Legacy National Seismic Hazard Model
The Legacy national seismic hazard model (NSHM) worksheet facilitates the importing and processing of USGS legacy datasets. Two options, discussed in USGS Earthquake Hazard Toolbox and Hazard Curve Input, are provided, depending on the applicable data source.
Site and Model Input
Use the drop-down list to select the data source as either “USGS Earthquake Hazard Toolbox” or “USGS Gridded Hazard Curve Data File.” Use the drop-down list to select the USGS NSHM. Information about the models for each data source is customizable at the bottom of the worksheet.
The input for site coordinates is the same as for the Modern NSHM worksheet. Ensure the site coordinates for short-term NSHM match the input for the corresponding long-term NSHM on the Modern NSHM worksheet.
The NEHRP site class is not user-specified because only NEHRP site class boundary B/C is available for legacy datasets. The site and model input are illustrated in Figure.
USGS Earthquake Hazard Toolbox
Option 1 processes hazard curve data from the USGS Earthquake Hazard Toolbox. The GM and AFE input are the same as the NSHM Modern worksheet but for PGA, 0.2-second SA, and 1-second SA only. Conditional formatting applies a gray background to this option if “USGS Gridded Hazard Curve Data File” is selected for the data source.
Hazard Curve Input
Option 2 processes hazard curve data from USGS gridded hazard curve data files. Conditional formatting applies a gray background to this option if “USGS Earthquake Hazard Toolbox” is selected for the data source.
The ground motion (GM) and corresponding annual frequency of exceedance (AFE) are obtained from files containing gridded seismic hazard curve data. Download the archive (.zip file) from the ScienceBase Catalog to obtain the gridded data for the hazard curves. Each archive contains a READ_ME.txt file that provides a detailed description of the files.
Each gridded data file has a row of constant GM values that must be copied from the gridded data file and pasted as values in the appropriate location on the worksheet. All other rows contain AFE values for every latitude-longitude pair based on the grid spacing and geographic limits of the selected model. The user-specified site coordinates are bounded by the coordinates of a square grid (latitude-longitude pairs). The AFE values for each of the four coordinate pairs must be copied from the gridded data file and pasted as values in the appropriate location on the worksheet. Detailed instructions on how to open the files and find the four coordinate pairs to import are at the top of the worksheet.
Figure illustrates how the four coordinate pairs are displayed for each ground motion intensity measure. If incorrect AFE data is pasted from the gridded data file for any of the four pairs, conditional formatting applies an orange background to the incorrect longitude or latitude, and a warning message is displayed as illustrated in Figure. If this occurs, return to the gridded data file and select the correct data. For some legacy datasets, it may be necessary to reverse the order of the latitude and longitude columns so longitude appears first. Repeat this import procedure for each ground motion intensity measure: PGA, 0.2-second SA, and 1-second SA.
Because the user-specified coordinates are bounded by the coordinates of a square grid (latitude-longitude pairs) based on the grid spacing of the selected model, a series of three linear interpolations estimates the hazard curve for the user-specified coordinates, as Figure illustrates.
The AEP is calculated from the interpolated AFE as on the Modern NSHM worksheet.
Plots of GM as a function of AEP for NEHRP site class boundary B/C are at the bottom of the worksheet as illustrated in Figure.
