October 26, 2010 Web-ex
West-2 NGA Nonlinear Site Factors Meeting-2
Duration of Project: 30 months from March 1st to be finished at the end of Feb 2012
Group: Badie Rowshandel, Donald Anderson , Emel Seyhan, Jonathan P. Stewart, Ken Campbell, Maury
Power, M. Idriss, Walt Silva, Tim Ancheta, Robert W Graves, Roger Borcherdt, Yousef Bozorgnia (Names
in bold are not joined)
Minutes
Work done so far :
- Proposal drafted and funded 1 month ago,
- Comparison of NEHRP Site Factors (SFs) and NGA SFs updated as follows:
o We evaluated average SFs across period ranges for Fa and Fv. The average of SFs in log
units are shown in addition to upper and lower limits along with SFs at the middle
frequency (0.3sec for Fa, 1.2 sec for Fv)
o To identify a representative Vs30 in each site class, the NGA site database was used to
evaluate histograms within NEHRP categories B, C, and D. Weighted average value Vs30s
were calculated for each category and were used for SF evaluation with the NGA
models.
o Since the SFs were coded as part of this project, spot checks of the results were done
using the full NGA models as coded by Linda Al-Atik.
- NGA errata was checked to ensure that the AS coding is up to date
Database
- > 6000 recordings
- hor1, hor2, ver components for each case more than 100 periods, 11 damping ratios from 0.130%. GMRotI50 to be included soon
- The data is organized into a Flatfile format. Unlike original NGA, the site and source database
files have not been kept up to date. Chiou will be doing a critical review of source parameters.
We (Emel and JPS) offered to help with the site database. Possibly use virtual geotechnical data
center in this process (contact Tom Shantz for status).
- Need to update basin depths for LA and SF.
- Wenchuan mainshock earthquake data will be included (IMs, but possibly not full waveforms)
- Italian data included
- Events not included:
o Wenchuan aftershocks
o some Greek eqs
o Baja eq (will be added)
o New Zealand (will be added)
- Contact Tim Ancheta and cc Walt Silva if any typo or errors are found in database
- There is still a need to coordinate the site database work with the work of A. Yong of Pasadena.
Either Walt Silva or Stewart/Seyhan will do this.
Discussion of Task 2: Evaluation of Site Factors for Comparison to NGA & NEHRP models
Reference Site Approach
- Identify soil/rock pairs (need to develop criteria for selection)
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Use original Loma Prieta sites that Roger used in his analyses
Identify additional pairs from Taiwan, Parkfield, Wenchuan, etc. When site database ready,
compile locations in Google Earth to find good pairs
Compare SFs to
o Site factors from non-reference site approach
o SFs from NEHRP and NGA models
Look at standard deviations
Non-Reference Site Approach
- Evaluate residuals as of new and original NGA data relative to:
o GMPEs for VS30=760 m/s. Use event terms to evaluate intra-event residuals.
o GMPEs for VS30=760 with modification by NEHRP factors
- For first case above, compare results to NGA and NEHRP factors. For second case above, look for
biases.
- Look at site metrics beyond Vs30 such as f0, basin depths.
General Considerations
- Emphasize soft soil sites. Site period f0 could be a powerful predictor for such sites.
- Provide recommendations to NGA developers about site factors
- Possibly recommend revised NEHRP factors. These may be expressed in terms of values in tables
(as they are now) and also as equations.
- Include epistemic uncertainty on site factors in final tables/equations.
Other discussions:
- Project website: Marcos will set up
- W9 forms needed to set up contracts.