Guide · Hazard & risk
Understanding Seismic Risk Maps
USGS seismic hazard maps drive building codes, insurance rates, and land-use decisions across America. Here is how to read them, and what they cannot tell you.
The short answer
Seismic hazard maps show the probability of ground shaking, not earthquake predictions, and high hazard is not the same as high risk, which also depends on buildings and people.
- 2% / 50yr
- Standard exceedance probability
- Every 6 yrs
- USGS National Seismic Hazard Model update
- SDC A–F
- Building-code categories the maps drive
Key Takeaway
Seismic hazard maps show the probability of ground shaking exceeding a certain level over a given time period, not predictions of individual earthquakes. High-hazard areas are not necessarily high-risk areas, because risk depends on building quality and population. The USGS National Seismic Hazard Model is updated every 6 years and directly shapes building codes via the International Building Code.
What Seismic Hazard Maps Actually Show
The USGS publishes the National Seismic Hazard Model (NSHM), which produces maps showing the expected level of ground shaking at every location in the US. The standard metric is peak ground acceleration (PGA) - how fast the ground accelerates during shaking, expressed as a percentage of gravity (%g).
The maps are probabilistic, not deterministic. The most common format shows shaking with a 2% probability of being exceeded in 50 years, roughly equivalent to a 1-in-2,500 annual chance. This time horizon was chosen because it matches the typical design life of a building. A location showing 0.4g PGA does not mean it will experience that shaking, it means there is a small but non-negligible chance of it over a building's lifetime.
How the Maps Are Built
The NSHM combines three major inputs to estimate shaking probability at each location:
- Earthquake source models: Known fault locations, their slip rates, and the maximum magnitude each fault can produce. For the western US, this includes detailed models of the San Andreas, Cascadia, Wasatch, and hundreds of smaller faults. For the central and eastern US, where faults are less well-mapped, the model relies more heavily on historical seismicity patterns.
- Earthquake rate models: How frequently earthquakes of various magnitudes occur in each region, based on the historical catalog. PlainQuake's yearly statistics show these patterns over the 2005-2025 period.
- Ground motion models: Given an earthquake of a certain magnitude and distance, how strong will the shaking be at a specific site? These models account for the attenuation of seismic waves with distance and amplification by local soil conditions.
The USGS runs thousands of simulations combining these inputs to calculate the probability distribution of shaking at each grid point across the country.
PGA Tier Reference Table
USGS PGA tiers translate directly to FEMA Seismic Design Category and IBC base-shear coefficients. The table below shows the standard mapping used by structural engineers and code officials:
| PGA Tier | PGA range | FEMA SDC | Typical jurisdictions |
|---|---|---|---|
| Low | <0.05g | A-B | North Dakota, Florida, Wisconsin |
| Moderate | 0.05-0.20g | C | South Carolina, Massachusetts, Tennessee |
| High | 0.20-0.40g | D | Utah, Nevada, Oregon, Washington |
| Very High | >0.40g | E | California (coastal), Alaska (south) |
SDC C vs SDC D, what changes
An SDC C site requires basic seismic detailing per ASCE 7-22 §11.4. An SDC D site triggers intermediate moment frames or special concentrically braced frames in steel construction, plus mandatory soil-amplification factors. The cost delta typically runs 8% to 15% of structural-frame cost on a mid-rise building.
Why two adjacent ZIP codes can sit in different tiers
PGA gradients are steepest near mapped faults and at basin edges. The Los Angeles Basin's western edge transitions from 0.6g (SDC E) to 0.35g (SDC D) within roughly five miles. Site-class soil correction (Vs30) further modifies the design value before it reaches a permit office.
Worked example: a 4-story office in Salt Lake City
A 4-story office building on the Wasatch Front sits at PGA 0.35g (SDC D) on Class C soil. Compare that to the same building moved to a Class C site in Phoenix at PGA 0.10g (SDC C). The Salt Lake site requires a special concentrically braced frame and a base-shear coefficient near 18% of building weight; the Phoenix site uses an ordinary moment frame at 6% base shear. Estimated structural-frame cost: $1.4M for the SDC D version vs $850K for the SDC C version on the same plan, roughly 65% premium for the higher seismic tier. Insurance premiums diverge similarly: 35% loaded earthquake-property rates in SDC D zones vs 12% in SDC C zones for unreinforced-masonry exposures.
When to commission a site-specific study
ASCE 7-22 §21 requires a site-specific ground-motion study for buildings on Site Class F soils, for SDC D-F buildings near a fault rupture surface, and for any Risk Category IV facility (hospital, fire station, emergency response). The study replaces the map-derived value with measured shear-wave profiles and probabilistic hazard de-aggregation.
The US Seismic Hazard Landscape
The seismic hazard map of the US reveals several distinct zones, each with different earthquake sources and characteristics:
- Pacific Coast (highest hazard): California, Oregon, and Washington sit on or near major plate boundaries. The San Andreas fault system, Cascadia subduction zone, and numerous smaller faults produce the highest PGA values in the country. Explore state-by-state data to see earthquake frequency in these states.
- Intermountain West (moderate-high): Utah (Wasatch fault), Nevada, and parts of Montana and Idaho have significant hazard from Basin and Range extension and associated fault systems.
- Central US (moderate, episodic): The New Madrid Seismic Zone (Missouri/Tennessee/Arkansas) produced three M7.5+ earthquakes in 1811-1812. It remains seismically active at lower magnitudes. The Charleston, South Carolina zone produced an M7 in 1886. These zones are dangerous because buildings are not designed for strong shaking.
- Alaska and Hawaii (very high): Alaska has the highest overall seismicity of any US state, including M9+ subduction zone potential. Hawaii has volcanic seismicity associated with Kilauea and Mauna Loa.
- Central and eastern US (low-moderate): Most of the eastern US has low seismic hazard but is not zero. Earthquakes in the eastern US are felt over much larger areas than equivalent western US events because the older, colder crust transmits seismic waves more efficiently.
How Hazard Maps Shape Building Codes
The most consequential use of seismic hazard maps is in the International Building Code (IBC), adopted by all 50 states. The IBC references USGS hazard data to define Seismic Design Categories (SDC) - A through F, that determine how much earthquake resistance a new building must have.
| SDC | Hazard Level | Engineering Requirements |
|---|---|---|
| A | Very low | Minimal seismic provisions |
| B | Low | Basic lateral force resistance |
| C | Moderate | Enhanced structural analysis required |
| D | High | Full seismic design with ductile detailing |
| E-F | Very high / near major fault | Most stringent design; fault setback requirements |
This means a home built in San Francisco (SDC D or E) is engineered to survive ground shaking that would collapse an identical home built to SDC A standards in central Texas. The hazard map is the reason California buildings survive M7 events that would devastate regions without comparable codes.
Limitations of Hazard Maps
Seismic hazard maps are powerful tools but have important limitations:
- Unknown faults: The maps can only model known faults. The 1994 Northridge earthquake occurred on a previously unrecognized blind thrust fault, a fault that does not break the surface and was invisible to mapping before the earthquake.
- Site-specific effects: National maps use a reference soil condition. Your specific lot may sit on soft clay (intensifying shaking 2-5 times) or hard bedrock (reducing shaking). Site-specific geotechnical studies are needed for precise risk assessment.
- Time-dependent hazard: Standard maps assume earthquakes occur randomly in time. In reality, some faults are overdue for a large event, and stress from recent earthquakes can increase or decrease the likelihood of future ones on nearby faults.
- Secondary hazards: Hazard maps show shaking only. They do not capture tsunami risk, liquefaction potential, landslide susceptibility, or post-earthquake fire risk, all of which can be more lethal than the shaking itself.
Practical Framework: Using Hazard Maps with PlainQuake Data
To get a comprehensive picture of seismic risk for any US location, combine hazard maps with PlainQuake's historical earthquake data:
- Check the USGS hazard map for your location's expected shaking level and Seismic Design Category.
- Browse your state's historical earthquake record on PlainQuake's state pages to see how frequently earthquakes occur nearby.
- Look at yearly trends to understand whether seismicity in your area is increasing, decreasing, or stable.
- Check whether your area has experienced significant earthquakes (M6+) historically, and whether local building codes reflect that risk.
- Consider that a location with moderate hazard but no seismic building codes may pose more practical risk than a high-hazard location with strict codes and well-enforced construction standards.
Safety Note
Seismic hazard maps inform building codes but cannot predict individual earthquakes. Regardless of your location on the hazard map, ensure you have an earthquake emergency plan, secure heavy furniture to walls, and know the Drop, Cover, and Hold On protocol.
Frequently Asked Questions
What do the colors on a seismic hazard map represent?
Colors represent the intensity of ground shaking expected at each location over a given time period, typically expressed as peak ground acceleration (PGA) or spectral acceleration. Red and dark orange indicate the highest expected shaking (areas near active faults), while blue and green indicate lower expected shaking. The values are probabilities: a 2% chance of exceedance in 50 years means there is a 1-in-2,500 annual probability that shaking will exceed that level at that location.
Are seismic hazard maps the same as earthquake prediction?
No. Seismic hazard maps do not predict when or where the next earthquake will occur. They are probabilistic assessments based on historical earthquake data, known fault locations, and geologic models. They tell you how likely it is that a certain level of shaking will occur at a location over a long time window (typically 50 years). They are used for building code design, insurance risk assessment, and long-term planning, not for short-term forecasting.
How often are USGS seismic hazard maps updated?
The USGS National Seismic Hazard Model (NSHM) is updated roughly every 6 years. The current model (NSHM 2023) incorporates updated fault models, new earthquake catalogs, revised ground motion models, and site effects. Each update can significantly change hazard estimates for specific areas, for example, the 2023 update increased hazard estimates in parts of the Pacific Northwest based on new Cascadia subduction zone research.
Why does Oklahoma show high seismic hazard when it is not on a plate boundary?
Oklahoma experienced a dramatic surge in seismicity from 2009 to 2016, primarily caused by the injection of wastewater from oil and gas operations into deep underground wells. This induced seismicity produced thousands of felt earthquakes, including several M5+ events. The USGS seismic hazard model now incorporates induced seismicity for the central US. Since injection regulations were tightened, Oklahoma seismicity has declined significantly, but the hazard maps still reflect the elevated risk.
What is the difference between seismic hazard and seismic risk?
Seismic hazard is the probability that a certain level of ground shaking will occur at a location. Seismic risk combines hazard with exposure (population, buildings, infrastructure) and vulnerability (building quality, preparedness). A high-hazard area with no people has zero risk. A moderate-hazard area under a dense city with old unreinforced masonry buildings has very high risk. USGS hazard maps show hazard only, risk assessment requires combining them with population and building data.
Should I use seismic hazard maps to decide where to live?
Seismic hazard maps are one factor among many. High-hazard areas like California and the Pacific Northwest also have the strictest seismic building codes, meaning modern buildings are engineered to survive strong shaking. A moderate-hazard area with no seismic building codes (parts of the central and eastern US) may pose more practical risk to building occupants. Consider building age, construction type, local emergency preparedness, and personal risk tolerance alongside the hazard map.
Sources
- USGS, National Seismic Hazard Model (NSHM 2023)
- USGS Earthquake Hazards Program, Seismic Hazard Maps and Site-Specific Data
- International Code Council, International Building Code, Seismic Design Categories
- FEMA, NEHRP Recommended Seismic Provisions for New Buildings
This content is for educational purposes only. For official earthquake information, real-time alerts, and emergency preparedness guidance, visit earthquake.usgs.gov and ready.gov.