We present a new empirical ground motion model for PGA, PGV, PGD and 5% damped linear elastic response spectra for periods ranging from 0.01–10 s. The model was developed as part of the PEER Next Generation Attenuation (NGA) project. We used a subset of the PEER NGA database for which we excluded recordings and earthquakes that were believed to be inappropriate for estimating free-field ground motions from shallow earthquake mainshocks in active tectonic regimes. We developed relations for both the median and standard deviation of the geometric mean horizontal component of ground motion that we consider to be valid for magnitudes ranging from 4.0 up to 7.5–8.5 (depending on fault mechanism) and distances ranging from 0–200 km. The model explicitly includes the effects of magnitude saturation, magnitude-dependent attenuation, style of faulting, rupture depth, hanging-wall geometry, linear and nonlinear site response, 3-D basin response, and inter-event and intra-event variability. Soil nonlinearity causes the intra-event standard deviation to depend on the amplitude of PGA on reference rock rather than on magnitude, which leads to a decrease in aleatory uncertainty at high levels of ground shaking for sites located on soil.

We used an expanded PEER NGA-West2 database to develop a new ground motion prediction equation (GMPE) for the average horizontal components of PGA, PGV, and 5% damped linear pseudo-absolute acceleration response spectra at 21 periods ranging from 0.01 s to 10 s. In addition to those terms included in our now superseded 2008 GMPE, we include a more-detailed hanging wall model, scaling with hypocentral depth and fault dip, regionally independent geometric attenuation, regionally dependent anelastic attenuation and site conditions, and magnitude-dependent aleatory variability. The NGA-West2 database provides better constraints on magnitude scaling and attenuation of small-magnitude earthquakes, where our 2008 GMPE was known to be biased. We consider our new GMPE to be valid for estimating horizontal ground motion from shallow crustal continental earthquakes in an active tectonic domain for rupture distances ranging from 0 km to 300 km and magnitudes ranging from 3.3 to 7.5–8.5, depending on source mechanism.

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The NGA-West2 project is a large multidisciplinary, multi-year research program on the Next Generation Attenuation (NGA) models for shallow crustal earthquakes in active tectonic regions. The research project has been coordinated by the Pacific Earthquake Engineering Research Center (PEER), with extensive technical interactions among many individuals and organizations. NGA-West2 addresses several key issues in ground-motion seismic hazard, including updating the NGA database for a magnitude range of 3.0–7.9; updating NGA ground-motion prediction equations (GMPEs) for the “average” horizontal component; scaling response spectra for damping values other than 5%; quantifying the effects of directivity and directionality for horizontal ground motion; resolving discrepancies between the NGA and the National Earthquake Hazards Reduction Program (NEHRP) site amplification factors; analysis of epistemic uncertainty for NGA GMPEs; and developing GMPEs for vertical ground motion. This paper presents an overview of the NGA-West2 research program and its subprojects.

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The data sets, model parameterizations, and results from the five NGA models for shallow crustal earthquakes in active tectonic regions are compared. A key difference in the data sets is the inclusion or exclusion of aftershocks. A comparison of the median spectral values for strike-slip earthquakes shows that they are within a factor of 1.5 for magnitudes between 6.0 and 7.0 for distances less than 100 km. The differences increase to a factor of 2 for M5 and M8 earthquakes, for buried ruptures, and for distances greater than 100 km. For soil sites, the differences in the modeling of soil/sediment depth effects increase the range in the median long-period spectral values for M7 strike-slip earthquakes to a factor of 3. The five models have similar standard deviations for M6.5-M7.5 earthquakes for rock sites and for soil sites at distances greater than 50 km. Differences in the standard deviations of up to 0.2 natural log units for moderate magnitudes at all distances and for large magnitudes at short distances result from the treatment of the magnitude dependence and the effects of nonlinear site response on the standard deviation.

Measured and calculated values of the effective quality factor Q ef and the site attenuation parameter κ 0 for unconsolidated and semiconsolidated sediments in eastern North America (ENA) indicate that the latter is strongly dependent on sediment thickness. Estimates of κ 0 for National Earthquake Hazard Reduction Program (NEHRP) BC site profiles (sediment plus hard rock) in the Mississippi Embayment and the Atlantic Coastal Plain were found to increase from about 9 to 31 msec for sediment thicknesses ranging from 116 to 600 m. Stochastic simulations using the 175 m thick hypothetical NEHRP BC site profile used to estimate ENA ground motions in the national seismic hazard maps by the U.S. Geological Survey (USGS) indicate that κ 0 20 msec provides a smaller estimate of amplification that agrees more closely with the low-strain short-period site coefficients in the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (NEHRP Provisions) than the 10 msec value used by the USGS. A linear regression of the κ 0 estimates compiled in this study indicates that κ 0 20 msec corresponds to a relatively thick BC sediment thickness of 460 116 m. These same stochastic simulations indicate that the relatively shallow USGS site profile provides estimates of amplification that are smaller than the low-strain long-period site coefficients in the NEHRP Provisions. The dependence of both site attenuation and site amplification on sediment thickness suggests that the use of a single reference site condition for hazard mapping might not be appropriate. Instead, these results imply that either a regional set of reference site profiles should be developed or that a more uniform site condition such as hard rock should be used to define a more stable reference site condition in ENA. * Note: Values in italics represent the BC section of the sedimentary column. The values for Q ef were taken from data provided by Cramer et al. (2004) and Boore and Joyner (1991); all other data were taken from Gomberg et al. (2003) and Cramer et al. (2004). Intermediate Q ef values are intermediate to those of Cramer and Boore and Joyner. The values of κ 0 for each layer were calculated from equation (10).