the western United States into discrete regions (fig. O1) and treats each region as an independent set of gridded sources with common parameters (for example, maximum magnitude [Mmax], strike, and focal mechanism). The regions were identified on the basis of differences in b-value, the historical catalog (for example, quality and completeness) and tectonic style. Spatially uniform and and spatially variable or rates, constrain the Mmax b-values a-values, distribution (MFD) of each source in a region. In regions with little seismicity (a rarity in California), a minimum earthquake rate (or floor) also is applied. Source rates were derived by smoothing the rates of declustered events in each region using an isotropic Gaussian kernel with a 50-km radius. In California, several subregions received special treatment: Mendocino, Imperial, Brawley, and the creeping section of the San Andreas Fault. These regions either had fixed strike sources, and that differed from the broader regional value, or were Mmax b-values smoothed by using an anisotropic kernel. A primary challenge in building a gridded seismicity forecast is to avoid double counting at grid cells that overlap spatially with faults. The 2008 NSHMP addressed this issue by reducing of sources within 10 km of near-vertical faults Mmax and over more steeply dipping faults. The focal mechanisms of each source reflect the tectonic style of the parent region, with each source assigned a weighted combination of strike-slip (SS), reverse (R), and normal (N) motion with weights that sum to 1. For example, grid sources spanned by the coastal California catalog have SS weights of 0.5 and R weights of 0.5, whereas sources spanned by the western U.S. extensional catalog have SS weights of 0.5 and N weights of 0.5. This level of focal-mechanism specificity is accommodated by the ground-motion prediction equations used in the NSHMP, and it ultimately provides a better estimate of hazard. As for implementation, UCERF2 defines sources at the centers of the 7,636 grid cells in the 0.1 by 0.1 degree discretization of California
the western United States into discrete regions (fig. O1) and treats each region as an independent set of gridded sources with common parameters (for example, maximum magnitude [Mmax], strike, b-value, and focal mechanism). The regions were identified on the basis of differences in the historical catalog (for example, quality and completeness) and tectonic style. Spatially uniform Mmax and b-values and spatially variable a-values, or rates, constrain the distribution (MFD) of each source in a region. In regions with little seismicity (a rarity in California), a minimum earthquake rate (or floor) also is applied. Source rates were derived by smoothing the rates of declustered events in each region using an isotropic Gaussian kernel with a 50-km radius. In California, several subregions received special treatment: Mendocino, Imperial, Brawley, and the creeping section of the San Andreas Fault. These regions either had fixed strike sources, Mmax and b-values that differed from the broader regional value, or were smoothed by using an anisotropic kernel. A primary challenge in building a gridded seismicity forecast is to avoid double counting at grid cells that overlap spatially with faults. The 2008 NSHMP addressed this issue by reducing Mmax of sources within 10 km of near-vertical faults and over more steeply dipping faults. The focal mechanisms of each source reflect the tectonic style of the parent region, with each source assigned a weighted combination of strike-slip (SS), reverse (R), and normal (N) motion with weights that sum to 1. For example, grid sources spanned by the coastal California catalog have SS weights of 0.5 and R weights of 0.5, whereas sources spanned by the western U.S. extensional catalog have SS weights of 0.5 and N weights of 0.5. This level of focal-mechanism specificity is accommodated by the ground-motion prediction equations used in the NSHMP, and it ultimately provides a better estimate of hazard. As for implementation, UCERF2 defines sources at the centers of the 7,636 grid cells in the 0.1 by 0.1 degree discretization of California