Background
The paper Site-Specific MCER Response Spectra for Los Angeles Region based on 3-D Numerical Simulations and the NGA West2 Equations provides background information and the method used to obtain the site-specific MCER response spectra.
Abstract
The Utilization of Ground Motion Simulation (UGMS) committee of the Southern California Earthquake Center (SCEC) developed site-specific, risk-targeted Maximum Considered Earthquake (MCER) response spectra for the Los Angeles region. The long period (T ≥ 2-sec) MCER response spectra were computed as the weighted average of MCER spectral accelerations derived from (1) 3-D numerical ground-motion simulations using the CyberShake computational platform, and (2) empirical ground-motion prediction equations (GMPEs) from the Pacific Earthquake Engineering Research (PEER) Center NGAWest2 project. The short period (T < 2- sec) MCER response spectra were computed exclusively from the NGAWest2 GMPEs. A web- based lookup tool was also developed so users can obtain the MCER response spectrum for a specified latitude and longitude and for a specified site class or 30-m average shear-wave velocity, VS30. The tool provides acceleration ordinates of the MCER response spectrum at 21 natural periods in the 0 to 10-sec band.
Citation
Crouse, C., Jordan, T. H., Milner, K. R., Goulet, C. A., Callaghan, S., & Graves, R. W. (2018, 06). Site-Specific MCER Response Spectra for Los Angeles Region based on 3-D Numerical Simulations and the NGA West2 Equations. Presentation at 11th National Conference in Earthquake Engineering.
Important 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.
1.0 Introduction.
The Application Page is where the user specifies the inputs required to obtain the site-specific MCER response spectrum and site-specific design response spectrum (=2/3 x MCER spectrum), subject to the 80% minimum requirement described in the Important Note above.
2.0 Instructions for Specification of Inputs.
2.1 Site Location.
The user can either (1) locate the site on the map by using the cursor and zoom-in (+) or zoom-out (-) feature, or (2) enter the latitude and longitude in the boxes to the left of the map. The map, which initially shows the rectangular region where the lookup tool is valid, allows the user to zoom-in to the streets bounding the site of interest, once the general location within the region is identified. After the site is found, a left click of the mouse brings a pop-up with the site coordinates. The user can click on the box, “Use this point” or exit by clicking on the “x” in the upper right corner of the pop-up and search again.
If instead the user enters the latitude and longitude, the longitude must be a negative number. If the user only knows the site address, then the corresponding latitude and longitude can be obtained using the ASCE Lookup Tool
2.2 Site Geotechnical Classification.
Three options are provided: Site Class, Vs30 (m/s), or Unknown (Vs30 estimated from Wills et al., 2015). Clicking on the circle to the left enables the option.
2.2.1 Site Class. The drop-down menu of site classes appears by clicking on the Select box. The nomenclature following the site class letter designation is identical to that in the ASCE lookup tool.
2.2.2 Vs30 (m/s). This parameter is the average shear-wave velocity in the upper 30 meters at the site (See Chapter 20 of ASCE 7-16 for equation to compute the value). Enter the number in the Value box.
2.2.3. Unknown (Vs30 estimated from Wills et al., 2015). This option is used when the user does not yet know the site class or Vs30 value, but wants a preliminary estimate of the MCER response spectrum for the site. In this case the tool selects a Vs30 value based on the method in Wills et al. (2015). See citation for Wills et al. in the References section of Crouse et al. (2018).
After the appropriate site classification option is selected, click on the "Compute Response Spectra" box at the bottom of the page to view the output page.
3.0 Description of Outputs.
The output page opens to a one-page Summary. Below the page header to the left of the map appears the row “Summary Detailed Download all”. The last two are links to additional information. All three are described below.
3.1 Summary.
The Input Parameters (site coordinates & classification) specified by the user are displayed under “Summary”. If the Unknown site classification option was selected, the value of Vs30 used by the tool is displayed under “Values used in Computation”.
The MCER and Design Earthquake (=2/3 MCER) response spectral parameters (SMS, SM1, SDS, SD1) and the MCEG parameter (PGAM) returned by the tool are listed under Computed Results. The site-specific MCER response spectrum is listed and plotted under MCER Response Spectrum.
The user must check these results against the 80% minimum requirement in the Important Note at the top of this User Guide. The web link to the ASCE web tool (https://asce7hazardtool.online/) at the bottom of the page can be used to extract the response spectra derived from Chapter 11 of ASCE 7-16 that is required for this check.
3.2 Detailed.
The Detailed output page provides values of parameters (Vs30 and basin depth terms, Z1.0 & Z2.5) the tool used to obtain the MCER response spectrum. [Note: Z1.0 & Z2.5 are the depths beneath the site to the tops of the layers with shear-wave velocities of 1.0 km/s and 2.5 km/s, respectively.]
The left hand plot under MCER Response Spectrum provides the site-specific MCER and Design Earthquake response spectra. The right hand plot provides the same MCER response spectrum and the MCER response spectra obtained from the ground-motion prediction equations (GMPE) and the 3-D numerical simulations (CyberShake). See Crouse et al. (2018) for details on the derivation of the MCER response spectrum from these GMPE-based and CyberShake-based MCER response spectra. The table below the plots provides the spectral acceleration values for the three curves in the right hand plot.
3.3 Download all.
Clicking on this link gives the user the option to download a ZIP file containing the Summary and Detailed output pages in PDF format, and an Excel CSV (comma separated values) file with the tabular MCER and Design response spectral data.
UGMS MCER Tool