RECENT DEVELOPMENTS IN
STRONG MOTION SIMULATIONS
FOR CEUS
Paul Somerville and Robert Graves
URS Pasadena
MOTIVATION: CEUS ground motion models
are based on simulations validated against
sparse data
March 7, 2008
NGA-East 2nd Workshop
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OUTLINE
• Strong ground motion simulation & validation
• Seismic wave propagation
• Effect of crustal structure on ground motions
• Review of EPRI ground motion modeling
• Review of CEUS ground motion model (USGS),
and modeling of NSN data (NRC)
• Improvement of simulations and models
March 7, 2008
NGA-East 2nd Workshop
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NGA-E
NGA-East
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NGA-East 2nd Workshop
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Broadband Ground Motion Simulation
Elastodynamic Representation Theorem:
Ground motion U(t) can be calculated from the
convolution of the slip time function D(t) on the fault
with the Green's function G(t) for the appropriate
distance and depth, integrated over the fault rupture
surface
U(t) =  D(t) * G(t)
Combine long period and short period simulations to
generate broadband time history
March 7, 2008
NGA-East 2nd Workshop
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Validation of Ground Motion Simulations
Against Strong Motion Recordings
• Measure the difference between recorded and
simulated ground motion (response spectral
residuals)
• Bias: Do the simulations systematically underor over-predict the recorded ground motions?
• Standard Error: What is the average difference
between recorded and simulated values?
March 7, 2008
NGA-East 2nd Workshop
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Validation
against Arleta
recording of
Northridge
earthquake
Recording
Simulations
Validation
against a large
set of Northridge
earthquake
recordings
March 7, 2008
NGA-East 2nd Workshop
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Validation against WUS earthquakes
March 7, 2008
NGA-East 2nd Workshop
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Wave Propagation
• Critical reflections from the lower crust
• Contributions of different wave arrivals to
ground motion attenuation
March 7, 2008
NGA-East 2nd Workshop
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Critical Reflections from the Lower Crust
Direct, upgoing:
becomes weak
with increasing
distance
Reflected, downgoing:
becomes strong beyond
the critical distance
30 km
40 km
March 7, 2008
NGA-East 2nd Workshop
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Data
Simulation
Direct (S)
fades as
downgoing
(ScS, SmS)
strengthens
Long duration –
due to scattering
March 7, 2008
This simulation
has no scattering
NGA-East 2nd Workshop
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Direct (upgoing)
waves are
strongest at close
distances but their
amplitudes fade at
larger distances
Reflected
(downgoing)
waves are weak
at close distances
but dominate at
larger distances
(beyond about 50
km)
March 7, 2008
NGA-East 2nd Workshop
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1988 Saguenay earthquake
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NGA-East 2nd Workshop
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Harvard data and simulation
Simulations work well at long periods
sSms,
SmSSmS
SmS etc.
recorded
simulated
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NGA-East 2nd Workshop
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March 7, 2008
NGA-East 2nd Workshop
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Contributions of Different Waves
Individual
body waves
Combination
Complete
simulation
March 7, 2008
NGA-East 2nd Workshop
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Review of EPRI (1993) Ground
Motion Modeling
• Survey of crustal structure in CEUS
• Simulation analysis of the effect of crustal
structure on ground motion attenuation
March 7, 2008
NGA-East 2nd Workshop
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Grenville
N. Appalachian
granite
Conrad interface
gabbro
Moho interface
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NGA-East 2nd Workshop
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Crustal Structure Regions
Grenville
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NGA-East 2nd Workshop
Northern
Appalachian
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Simulated
attenuation in
16 different
regions
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NGA-East 2nd Workshop
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Crustal Structure Regions
Grenville
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NGA-East 2nd Workshop
Northern
Appalachian
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Effect of Crustal Structure
Eq Depth 11 km Eq Depth 15 km
N. Appalachians
Conrad Layer
Grenville - No
Conrad Layer
Eq Depth 7 km
March 7, 2008
NGA-East 2nd Workshop
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NRC Project (1998): Modeling
Attenuation of NSN Data in the Grenville
Province (5 yrs of data)
o Data
-
Simulation
-- Model
March 7, 2008
NGA-East 2nd Workshop
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USGS Project (2001):
CEUS Ground Motion Model from
Simulations
• Earthquake Source Scaling and Ground
Motion Attenuation Relations in Central
and Eastern North America, Somerville et
al. (2001)
• Comparison with results obtained using
the stochastic model: Atkinson and Boore
(1995); Toro et al. (1997)
March 7, 2008
NGA-East 2nd Workshop
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Generating Ground Motion Models
from Simulations
• Earthquake source scaling relations were
derived from the source parameters of 3 eastern
North American events (Hartzell, 1994)
• The source scaling relations were used to
generate a large suite of ground motion
simulations using a broadband Green’s function
method
• The simulated ground motions were used to
generate a ground motion prediction model
March 7, 2008
NGA-East 2nd Workshop
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Hartzell (1994) Rupture Models
March 7, 2008
NGA-East 2nd Workshop
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Rupture Area Scaling
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NGA-East 2nd Workshop
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Scaling of Rise Time (slip duration on fault)
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NGA-East 2nd Workshop
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Source Scaling Relations in the CEUS
• Rupture area is smaller (0.4) than for tectonically
active regions
• Average slip is 2.5 times larger than for
tectonically active regions
• Rise time is 1.85 times larger than for
tectonically active regions
• Slip velocity is 35% larger than for tectonically
active regions (higher dynamic stress drop)
March 7, 2008
NGA-East 2nd Workshop
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Comparison of Simulation-Based
Ground Motion Prediction Models
M 6.5
March 7, 2008
NGA-East 2nd Workshop
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Ways to Improve CEUS Ground
Motion Prediction Models
• Use 15 years of NSN / ANSS data from CEUS
to test earthquake source scaling and wave
propagation models
• Apply broadband simulation methods,
developed and validated in the WUS, to the
CEUS environment
• Rigorously validate predictions against
augmented CEUS data
March 7, 2008
NGA-East 2nd Workshop
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Resources for Simulation
• SCEC Broadband Strong Motion
Simulation Platform (Three Methods)
• USGS Golden: Set of Four Strong Motion
Simulation Methods (Steve Hartzell)
March 7, 2008
NGA-East 2nd Workshop
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SCEC Kinematic Broadband Ground
Motion Simulation Platform
Calculate Kinematic Broadband (0-10 Hz)
Waveforms for Scenario Earthquakes
• Validation of simulations
• Verification of simulations
• Platform brings objectivity,
transparency and repeatability to
strong motion simulations
March 7, 2008
NGA-East 2nd Workshop
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Source Description
Low Frequency
Simulation (< 1 Hz)
• CFM, ERF
• Mw, Dimension,
Geometry
•…
1. 1D / 3D GF
2. 3D AWM
3. Site Effects
4. …
Kinematic Rupture
Generator
1. Beroza
2. Archuleta
3. Graves
4. …
Standard
Rupture
Format
SCEC Broadband
Simulation Platform
March 7, 2008
•
•
•
•
GF Libraries
Site Lists
Velocity Models
…
Combine Low &
High Frequency
1. Filters
2. Time Shift
3. …
High Frequency
Simulation (> 1 Hz)
1. 1D / Ray GF
2. Scattering
3. Site Effects
4. …
NGA-East 2nd Workshop
Broadband
Time Histories
(0 – 10 Hz)
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SUMMARY
• CEUS ground motion prediction models are
based on simulations validated against sparse
data
• We now have 15 years of high quality ground
motion data from NSN / ANSS in the CEUS
• We have advanced broadband strong motion
simulation procedures that have been validated
in the WUS
• We have a platform for broadband strong motion
simulation that brings objectivity, transparency
and repeatability to strong motion simulations
March 7, 2008
NGA-East 2nd Workshop
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