News You Can Use The New Seattle Urban Seismic Hazard Maps

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News You Can Use? The New Seattle Urban Seismic
Hazard Maps

• Casualty Actuaries of the Northwest
• Craig Weaver
• Seattle Field Office
• February 29, 2008

USGS Open-File Report 2007-1175
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The Big Picture Earthquake Country
(Old Friends)
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Approximate 50 year probabilities
How Often?

• Cascadia M9 10-14
• Seattle Fault M ? 6.5 5 (from slip rate, GR
  model 1000 yr return time) ???
• Deep M ? 6.5 84 (from 1949, 1965, 2001)
• Random shallow M ? 6.5 in entire Puget Sound
  area 15 (mostly from rate of M ? 4 since 1963,
  b0.8) ???

Probabilities from Art Frankel
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Strong Partnership began 10 years ago

• USGS joined Seattle Project Impact in 1997
• USGS did not want moneywe needed help knocking
  on doors
• USGS pushed hard at national level to show
  connection to local communities
• By late 1996 very clear that Seattle fault was a
  big regional problemexpected high public
  interest in the story

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A Stream of new Products from the USGS and UW

• Digital geologic map, 2003 continuing
• Digital geotechnical database (tied to geologic
  map)
• Digital, lidar-based topography map, 2002
• EERI Seattle fault magnitude 6.7 scenario, June
  2005
• Landslide hazard maps, August 2006
• Urban seismic ground motion maps, June 2007
• ShakeMap and improved real-time earthquake
  locations, Fall 2007?

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Why so much USGS effort?
The Seattle fault and its friends are dangerous
urban faults.
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The Seattle Faultthe early science part

• Recognized as active in 1992 from movement of
  tidal marshesprevious assessments discounted
  Seattle fault-style events
• Northridge (1994) and Kobe (1995) showed the
  power of large urban earthquakes
• Marine seismic studies began identifying Seattle
  fault as key player in regional tectonic setting
  (what causes earthquakes)

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The Seattle faultthe luck part

• USGS picked Seattle area for special effort,
  1996 to 2000. (Bag of cash fell on my desk)
• Large geophysical experiments attracted attention
  and public interest
• Robinson Point and Duvall earthquakes reminded us
  that this is earthquake country
• Seattle picked by USGS for Urban Seismic Mapping
  and picked by FEMA for Project Impact

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USGS SHIPS experiments were designed to
understand the structural setting of Puget Sound
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Gravity data outlines a system of deep basins and
possible fault locations. Relation between
surface faults and located earthquakes is
uncertain.
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The USGS has conducted a series of experiments in
Seattle to better estimate ground motions from
future events
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The Seattle Fault Zone is Complex
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Bellevue Surface Faulting Sites
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Church Parking Lot
Bedrock
Sediments
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Vasa Park Trench
South
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How Often??
The Seattle fault zone is one of a series of
major, active faults that cut across the Puget
Sound basin
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Known Crustal Fault EventsHow Often??
3 or 4 events in 3,000 yrs
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Where it all ends up National Hazard Maps
USGS work has significantly raised hazard
estimates in last decade, maps are currently
being updated
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Improving Seattles chance in the next
earthquake detailed local hazard maps

• Modern, comprehensive geologic map with digital
  database
• High-resolution topography with vegetation,
  buildings removed
• Greatly expanded ground motion recordings
• Site characterization (stuff at the surface)
• Better models of Seattle basin
• Fault breaks at surface for recurrence, size

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Ground Motion Amplification
Nisqually ground motions recorded on all
operating strong motion recorders in Seattle. At
1 Hertz, ground motions at most soft sites in
Seattle show amplification.
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Seattle Fault Earthquake Scenario
Basis for Sound Shake 08
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The Seattle Urban Seismic Hazard maps are built
from our 2002 maps

• Use same fault sources and recurrence rates as in
  2002 National Seismic Hazard maps
• Use same rock-site attenuation relations as in
  2002 maps multiply these values by amplification
  factors determined from 3D simulations and
  soft-soil amplification factors
• Seattle urban seismic hazard maps are built on
  state-of-the art modeling of our local faults
  using numerical simulations (458 for Seattle
  fault!)

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And Add

• 3-D sedimentary basin effects
• Non-linear site response of soft soils
• Rupture directivity

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Procedure to Make Urban Seismic Hazard Maps
PSHA Probabilistic Seismic Hazard Assessment
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541 3D finite-difference simulations used in
Seattle urban seismic hazard maps

• 458 simulations for earthquakes in Seattle fault
  zone (M6.6-M7.2)
• 9 simulations for earthquakes on South Whidbey
  Island fault
• 10 simulations for point sources on Cascadia
  subduction zone
• 48 simulations for shallow earthquakes 8
  azimuths, 3 distances and two depths (10 and 15
  km)
• 16 simulations for deep earthquakes (50 km
  depth) 8 azimuths and 2 distances

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Float rupture zones along fault trace, do nine 3D
simulation for each rupture
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Simulations capture uncertainty
Examples of amplification patterns (top) for two
simulations of Seattle fault earthquakes with
different slip on fault (bottom)
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We estimate the effect of the Seattle basin
1 Hz amplification relative to local rock sites,
for point source on Cascadia subduction zone (red
dot)
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Simulate background seismicity
Black shallow events Depth 10 and 15 km Red
deep events Depth 50 km
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We add local geology from the new detailed
Seattle geologic maps using the borehole data
(dots) compiled by K. Troost and D. Booth
(UW). Add nonlinear amp. factors from Choi and
Stewart (2005) for the areas of fill and
alluvium
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1 Hz Spectral Acceleration, 2 probability of
exceedance in 50 yr
From 3D simulations and nonlinear
amplification of fill/alluvium
From national map rock-site condition
From 3D simulations
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What Next for the Maps?

• Detailed briefings for interested departments,
  engineering, insurance groups under way
• Maps given to Washington Department of
  Transportation for their preliminary engineering
  of the new 520 bridge
• Derivative productssingle family dwellings,
  other periods, simulate urban isolation following
  Seattle fault event
• Maps planned for greater Eastside, Portland

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Bonus Slides What Next?

• Reliable ShakeMaps for Seattle and surrounding
  areasReal Time Assessment
• Liquefaction monitoring
• Seattle fault surface rupture near Seward Park?
• Tsunami hazards in Lake Washington
• Seattle fault strip map, Bremerton to Issaquah

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Slide 3
Slide 2
Merged lidar high resolution bathymetry
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Basis for Lake Washington tsunami study, Seattle
fault strip map
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Coming Ground-penetrating radar lines, seismic
reflection and offshore magnetic survey
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Contact Information Craig S. Weaver Pacific
Northwest Earthquake Program Coordinator United
States Geological Survey _at_Department of Earth and
Space Sciences University of Washington, Box
351310 Seattle, WA 98195-1310 206-553-0627 craig_at_e
ss.washington.edu