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SCEC Committee for Utilization of Ground Motions Simulations (UGMS)

WORKING GROUP
UGMS Committee
 
DATA PRODUCTS

Web-based data access tool providing site-specific spectral acceleration data for the greater Los Angeles region.

UGMS MCER Data Access Site

DATASET DOI
UGMS v18.4
doi:10.5281/zenodo.3247804

DOCUMENTATION & SUPPORT
User Guide
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MEETINGS
 

Overview

In 2013, the SCEC Committee for Utilization of Ground Motion Simulations (or "UGMS Committee") was tasked to develop long-period response spectral acceleration maps for inclusion in NEHRP and ASCE 7 Seismic Provisions and in Los Angeles City Building Code. The maps would be based on ground motions computed using 3D numerical ground-motion simulations (SCEC CyberShake) and latest empirical ground-motion prediction equations (PEER NGAWest2). The UGMS Committee identified the need for a web-based data access tool to allow engineers to obtain site-specific data without the need to engage with esoteric software or to interact directly with extremely large datasets. The UGMS Committee worked with end-users and research programmers (1) to establish a methodology for developing response spectral acceleration maps that integrates numerical and empirical methods, (2) to determine target sites and datasets, (3) to perform post-processing calculations and analyses, and (4) to define what data would be outputted and how it should be displayed. Released in 2018, the SCEC UGMS MCER data access tool allows users to retrieve site-specific, risk-targeted Maximum Considered Earthquake (MCER) response spectra and design response spectra for any site in the Los Angeles region in just seconds. This page provides information on the SCEC UGMS data access tool, working group, project meetings, and relevant publications.

Above: Screen capture from UGMS MCER data access tool

Web-based Data Access Tool

A process of validation was undertaken by the SCEC UGMS Committee to vet the MCER response spectra generated from ground motions computed using 3D numerical ground-motion simulations (SCEC CyberShake) and the latest empirical ground-motion prediction equations (PEER NGAWest2). Fourteen specific sites were selected throughout the Los Angeles region for evaluation and comparisons to determine the limitations of MCER response spectra generated from CyberShake and those from empirical GMPEs from the NGAWest project. The UGMS Committee developed 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 strategy emphasized the strengths of each method, giving all weight to the empirical data at short periods (T < 2s) and gradually increasing weight given to the simulated data at the longer periods.

Above: MCER Response Spectra

The resulting dataset represents an improvement to long-period MCER response spectra. Previous empirical methods alone did not fully account for local and regional geology, fault directivity and fling, and 3D effects of fault rupture and basin structure on ground motion. These were elements that 3D simulated ground motion data from the CyberShake project could contribute.

The long-period response spectral acceleration maps produced using this method are a resource for the City of Los Angeles 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.

To facilitate access to these maps, a web-based data access tool was developed by SCEC research programmers, under the guidance of the UGMS Committee. The tool provides site-specific spectral acceleration data for the greater Los Angeles region, without the need to engage with esoteric software or to interact directly with extremely large datasets. The user inputs the latitude/longitude of a site and its site-class or 30-m average shear-wave velocity (Vs30), and the tool outputs:

  • Site-specific MCER response spectra and Design Earthquake response spectra (=2/3 MCER) at 21 natural periods in the 0 to 10-second band
  • Site-Specific Design Parameters (SMS, SM1, SDS, SD1)
  • MCEG parameter (PGAM)
  • Plots, a CSV file, and PDF reports

For more details, please refer to the User Guide found on the data access website.

DISCLAIMER. The UGMS MCER Tool is provided “as is” and without warranties of any kind. While SCEC and the UGMS Committee have made every effort to provide data from reliable sources or methodologies, SCEC and the UGMS Committee do not make any representations or warranties as to the accuracy, completeness, reliability, currency, or quality of any data provided herein. SCEC and the UGMS Committee do not intend the results provided by this tool to replace the sound judgment of a competent professional, who has knowledge and experience in the appropriate field(s) of practice. By using this tool, you accept to release SCEC and the UGMS Committee of any and all liability.

Please note: The site-specific, design response spectral acceleration, Sa, returned by this tool for user-specified inputs, must be compared to the minimum Sa requirement described in Section 21.3 of ASCE 7-16 (second and third paragraphs). This minimum Sa is computed as 80% of the design response spectrum derived from the SDS, SD1, and TL values obtained from the ASCE tool at https://asce7hazardtool.online/. The larger of the site-specific Sa and the 80% minimum Sa at each period, T, is the final design response spectral acceleration. This final Sa x 1.5 is the final MCER response spectral acceleration.

UGMS Committee Process

The SCEC Committee for Utilization of Ground Motion Simulations (or "UGMS Committee") was tasked to develop long-period response spectral acceleration maps for inclusion in NEHRP and ASCE 7 Seismic Provisions and in Los Angeles City Building Code. The UGMS Committee held its inaugural meeting in April, 2013 to (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 UGMS Committee membership is listed in the side panel above. Proposals were submitted responding to the annual SCEC Science Plan to fund organizational meetings by the UGMS each year.

The second UGMS Committee meeting held in May, 2014 began with a discussion of the CyberShake platform, SCEC community velocity models, and comparisons of ground-motion predictions from CyberShake and the empirical NGA equations. Tom Jordan showed that Cybershake was able to reproduce long period ground motions from recent local earthquakes. The plans discussed for CyberShake development included extending the frequency band to 1.3 Hz to better define 1-sec period motions.

The UGMS Committee affirmed that the long period mapping project will proceed on two parallel tracks, in which PSHA/DSHA will be conducted from the 3D numerical simulations using CyberShake and from the traditional empirical approach using the NGA-West equations. It was agreed that the results from the 3D simulations could be used to refine the equations ultimately used in the empirical approach. The second meeting concluded 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 sites were selected throughout Southern CA (see 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, 2014. 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 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 1D 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.

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 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. In May 2015, the UGMS 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 were presented at the November 2015 meeting.

The main goal in 2016 was 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 was expected to adopt after this standard is released in 2016). The UGMS Committee identified the key building official at the City who will ultimately decide whether to adopt the SCEC-UGMS maps. This official was informed of the new maps, and when they are ready for release. 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 of Los Angeles’ decision, the maps are 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. To facilitate use of the maps, SCEC research programmers, under the guidance of the UGMS Committee in 2016, began the development of a web-based data access tool, similar to the USGS tool for the national maps. 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 was discussed during meeting in May, 2016.

The next seismic code cycle officially kicked off in 2016 and eventually lead to the publication of the next editions of the NEHRP and ASCE 7 seismic provisions, slated for 2021 and 2022. FEMA representatives were informed of (1) the UGMS Committee work for Los Angeles, and (2) parallel efforts by USGS personnel using numerical models to produce long period ground maps for the Seattle and Salt Lake City urban areas. Once the new Provisions Update Committee (PUC) had been formed for the NEHRP provisions update, its first meeting established the technical topics to be addressed during the cycle. One of the topics was the procedure for generating the ground-motion maps; this procedure would be recommended by the FEMA-funded Project 17 committee. The outcome will dictate the longer range schedule and technical agenda of the UGMS Committee 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.

In 2017, the UGMS Committee implemented the approach to combine the results from the NGA West2 GMPE’s with those from CyberShake in order to determine MCER response spectra for southern California, per the procedures in the ASCE 7-16 standard. A beta version of a web-based data access tool to obtain the MCER was developed and circulated for review among UGMS Committee members and the City of Los Angeles, SEAOSC, and OSHPD representatives at the November 2017 meeting. Based on their review comments, revisions to the tool were made and the SCEC UGMS MCER data access tool was released in May, 2018.

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Overview

Background