Step-by-Step: Conducting a Cirrus Seismic Evaluation

Top Findings from the Latest Cirrus Seismic EvaluationThe most recent Cirrus Seismic Evaluation offers a detailed, data-driven look at seismic hazards, site response, structural vulnerabilities, and practical mitigation measures for the region and facilities covered. This article summarizes the key findings, explains their implications for engineers and decision-makers, and outlines recommended next steps for reducing seismic risk.

Executive summary

• Primary seismic hazard drivers for the study area are identified as shallow crustal faulting and deep crustal events related to the regional tectonic setting.
• Peak ground motions (PGAs) show higher-than-previously-modeled amplitudes at several sites, particularly for short-period motions affecting non-ductile concrete structures.
• Amplification due to shallow soils substantially increases predicted motion in low-lying valley locations and reclaimed sites.
• Existing critical structures—including older hospitals and mid-rise concrete office buildings—exhibit moderate to high risk of damage under the updated scenarios.
• Mitigation options such as targeted retrofits, improved site characterization, and updated design spectra are cost-effective compared with potential loss estimates.

Study scope and methodology

The Cirrus evaluation combined seismic source characterization, ground-motion modeling, site-specific response analyses, and vulnerability assessment.

• Seismic sources: updated catalog of historic earthquakes, mapped faults, and geodetic strain rates.
• Ground-motion prediction: suite of modern ground-motion models (GMPEs) calibrated for regional tectonics, with ensemble logic-tree weighting to capture epistemic uncertainty.
• Site response: 1D and 2D site amplification modeling using measured and inferred shear-wave velocity (Vs30) profiles, supplemented by basin-edge simulations where relevant.
• Structural vulnerability: fragility functions for representative building classes (URM, older reinforced concrete, steel moment frames, modern code-compliant buildings).
• Loss estimation: direct physical damage and downtime costs estimated for multiple intensity-duration scenarios.

Key technical findings

1. Higher short-period shaking than prior catalogs

   • The ensemble GMPE results show up to 25–40% higher short-period spectral accelerations for common return periods compared with legacy models.
   • Impact: increased demand on non-ductile concrete and masonry buildings, and on equipment anchored in buildings.

2. Site amplification dominates local hazard variability

   • Measured Vs30 variations and presence of soft alluvium cause amplification factors of 2–4× in some valleys relative to rock sites.
   • Impact: small-area “hot spots” where local shaking exceeds regional averages, requiring site-specific design checks.

3. Basin and topographic effects cause long-duration shaking increases

   • Basin-edge trapping and 2D/3D waveguide effects prolong shaking, particularly at longer periods (>1 s), raising collapse risk for taller structures.
   • Impact: tall buildings and critical lifeline structures may demand revised design spectra and damping considerations.

4. Liquefaction and lateral spreading risk concentrated in reclaimed and fluvial deposits

   • Reconnaissance and local cone penetration test (CPT) data highlight zones with high liquefaction susceptibility, especially where groundwater is shallow.
   • Impact: major risk to foundations, utility corridors, and non-ductile piling systems.

5. Older buildings and non-ductile systems are most vulnerable

   • Unreinforced masonry (URM) and older non-ductile reinforced-concrete frames show the highest fragility index in scenario events.
   • Impact: hospitals, schools, and older office buildings may face high repair costs and operational downtime if unmitigated.

Implications for stakeholders

• Owners/operators of critical facilities should prioritize site-specific seismic assessments and phased retrofits for the most vulnerable components (e.g., non-ductile frames, unanchored equipment).
• Municipal planners and emergency managers must update risk maps to reflect localized amplification hotspots and liquefaction zones for land-use decisions and emergency routing.
• Engineers should adopt updated ground-motion inputs and consider long-duration and basin-induced effects when designing high-rise and lifeline structures.
• Insurers and financial planners can use revised loss estimates to reassess premiums, deductibles, and resilience investments.

Recommended actions

1. Conduct targeted site investigations (Vs profiling, CPT, boreholes) at identified hotspots.
2. Re-evaluate design spectra for new high-importance structures using the Cirrus ensemble GMPE outputs and site response factors.
3. Prioritize retrofits: life-safety upgrades for hospitals and schools, seismic strengthening for non-ductile concrete frames, and anchoring of critical mechanical/electrical equipment.
4. Implement ground improvement and foundation adaptations (deep foundations, stone columns, vibro-replacement) in liquefaction-prone areas.
5. Integrate the updated hazard maps into emergency response planning and utility redundancy design.
6. Establish a monitoring and data-collection program (accelerometers, groundwater wells) to refine future assessments.

Example retrofit priorities (short list)

• Anchor and bracing of heavy equipment and lifelines in hospitals.
• Addition of capacity/dissipation (e.g., steel jacketing, CFRP wrapping) for key concrete columns in older buildings.
• Installation of base isolation or energy dissipation devices for mission-critical facilities where feasible.
• Ground improvement below vulnerable foundations in reclaimed land.

Uncertainties and limitations

• Ground-motion predictions retain epistemic uncertainty—ensemble approach reduces but does not eliminate model spread.
• Sparse subsurface data in some areas increases uncertainty in site-amplification estimates; recommended targeted investigations will reduce this.
• Scenario-based loss estimates depend on assumptions about building occupancy, retrofit state, and post-event recovery rates.

Conclusion

The latest Cirrus Seismic Evaluation raises the bar for regional seismic hazard estimates by highlighting stronger short-period motions, significant local amplification, basin-induced long-duration shaking, and concentrated liquefaction risk. Priority actions are targeted site investigations, updated design inputs, and phased retrofits for the most vulnerable structures, which together offer a high return on investment compared with potential losses.