1.Summary
Our seismic risk intelligence engine explicitly models advanced hazard and structural-response effects that off-the-shelf commercial catalogs typically do not document at this level of detail. This paper enumerates four such effects, the literature on which each is grounded, and the implication for risk quantification.
2.Rupture directivity
The model captures rupture-directivity effects through hypocenter randomization across each rupture location and magnitude, following the formulation of Bayless & Somerville (2013). Hypocenter position is drawn per rupture realization rather than fixed at the geometric centroid, so along-strike and along-dip variability of slip propagation is reflected directly in the resulting ground-motion distribution.
This produces a more realistic representation of near-fault ground-motion variability and removes the systematic bias that fixed-hypocenter modelling introduces against sites located in the forward-directivity quadrant of dipping or strike-slip ruptures.
Implication for site response
- Near-fault sites no longer receive a single deterministic directivity adjustment; instead, directivity is integrated probabilistically over the rupture population.
- The conditional distribution of spectral demand at long periods (T > 1.0 s) reflects the actual variability that a structure will see across the seismogenic source.
- Tail behavior (1-in-500, 1-in-1000) on near-fault sites differs materially from a centroid-only treatment.
3.Maximum-direction motion
The engine converts ground motion to the maximum-direction (RotD100) component, applying the period-dependent scaling factors of Shahi & Baker (2014) over the geometric mean (GMRotI50 / RotD50). The maximum-direction component represents the largest spectral response across all horizontal orientations, which is the demand that controls the failure mode for most modern structural and non-structural systems.
This delivers seismic demands that are more demanding and more consistent with modern design and assessment criteria, including the orientation-independent definitions used in current performance-based seismic engineering (PBSE) practice.
| Component | Definition | Use case |
|---|---|---|
| GMRotI50 / RotD50 | Median across horizontal orientations | Legacy GMPE training and historical hazard maps |
| RotD100 | Maximum across horizontal orientations | Modern code design (ASCE 7-22), PBSE, near-fault assessment |
| Shahi & Baker (2014) factors | Period-dependent ratio RotD100 / RotD50 | Conversion path used by our engine |
4.Fling-step in fault-parallel direction
The engine anticipates fling-step effects in the fault-parallel direction following the recommendations of TBI 2017 (PEER Tall Buildings Initiative — Guidelines for Performance-Based Seismic Design of Tall Buildings). Fling-step is a near-fault phenomenon: a one-sided, pulse-like permanent displacement caused by tectonic offset across the fault, distinct from forward-directivity velocity pulses.
Most off-the-shelf hazard catalogs treat ground motion as zero-mean and stationary — fling-step, by contrast, leaves a residual displacement that drives base-shear demand and inelastic deformation in stiff, displacement-sensitive systems (long-period buildings, base-isolated structures, bridges, pipelines, and storage tanks).
Where it matters
- Structures sited within ~10 km of the surface trace of an active fault.
- Long-period assets with significant residual-displacement sensitivity: tall buildings, base-isolated hospitals and data centers, long bridges, large-diameter tanks.
- Lifeline networks crossing or paralleling active faults — water, gas, transmission.
5.Effective-period (Teff) calibration
Our framework integrates effective-period (Teff) evaluation per ASCE 7-22, calibrated against more than 100 non-linear time-history analyses (NLTHA) developed in-house across the project portfolio. Teff shifts the elastic period to a value representative of the effective stiffness at the level of damage being assessed, closing the loop between hazard input and structural response.
The calibration anchors hazard outputs to observed structural behavior rather than to textbook elastic periods, producing PML, AAL and exceedance-probability outputs that reflect how a real structure responds at the limit states that drive insurance loss.
6.Why it matters for risk
Together, these four developments position the engine as a next-generation seismic risk intelligence tool, with a more complete characterization of near-fault effects, directional demands and structural response than off-the-shelf commercial catalogs deliver.
7.References
- Bayless, J. R., & Somerville, P. G. (2013). Updated Directivity Models for the Pacific Earthquake Engineering Research Center NGA-West2 GMPE Project. Pacific Earthquake Engineering Research Center, PEER Report 2013/09.
- Shahi, S. K., & Baker, J. W. (2014). NGA-West2 Models for Ground Motion Directionality. Earthquake Spectra, 30(3), 1285–1300.
- TBI (2017). Guidelines for Performance-Based Seismic Design of Tall Buildings, v2.03. Pacific Earthquake Engineering Research Center, Tall Buildings Initiative.
- ASCE/SEI 7-22 (2022). Minimum Design Loads and Associated Criteria for Buildings and Other Structures. American Society of Civil Engineers.
- Internal in-house NLTHA suite — 100+ non-linear time-history analyses developed by Dynamis across the project portfolio. Confidential, not public.