Earthquake Ground-Motion Risk Screening

Lead with modeled ground motion

The earthquake module uses USGS National Seismic Hazard Model peak ground acceleration at selected return periods as its primary modeled signal. This provides location-specific variation even where FEMA NRI community context is unavailable.

• PGA is expressed as a fraction of gravity for the model's reference site condition.
• The report compares more frequent and rarer modeled ground-motion levels.
• The model describes probabilistic hazard; it does not predict when an earthquake will occur.

Read spectral acceleration as technical context

The report also shows spectral acceleration at short and longer structural periods. These values can help frame professional questions, but they are not converted into building-performance claims because the report does not know the structure's complete system and condition.

• Short- and longer-period values relate differently to structural response.
• The displayed values use the dataset reference site class and Vs30.
• A structural engineer should interpret project-specific design or retrofit implications.

Use community risk and mapped faults as support

FEMA NRI adds community-scale loss and exposure context, while USGS Quaternary faults add mapped proximity and classification evidence. Neither replaces the modeled shaking values or a site-specific investigation.

• A nearby mapped fault is relevant context, but distant sources can also produce strong shaking.
• Community expected annual loss is not a parcel or building loss estimate.
• Unavailable sources are excluded rather than interpreted as zero earthquake risk.

Separate modeled local Vs30 from the reference site condition

The additional ground-condition panel samples the USGS Global Vs30 Mosaic median and logarithmic uncertainty at the location. Vs30 describes average shear-wave velocity in the upper 30 metres. The 30-arcsecond grid is regional modeling context, not a measured parcel or building-code site class.

• The mosaic uses a terrain-based background with regional models where available; values have modeled uncertainty.
• Local modeled Vs30 does not replace the NSHM reference-site Vs30 or adjust the displayed PGA, spectral acceleration or earthquake score.
• Masked water and ice cells and unavailable samples remain explicit. Do not assign a soil class from missing evidence.
• For design, retrofits or consequential insurance questions, obtain locally validated ground conditions and professional structural or geotechnical review.

Know what still requires professional review

Local soils, liquefaction, slope stability, foundation type, building age, structural system, retrofit history, and code requirements remain outside a national screening score.

• Review local liquefaction, fault-rupture, and seismic-hazard maps where applicable.
• Confirm building construction, retrofit history, and adopted code era.
• Use geotechnical and structural professionals when the decision or signal warrants it.

Sources and further reading

• USGS Global Vs30 Mosaic
• USGS National Seismic Hazard Model

Source publications and live agency pages may differ from the snapshot used in a report. Check the report's source dates and confirm material findings through the responsible authority.