Rajesh Rupakhety

Physics-based ground-motion simulation can reduce epistemic uncertainty in regions with sparse strong-motion data, but hazard applications require fault rupture ensembles that are physically plausible, statistically controlled, and computationally efficient. We present a modular kinematic rupture generator for physics-based simulations (PBS) in which final slip, rupture speed ratio (VR/VS), and peak slip velocity (Vmax) are modeled as heterogeneous, mutually correlated spatial fields governed by prescribed one-point statistics and covariance-based two-point structure. The generator supports both event-constrained ruptures (for validation) and fully stochastic scenario ruptures, while enabling systematic propagation of source uncertainty through an effective stress-parameter scaling with Vmax. We demonstrate the approach using a regional three-dimensional numerical model of South Iceland (accurate up to 1.9 Hz) solved with the spectral-element code SPEED. A broadband extension is obtained via the artificial neural network to broadband (ANN2BB) method. Validation against the June 2000 moment magnitude (Mw) 6.5 and 6.4 earthquakes shows good agreement between recorded and simulated low-frequency waveforms and response spectra, supported by goodness-of-fit metrics across duration, peak measures, and long-period spectral ordinates. We then generate Mw 6.5 and Mw 7 scenario ensembles and compare their spectral-acceleration attenuation and variability with local and global ground-motion models. Mw 6.5 scenarios track the Icelandic model within its calibration range, whereas Mw 7 scenarios diverge from extrapolated local predictions, indicating substantial epistemic uncertainty in large-magnitude, short-distance scaling where observations are unavailable. The simulated variability is consistent with empirical expectations, supporting the use of the proposed rupture-generator–PBS framework to produce region-specific, nonergodic ground-motion ensembles for scenario-based risk analysis and PBS-informed hazard assessment.

In many design procedures, a scalar response spectrum like RotD50 or RotD100 is interpreted as a component spectrum applied independently along two orthogonal structural directions. When a maximum‐direction measure such as RotD100 is used in this way, an implicit assumption is introduced: both structural axes experience the worst possible orientation of ground motion. This has contributed to the view that RotD100 is overly conservative for structural design. This paper clarifies that RotD100 is not inherently conservative but that it represents a different type of quantity than a typical component response. Unlike RotD50, which represents a typical directional component, RotD100 represents an instantaneous peak response occurring in some direction of the horizontal plane. The properties of different rotation‐invariant spectra are examined using a geometric interpretation in which the directional response field of ground motion is approximated by an ellipse. Selecting a scalar design spectrum can then be viewed as replacing this ellipse with a circle. Within this geometric framework, several common misconceptions regarding RotD100 can be clarified and avoided in seismic design. The paper concludes that the suitability of a scalar spectrum for design depends on how it is deployed in structural analysis. Ensuring consistency between the chosen intensity measure and the design‐action model is therefore essential for the meaningful use of rotation‐invariant spectra in structural design.