Strong ground motion

​ (with $d$d as hypocentral distance and $h$h as a fictitious depth), $\beta$β scales magnitude effects, and $b r$br approximates anelastic attenuation. This model incorporated site amplification based on $V_{S30}$VS30​ and nonlinear soil response. Subsequent advancements in the Next Generation Attenuation (NGA) project led to the NGA-West2 suite of GMPEs in 2014, which expanded on BJF by integrating effects such as hanging-wall amplification (stronger motions on the hanging-wall side of dipping faults) and basin depth (via sediment depth parameter $z_{1.0}$z1.0​ for long-period amplification in deep basins). The NGA-West2 models, including those by Boore et al. and Abrahamson et al., were developed using an extensive database of over 20,000 recordings from active tectonic regions.^[38][39][40] The functional components of GMPEs generally include magnitude scaling, which often follows an exponential form like $c_1 (M - M_h) + c_2 (8.5 - M)$c1​(M−Mh​)+c2​(8.5−M) for $M > M_h$M>Mh​ (a hinge magnitude around 6.0) to capture saturation at high magnitudes for PGA, and distance attenuation comprising geometric spreading (approximating point-source $1/R$1/R or extended-source $1/\sqrt{R}$1/R

​ for body waves) and anelastic damping (e.g., $\exp(-\alpha f R)$exp(−αfR), where $\alpha$α is frequency-dependent and $f$f is spectral period). For example, in NGA-West2 models, the distance term is $\left[ c_1 + c_2 (M - M_{ref}) \right] \ln(R_{JB}/R_{ref}) + c_3 (R_{JB} - R_{ref})$[c1​+c2​(M−Mref​)]ln(RJB​/Rref​)+c3​(RJB​−Rref​), balancing near-source nonlinear effects with far-field decay. Site terms add amplification, typically nonlinear for soft soils, using $V_{S30}$VS30​ to scale motions relative to a reference rock condition.^[37][40] Regional adaptations of GMPEs account for tectonic regime differences, with subduction zone models showing elevated long-period (T > 1 s) motions compared to crustal fault models due to deeper rupture interfaces and slower near-source attenuation in oceanic slabs. For instance, the Youngs et al. (1997) model for subduction zones, based on global data from Japan and other regions, predicts higher spectral accelerations at longer periods for interface events (e.g., up to 110 cm/s² at 0.5 Hz for M ≥ 8 at 100 km) than crustal equivalents like BJF, while in-slab events attenuate more rapidly beyond 100 km. Crustal GMPEs, such as those in NGA-West2, emphasize shallow active faults with hanging-wall effects, whereas subduction models incorporate slab geometry and hypocentral depth. More recent models, such as those from the NGA-Subduction project (2022), build on this foundation using larger global datasets to better capture regional variations in subduction zone shaking. Uncertainty is quantified via the total standard deviation $\sigma \approx 0.5$σ≈0.5 in natural log units for PGA (often split into between-event $\tau \approx 0.3$τ≈0.3 and within-event $\phi \approx 0.4$ϕ≈0.4 components), with values decreasing slightly for larger magnitudes or longer periods due to ergodic assumptions in mixed-effects regressions.^{[41][37][40][42]} Validation of GMPEs relies on residual analysis against observed data from global databases, such as the PEER NGA-West2 database containing recordings from over 600 events in California and other active regions. Residuals, defined as $\ln(Y_{obs}/Y_{pred})$ln(Yobs​/Ypred​), are examined for bias (mean ≈ 0) and trends across magnitude, distance, and site classes using mixed-effects models to separate aleatory and epistemic uncertainties; for NGA-West2, residuals show no significant bias, confirming model reliability for probabilistic seismic hazard analysis.^[43][40]

Simulation Methods

Impacts

Structural Damage

Correlation with Intensity Scales

Notable Examples

Historical Earthquakes

Modern Records