SCEC Committee for Utilization of Ground Motions Simulations
Goal and Charter
The SCEC Committee for Utilization of Ground Motions Simulations (or "UGMS Committee" for short) was chartered the SCEC Earthquake Engineering Implementation Interface Focus Group.
The SCEC UGMS Committee is tasked to develop long-period response spectral acceleration maps for Los Angeles region for inclusion in NEHRP and ASCE 7 Seismic Provisions and in Los Angeles City Building Code. The maps would be based on 3-D numerical ground-motion simulations, and ground motions computed using latest empirical ground-motion prediction equations from the PEER NGA project. This project is coordinated with (1) the SCEC Ground Motion Simulation Validation Technical Activity Group (GMSV-TAG), (2) other SCEC projects, such as CyberShake and UCERF, and (3) the USGS national seismic hazard mapping project.
- Norm Abrahamson (PG&E/UCB)
- John Anderson (UNR)
- Bob Bachman (R.E. Bachman Consulting Structural Engineers)
- Jack Baker (Stanford)
- Jacobo Bielak (CMU)
- C. B. Crouse (URS, chair)
- Art Frankel (USGS)
- Rob Graves (USGS)
- Ron Hamburger (SGH)
- Curt Haselton (CSUC)
- John Hooper (MKA)
- Marty Hudson (AMEC)
- Charlie Kircher (Kircher & Associates)
- Marshall Lew (AMEC)
- Nico Luco (USGS)
- Farzad Naeim (Farzad Naeim, Inc.)
- Paul Somerville (URS)
In the future the UGMS Committee plans to invite structural engineers (SE’s) in Southern CA to participate as Corresponding Members. Possibilities include SE’s from the City of L.A., L.A. County, and Structural Engineers Association of Southern California (SEASC). The addition of these SE’s is important for building a consensus on the long period ground motion maps for the Los Angeles region.
Besides SCEC, the Federal Emergency Management Agency (FEMA) and USGS Golden, Colorado, office would participate. FEMA would enter the project during the next NEHRP code cycle, which would likely begin in late 2015. The USGS would prepare the long period maps and submit them to the Provisions Update Committee (PUC). The PUC membership would be established by the organization responsible for preparing the NEHRP seismic provisions. FEMA will select this organization, which in years past, has been the Building Seismic Safety Council of the National Institute of Building Sciences. The SCEC pilot study would then also become an Issue Team project under the PUC with the specific goal of developing the long-period maps for the Los Angeles region. If the PUC approves the maps and they become part of the NEHRP seismic provisions, then the maps would be submitted to the ASCE Seismic Subcommittee for possible inclusion in the ASCE 7 standard.
The 1st-year scope consisted of one 2-hour meeting (3 – 5 p.m.) at SCEC on April 3, 2013; the meeting was held in conjunction with the GMSV-TAG committee meeting earlier in the day. The 1st meeting covered the following agenda items: (1) finalize committee membership, (2) establish technical tasks and the resources to accomplish these tasks, (3) create organization chart and identify the committee members to lead and participate in each task, and (4) establish a schedule for the project.
The second UGMS meeting was held on May 12 at SCEC. Much of the meeting was devoted to a discussion of the CyberShake platform for generating ground-motion simulations throughout Southern California. T. Jordan first presented an overview of CyberShake and the SCEC ground motion simulation program. Jordan followed with presentations on the SCEC community velocity models, and comparisons of ground-motion predictions from CyberShake and the empirical NGA equations. Jordan also showed that Cybershake was able to reproduce long period ground motions from recent local earthquakes. Jordan’s final presentation was plans for future CyberShake development, which included extending the frequency band to 1.3 Hz to better define 1-sec period motions.
The UGMS affirmed that the long period mapping project will proceed on two parallel tracks, in which PSHA/DSHA will be conducted from the 3-D numerical simulations using CyberShake and from the traditional empirical approach using the NGA-West equations. It was agreed that the results from the 3-D simulations could be used to refine the equations ultimately used in the empirical approach. The second meeting concluded with an action item that SCEC would generate long period response spectra at selected sites using CyberShake and the NGA-West equations as a benchmark to guide the future direction of the committee.
Fourteen (14) sites were selected throughout Southern CA (see attached map) and risk targeted Maximum Considered Earthquake (MCER) response spectra were computed at each site by following the procedures in ASCE 7-10. Plots of the probabilistic and deterministic MCER response spectra, as well as the resulting MCER response spectra (lower of the probabilistic and deterministic MCER) were presented at the third UGMS committee meeting on November 3 at SCEC. Each plot showed the MCER response spectra from CyberShake and NGA-West. For some sites the two spectra were similar and at other sites significant differences were observed. The observations were discussed and suggestions for the third year research program were proposed.
The reasons for the similarities and differences in the response spectra at the 14 sites were investigated during the first half of 2015. Possible limitations of both the NGA and CyberShake models were explored. Sensitivity studies using CyberShake and the widely used 1-D SHAKE site-response code were conducted to examine the effects of the shallow velocity structure at a deep soil site in Carson. The CyberShake model uses a constant shear-wave velocity (Vs) model in the upper 200m, while the SHAKE model used a more realistic depth-varying Vs based on measured Vs data at a nearby site. Under the assumption of linear response, SHAKE and CyberShake predicted similar surface motions for periods, T > ~ 1.5 sec. When SHAKE was run using nonlinear response (i.e., strain-dependent modulus-reduction and material-damping values) the comparison with the CyberShake linear model were much less favorable. Domniki Asimaki, who attended the May 2014 meeting, noted that better soil constitutive models were available and during the latter half of 2015, she conducted additional site-response studies using these alternative models and different levels of input earthquake ground motion. Results will be reported during the next UGMS meeting, which is to be scheduled in early December.
The CyberShake MCER response spectra computed at the 14 sites in 2014 were underestimated at 2-sec period, which at the time was the lower limit of the period band in the CyberShake model. In the first half of 2015, SCEC expanded the valid period range of CyberShake and eliminated the underestimation at 2-sec period. Revised MCER response spectra at the 14 sites showed the 2-sec spectral ordinates increased by a factor of ~2 as a result of the period-band expansion.
During the May 2015 UGMS meeting, the committee noted that the algorithm for CyberShake’s deterministic MCER calculation needed to be revised. This revision was subsequently made by SCEC. During the last half of 2015, SCEC worked with the USGS to obtain the fault and maximum magnitude database the USGS used in its deterministic MCER calculation for the 2008 and 2014 national ground-motion maps.
Comparisons of the CyberShake-based and NGA-West2-based MCER response spectra at the 14 sites indicated a period-dependent weighting scheme to combine the results from these two approaches to produce multi-period MCER response spectra from 0 to 10-sec for use in design. This scheme was tested for the Compton site (i.e., the site in the deep part of the Los Angeles basin) and the result was favorable. Results at the other 13 sites are planned to be presented at the December 2015 meeting.
In progress: The main goal for 2016 is to finalize the procedure for generating MCER maps for southern California and produce preliminary maps to be presented to the City of Los Angeles for possible inclusion as an amendment to the maps in ASCE 7-16 standard, which the City is expected to adopt after this standard is released in 2016. The UGMS has identified the key building official at the City, Ifa Kashefi, PhD, SE, Engineering Bureau Chief, who will ultimately decide whether to adopt the SCEC-UGMS maps. This official will be informed of the new maps, and when they are ready for release. A presentation will then be made to the City, which would tentatively occur toward the end of 2016. Thus, the first half of 2016 would be devoted to finalizing the procedure and the second half to preparing the maps. This action plan will be discussed at the December 2015 UGMS committee meeting. If the City adopts the new maps, then other cities in southern California would more likely be inclined to adopt them also.
Regardless of the City’s decision, the maps will be a resource for the City and other organizations, for immediate use on future projects involving long period structures, such as high-rise buildings, which would be designed using ASCE 7-16 and the provisions in the Los Angeles Tall Buildings Initiative document. Therefore, to facilitate use of the maps, SCEC, under the guidance of the UGMS, would develop a web-based lookup tool, similar to the USGS tool for the national maps. SCEC can begin this task in 2016. The maps would cover the greater Los Angeles region and thus provide a resource for other cities and counties. The plan for publicizing the maps would be discussed during the first UGMS meeting in 2016.
The next seismic code cycle will officially kick-off in 2016 and will eventually result in the publication of the next editions of the NEHRP and ASCE 7 seismic provisions, slated for 2021 and 2022. FEMA representatives have been informed of (1) the USMS committee work for Los Angeles, and (2) parallel efforts by USGS personnel to use numerical models to produce long period ground maps for the Seattle and Salt Lake City urban areas. Once the new Provisions Update Committee (PUC) has been formed for the NEHRP provisions update, its first meeting will establish the technical topics that will be addressed during the cycle. One of the topics will be the procedure for generating the ground-motion maps; this procedure will be recommended by the FEMA-funded Project 17 committee. The outcome will dictate the longer range schedule and technical agenda of the UGMS beyond 2016 for submitting a proposal to the PUC for MCER maps of the greater Los Angeles area. This activity would be coordinated with the USGS Golden, Colorado, office.
April 3, 2013
May 12, 2014
November 3, 2014
May 4, 2015
November 30, 2015
May 16, 2016
View action items summary | presentation materials