqually Earthquake 2001 - $34.4 million in damages from landsliding (Highland, 2003)Other Puget sound area eq’s in 1949 (M7.1) and 1965 (M6.5) and others also caused landslides
- The most ominous evidence is in the geologic evidence from the last major quake on this faultin the form of the remains of giant block landslides in the bottom of lake washington7 quakes over past 3500 years
- many from last quake in 900 AD, black ones on this map are actually forests that slid right into the water
- -shallow only, few meters thick-deep seated requires site specific info/detail we don't have/difficult on the scale
- since salish didn't have seismometers
- Mean magnitude of 4.6, moment and slip distribution modeled as a spatial random field with a correlation length corr to magnitude (mai and beroza, 2002)Rupture velocity is 70% of Vs, slows toward surface. Rupture vel randomized by 25%, never exceed Vs, rake varies by +-20degrees. Rise time depends on shear wave velocity at point of rupture.
- -First, put rupture model into a 3D velocity model that includes the Seattle basin developed by Andy Delorey-finite diff code propagates waves through the model and we record it on a fine grid throughout the city-computational limitations restrict us to running it only up to 1Hz-So for the short period portion of the signal, we use a stochastic method developed by Boore where each subevent is assigned a theoretical spectrum which is then multiplied by gaussian noise in the frequency domain to make it more realistic when transformed to the time domain-and then these subevent time series are sent off as each subevent occurs and they are added up at each station. -Then we combine the long and short periods with a matched filter.
- Available on 210 meter grid throughout the city if anyone wants to use them
- Validation of 1D amplificationCould be improved, but do a surprisingly good job
- Soil types were different
- -Nisqually didn't trigger much landsliding at all in the city because soils were dry-so we ran the landslide triggering simulation using the Nisqually ground motions as input-and found that it triggered hardly any landsliding-The few landslides triggered were in -shows that our model gives reasonable results
- Google earth flyover of results for dry scenarioInstead of individual failures, I show hazard zones for more complete pictureLandslide areas interspersed with private residences upslope and downslopeCritical roads potentially at risk
- Herren home in 1997 landslide set1916 Alki landslide
- 1965 rail damage
- Train derailed Dec 18, 2012
- We cannot assume that seismically induced landslides will only be triggered in areas already designated as hazardous by studies focusing on static slope stability
- Can we get the same results with way less work?
- Including site amplification