Ground Motion Prediction Equations for Eastern North America

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Ground Motion Prediction Equations for Eastern
North America

• Gail M. Atkinson, UWO
• David M. Boore, USGS
• (BSSA, 2006)

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Atkinson and Boore 2006 ENA relations

• Based on stochastic finite-fault model (of
  Motazedian and Atkinson, 2005)
• Key source parameter is stress drop determined
  from compilation of instrumental and historical
  data
• Attenuation model based on empirical trilinear
  model of Atkinson, 2004 for ENA

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Stochastic finite fault model (Silva Beresnev
and Atkinson Motazedian and Atkinson, 2005)Key
features - Subsources are Brune point sources-
long-period controlled by moment, short-period
controlled by stress - Results independent of
subfault size
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Parameters needed to apply stochastic
finite-fault model

• All parameters needed for stochastic point source
  model stress drop, attenuation with distance,
  site amplifications, physical properties
• Geometry of source (can assume fault plane based
  on empirical relations such as Wells and
  Coppersmith on fault length and width vs. M, in
  this case assumed to be 1/3 of western fault area
  for a given M)
• Direction of rupture propagation (assume random)
• Slip distribution on fault (assume random)

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High-frequency spectral level depends on stress
drop (as in point-source model)
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ENA Model Parameters and their aleatory
variability (AB06)
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Illustration of typical decay of spectral
amplitudes for m13.75. Line is trilinear shape
fitted to the data. Transition distances are
r0170 km, ro2 140 km. Slopes of geometric
attenuation -1.3, 0.2, -0.5
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Amplitudes decay faster than 1/R at Rlt70 km.
This has important implications for ENA ground
motion relations.
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ENA stress drops, based on high-frequency
spectral level. Mean 140 bars.
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AB06 simulationsLog values of horiz, component
5 PSA at frequencies 0.5, 1, 5 Hz, and PGA, for
ENA rock sites. Dots show PSA from simulations,
including aleatory uncertainty, for M 5 (light)
and M 8 (dark). Solid lines show predicted
amplitudes from regression equations developed
from simulated database, for M5, 6, 7, 8
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AB06 rock simulations (dark greymid-point M,
light grey/-0.5M) equations (black) data
(M-0.5 green, M0.5 red)
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AB06 rock simulations (dark greymid-point M,
light grey/-0.5M) equations (black) data
(M-0.5 green, M0.5 red)
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Comparison of AB06 eqns to M4.9 2005 Riviere du
Loup data
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Comparison of AB06 eqns to several M4.5 events
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Comparison of equations to data for M5.8 and M7.6
Saguenay and Bhuj events note high PGA for
Saguenay, well above AB06 predictions for M5.8
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Comparison of ground-motion equations of this
study (solid red lines) for M 5.5 and 7.5, with
previous predictions (Atkinson and Boore, 1995,
black), and mean and standard deviation of
alternative EPRI (2004) predictions (blue), all
for hard-rock site conditions in ENA.
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Main epistemic uncertainty is in stress drop
our preferred average value is 140 bars, but
alternative interpretations of data could be
higher (200 bars?). We include factors to adjust
the predictions for higher or lower stress drops.
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AB06 ground-motion equations are given for 2 site
conditions

• Base condition for development and comparison to
  ENA data is hard-rock (vs30gt2000 m/s)
• We also perform the same set of simulations for
  B/C boundary site conditions, for an assumed B/C
  profile (from Frankel et al., 1996)
• Separate sets of equation coefficients for each
  set of simulations

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AB06 predictions for BC for ENA vs. BA07 active
regions- note plotting error in AB06 (rock vs.
B/C)
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Conclusions

• New ground motion relations for ENA, based on a
  stochastic finite fault model
• Results in good agreement with ENA ground motion
  data
• Factors given to adjust to all site conditions,
  based on empirical non-linear amplification
  studies in California
• Factors given to adjust for alternative stress
  drop values

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Significant Issues

• Limited M gt 4 ground motion database
• Vertical vs. horizontal ground motion
• Stress drop of ENA source spectrum
• Near-source geometrical attenuation
• Kappa of NEHRP B-C site profile