Technical Updates: 2013, Third Quarter

Combined efforts

NGA-East Special Session at SMiRT22, San Francisco, August 23

Data Sets (Task A)

The initial reprocessing of the database was completed in early September. That was followed by a round of QA/QC. Interactions between Dr. Goulet (PEER) and Drs. Kishida (PEER), Silva and Darragh (Pacific Engineering Analysis) led to a change in the coda window length. The data is being reprocessed using the new windows.
A PEER report documenting the data collection and processing is under preparation.

Reference rock and site amplification models (Task B)

All the tasks below are being led by the Geotechnical Working Group (GWG). The group continues to be very active with their tasks and have monthly meetings attended by the TI Lead Dr. Goulet.
- Revisions are being made to a journal paper submitted on reference rock condition.
- Revisions are being made to the report on reference rock condition after review by the TI team.
- Revisions are being made to a manuscript on using a Vs30 proxy at recording stations using P-wave seismograms for submission.
- The group is developing a paper on a Vs30 proxy method based on topographic slope and conditioned by regional geology in CENA.
- The group continues its work on establishing thresholds for when a nonlinear site response analysis is needed in addition to equivalent linear site response analysis. These thresholds will be used in defining the response analysis required for the large-scale simulations described next. The group is developing a paper on these thresholds.
- The group continued their development of protocols of the large scale site response simulations that will be conducted using input ground motions and typical soil profiles. The group is finalizing the development of idealized Vs profiles from about 900 measured Vs profiles from various NPP license applications and the open literature for CENA. The idealized Vs profiles are geology-dependent and divided in to 9 categories: Old glacial, Old non-glacial, Young glacial marine, Young glacial non-marine, Young non-glacial marine, Young non-glacial non-marine, Residual from metamorphic rock, Residual from sedimentary rock, and Residual from soil. The large scale simulations will account for variations in sediment depth in the profiles, variations in properties of the weathered rock zone above the reference rock, and include a randomization scheme for soil properties and velocity profiles. The analysis will include approximately 2 million site response simulations. The output of these simulations will be used for development of nonlinear site amplification functions or models.
Dr. Boore (USGS) has revisited the amplification factors he originally developed for the Atkinson and Boore (2006) (AB06) GMPE. The site amplification factors consist of very hard rock (VHR) to NEHRP BC-boundary conditions ratios (VHR/BC). These factors were part of the preliminary site correction model prepared by the Geotech WG to be used in the simulations validation exercises for CENA. The original ratios from AB06 showed kinks with distance where the attenuation function also had kinks. Using his SMSIM code (Boore, 2009) to generate sets of ground motions, Dr. Boore developed updated ratios to be used in conjunction with the AB06 GMPE.

Regionalization & Source/Path Studies (Tasks C and D)

The two TI leads (Drs. Goulet and Abrahamson) organized a regionalization meeting in San Francisco on August 22. Drs. Mooney (USGS) and Chapman (Virginia Tech.), Ms. Dreiling and Mr. M. Isken (both from USGS) were present while Dr. Pezeshk (U. Memphis) participated remotely. The focus of the meeting was to provide an update on recent progress by the researchers and to establish a set of tasks to be completed by the group on regionalization. The group also started weekly calls to support the increased rate of activity and interaction between Dr. Mooney’s group and the team formed of Drs. Chapman and Pezeshk.

Drs. Mooney and Chapman met on September 26 to combine the results of their respective work and to define the regional boundaries for CENA. Dr. Mooney’s research group defined broad regions from their large database of crustal structure measurements. Dr. Champan and his students have also developed regions on the basis of regression and residuals analysis from TA records. The final regional boundaries will depend on both of these complementary approaches. The preliminary boundaries were consistent for both approaches, defining four regions: eastern North America, the Applachians, central North America, and the Gulf Coast. Issues are to be resolved for placement of the boundary near the northern end of Florida. Dr. Chapman is working on data from recent events to resolve this issue.
Sensitivity studies are underway to evaluate the significance of differences in crustal structure in terms of estimated ground motions.
- The methodology has further been developed in the last two months. The crustal structure in each of the four regions listed above is quantitatively defined using search algorithm in velocity-depth space. The input to the search algorithm is the USGS database of crustal structure. The result is four representative crustal velocity models, one for each region. Synthetic seismograms are then computer for a selected model, allowing variability in the layer boundaries, to define the within-region variability effect on ground motions. This level of ground motions change will be used as guidance in lieu of the originally proposed 20% PSA difference threshold required to define a distinct region. Simulations are then run for the different regions to test whether ground motions are significantly different enough to warrant the definition of distinct regions.
- The TI leads were interested in using the simulations to develop Q(f) models for the various regions in lieu of the regular fixed-Q models. The inclusion of frequency dependent Q is not a routine matter when computing synthetic seismograms, hence the group spent a considerable effort to evaluate two independent codes based on the Zhu and Rivera (2002) software. Discrepancies that were not easily resolved lead to the decision of having the USGS team run the fixed-Q FK code (Zhu and Rivera, 2002) while Dr. Chapman’s team will run a modified version of Prof. Herrmann’s code (St-Louis University) that went through prior testing.

Dr. Frankel is also continuing to write a paper describing and modeling the decay with distance of S-wave amplitudes in the Charlevoix region.
Dr. Boatwright (USGS) continued his work on the 2011 Mineral, VA event. He also obtained records and begun the analysis of the 2013 M4.4 Shawville earthquake.

Finite Fault and Stochastic Simulations (Tasks E and F)

The collaborative simulations group (PEER, SWUS and SCEC) kept very active in the last few months and continued to have their weekly phone calls.
- The first round of validation has been completed and the review panel report was submitted to SCEC on August 1. The focus of this first round was on minimized simulated PSA bias relative to: Part A. seven recorded Active Tectonic Region events and Part B. average WUS GMPE predictions. The review panel, formed by SCEC, was led by Prof. Dreger (UC Berkeley) and consisted of Prof. Beroza (Stanford), Prof. Day (SDSU), Dr. Goulet (PEER), Dr. Spudich (USGS), Prof. Stewart (UCLA) and Prof. Jordan (SCEC, USC). Two posters summarizing the validation process (abstract here) and the evaluation results (abstract here) were presented at the SCEC AM.
- Three methods have passed the first round of evaluation and are being used by the SWUS project.
  - Graves and Pitarka (led by Dr. Graves, USGS and Mr. Bayless, URS)
  - San Diego State University method (led by Prof. Olsen, SDSU)
  - EXSIM (led by Dr. Assatourians and Prof. Atkinson, University of Western Ontario)
- NGA-East is putting the final touch to the velocity models for the following scenarios: Saguenay, Riviere-du-loup, Mineral. All the models will be evaluated based on those CENA scenarios. In the meantime, two additional models are allowed to be modified before all the validation runs are re-done, before the end of the last quarter:
  - University of California Santa Barbara method (led by Prof. Archuleta, UCSB)
  - University of Nevada Reno method (led by Prof. Anderson, U. Nevada Reno)
- A special workshop on simulations was also organized by the two TI Leads (Drs. Abrahamson and Goulet) at the SCEC Annual Meeting. The workshop summarized the results from the first round of validation and covered various topics relevant to the future validation efforts. See agenda with presentations here.
Dr. Frankel (USGS) continued the development of new GMPE’s for eastern North America earthquakes. He has been running programs to make long-period determinstic and short-period stochastic synthetic seismograms, with finite faulting, for earthquakes with magnitudes of 5.5, 6.5, and 7.5.The initial simulations were done for a hard-rock site condition. The research work included evaluating the effects of changing the stress drop and other rupture parameters on the ground motions, for the broadband synthetics. Changing the stress drop for the long-period synthetics involves changing the average slip velocity. Of particular interest is a comparison of the 1 sec spectral accelerations from the frequency-wave number (FK) integration method with those derived from the stochastic method. Initial analysis indicates that the 1 and 3 sec spectral accelerations from the FK integration are somewhat less than those from stochastic simulations, for close-in distances. Frankel is investigating how the average slip velocity specified in the deterministic synthetics affects the spectral accelerations at 1 sec and longer periods.
Dr. Atkinson and her student Mr. Yenier (University of Western Ontario) are looking at equivalence of point-source and finite-fault simulations. The are currently working on revising the parameters as needed for ENA events. They presented a poster on this work at the SCEC AM (abstract)

References

Atkinson, G.M. and D.M. Boore (2006). Earthquake ground-motion prediction equations for eastern North America, Bull. Seismol. Soc. Am. 96, 2181—2205.

Boore, D. M. (2009). Comparing stochastic point-source and finite-source ground-motion simulations: SMSIM and EXSIM, Bull. Seismol. Soc. Am. 99, 3202-3216.

Zhu, L. & Rivera, L. A. (2002) A note on the dynamic and static displacements from a point source in multilayered media, Geophysical Journal International, Volume 148, Issue 3, pp. 619-627.