Determining Near‐Fault Fault‐Normal And Fault‐Parallel Site‐Specific Pulse Response Spectra And Selecting/Scaling Site‐Specific Fault‐Normal And Fault‐
Parallel Motions Including Simulated Motions & Spectral Matching Programs That Acceptably Include Pulse Motions Comments on Near‐Fault Ground Motions
N. Abrahamson
PG&E COSMOS Annual Meeting, Nov 14, 2014
Disclaimer
• Topic of the meeting is new Chapter 16 of ASCE 7‐2014
• My comments may not fit into the new Chapter 16 framework
Topics
• Near‐Fault Effects – Velocity Pulses
– Directivity
– Fling
• Deterministic near‐fault response spectra
– Accounting for directivity and fling
• Selecting near‐fault time histories
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How many have pulses?
What is the appropriate pulse period?
FN & FP directions or principal axes?
Use of finite‐fault simulations
• Spectral matching for pulse ground motions
Near Fault Effects
• Directivity
– Related to the direction of the rupture front
• Forward directivity: rupture toward the site
(site away from the epicenter)
• Backward directivity: rupture away from the site (site near the epicenter)
• Fling
– Related to the permanent tectonic deformation at the site
Velocity Pulses
• Forward Directivity
– Two‐sided velocity pulse due to constructive interference of SH waves from generated from parts of the rupture located between the site and epicenter
• Constructive interference occurs if slip direction is aligned with the rupture direction
• Occurs at sites located close to the fault but away from the epicenter for strike‐slip
• Fling
– One‐sided velocity pulse due to tectonic deformation
– Occurs at sites located near the fault rupture independent of the epicenter location
Directivity Parameters for Strike‐Slip Faults
Abrahamson (2000) Directivity Factors 5% damping, Ave Horiz, Strike‐Slip
Directivity 2008 models
(not centered on GMPE data) Centering addressed
by Choiu and Youngs
2014 GMPE
Directivity and Deterministic GM
• How to include directivity?
– Choose a worst‐case hypocenter?
– Choose an representative hypocenter?
• Based on deaggregation of hazard
– Randomize hypocenters?
• But doing probabilistic
Parameters for Simplified Directivity for NGA‐West2 Model Parameters
Strike‐Slip:
Rupture Length
Rx, Ry
Dip‐Slip:
Rupture Width
Rx, Ry
Watson‐Lamprey (PEER Report 2014) Simplified Directivity for NGA‐West2 • Model Results
– Correction to median GMPE (ln units)
– Correction to the standard deviation (ln units
• Use of Results
– Compute new median and standard deviation
– Compute new 84th percentile ground motion
Median with Directivity  GMPEmed exp Med _ Fac(Rx, Ry, L,W , Dip)
2
Sigma with Directivity   GMPE
 Sig _ Fac Rx, Ry, L,W , Dip 
Example Directivity Factors
SS, M7, Rx=3 km, L=80 km
Example 84th Percentile with Directivity Correction: SS, M7, Rx=3 km
Example 84th Percentile with Directivity Correction: SS, M8, Rx=3 km
Fling Effects
• Typically not addressed
• Fling effects on the response spectral amplitudes are captured by NGA‐W2 GMPEs for periods up to 5 sec
– Discussed at COSMOS 2012
• We know fling effects will happen for sites located close to the rupture of larger earthquakes
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There will be a permanent displacement across the fault that will take 2‐10 seconds to slip for M>7
– Methods for adding fling effects
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Kamai et al (2014) – BBSA
If fling effects are added, check the amplitude at periods < 5 seconds to avoid double counting of the effects
Example of Fling and Processing
Red=
fling Retained
Blue = standard processing
Example of Adding Fling into Time Histories
Selecting Pulse Time Histories
• How many should have a pulse?
• Model probability of pulse based on source/site rupture geometry
– Shahi and Baker (2011), use Rrup, s, theta
• Model probability of pulse based on the PGV epsilon
– Hayden et al (2014) use idea that the above average PGV are more likely to be from pulse records
Probability of Pulse Motions
From Hayden et al (2014). J. Geotechnical and Geoenvironmental Engineering
Model of Fraction of Pulse Motions Example
R=5 km and epsilon=1
7 time histories total
Use 65% pulse motions:
5 out of 7 should be pulse
From Hayden et al (2014)
Selecting Pulse Time Histories
• Classified pulse motions
– Lists by Shahi and Baker & Hayden et al
• Pulse period
– What range pulse periods to use?
• Shahi and Baker provide Tpulse(M) model
• Hayden et al uses two period ranges ( break at 2.2 sec)
• Finite‐Fault Simulations
– Are they ready to be used for pulse motions?
Pulse Period
Orientation of Pulse (Max PGV)
• Since 1990s, directivity has been explained as being polarized on the FN component
– Over sold this concept
• Wide range of orientation of maximum PGV
– Tendency for FN to be the largest PGV for forward directivity, but large range
• Consider using principal axes (max PGV direction) for time histories for the small set of selected GM
– Then capture the range of angles using larger set for statistical properties
Orientation of Maximum PGV
Larger IDP means
Forward directivity
From Hayden et al (2014)
FN/FP scaling
• Somerville et al (1999) FN/Ave_H model is still reasonable, but does not address maximum direction
• Earlier, Baker already discussed directionality
Example Pulse Record
Northridge: Sylmar Conv East FN
Scaling Pulse Records
84th Percentile GM and Pulses
• Deterministic 84th percentile GM
– Ground motion at the 1/6 level
• At long periods, above average ground motions (epsilon=1) are cause by pulses
– 84th percentile is an envelope of the 1/6 ground motions with a range of pulse periods
Spectral Matching Pulse Records
• Strongly held opinions:
• Method to reduce double‐counting of variability
– Keeps the level of shaking consistent with the number of epsilons (deterministic) or return period (probabilistic)
– Amplitude of the pulse is what caused the above average ground motion
• Tool of the devil that must be stopped
– Fitting narrow band pulse records to a broad spectrum reduces the amplitude of the pulse
– Destroying the key features of the ground motion
Matching Pulse Records
Example Matching Pulse Records
Evaluating Time History Methods
• Using small number of recordings (scaled or spectrum compatible) is a simplified approach
• To evaluate these simplified methods, compare risk using a full set of ground motions
• Full Set – conditional spectra
– 500‐1000 3‐component time histories, scaled to capture the hazard over wide period range (e.g. 0.1 to 5 sec) and a wide hazard range (e.g. 1E‐2 to 1‐E5)
– These will full represent the hazard at the site
– Compute hazard curve for drift (or other structural response parameter)
• Find the probability of exceedance of the mean structural response from simplified methods
– Is this small enough?