Seismic Zone USA — ASCE 7-22 · PGA and kh by region
ASCE 7-22 Chapter 11 and the 2021 International Building Code define seismic design criteria across the United States using the USGS National Seismic Hazard Model. Ground motion is mapped as mapped spectral accelerations SS (short period, 0.2 s) and S1 (1-second period), from which PGA is derived per ASCE 7-22 Section 11.8.3. Hazard ranges from very low in the Midwest plains to very high along the Pacific Coast and around the New Madrid seismic zone. This tool returns regional PGA values and the pseudo-static horizontal coefficient kh used for retaining walls, slope stability and foundations under seismic loading.
What it is and when it applies
ASCE 7-22 requires every new building to be designed for the mapped ground motions at the site, adjusted by Site Class (A through F) and Risk Category (I to IV). USGS hazard maps published in 2018 and updated in 2023 provide SS, S1 and PGA for a 2% probability of exceedance in 50 years (2,475-year return period). For geotechnical design, PGA is used directly to derive the pseudo-static horizontal coefficient kh in simplified stability analyses: kh = 0.5·PGA/g for retaining walls and slopes that tolerate limited Newmark displacement (10-25 cm per Hynes-Griffin & Franklin 1984), and kh = PGA/g for rigid structures with no allowable deformation. Application is mandatory for buildings, bridges (AASHTO LRFD), industrial facilities, embankments, MSE walls and machine foundations across all 50 states and territories.
Governing equations
Peak ground acceleration (ASCE 7-22 Section 11.8.3):
PGAM = FPGA · PGA, where FPGA is the site coefficient and PGA is taken from USGS
Pseudo-static horizontal coefficient:
kh = 0.5·(PGA/g) → retaining walls and slopes with acceptable deformation
kh = 1.0·(PGA/g) → rigid structures with no tolerated deformation
kv = 0 (typical) or kv = ±0.5·kh (near-fault and critical projects)
Surface acceleration with site factor (ASCE 7-22 Table 11.4-1):
amax = FPGA · PGA, with FPGA: A=0.8, B=1.0, C=1.2, D=1.4, E=1.7
Mononobe-Okabe seismic earth pressure:
θ = arctan(kh/(1−kv)), then Kae follows the classic formulation
Worked example
| Parameter | Value |
|---|---|
| Location | Oakland, CA — very high hazard (USGS) |
| PGA | 0.60 g |
| Site Class D (stiff soil) | FPGA = 1.4 |
| Use | Retaining wall with acceptable deformation |
| Pseudo-static kh | 0.5 × 0.60 = 0.30 |
| Surface amax | 1.4 × 0.60 = 0.84 g |
With kh = 0.30 the Mononobe-Okabe seismic earth pressure gives θ = arctan(0.30/(1−0)) = 16.7°. That Kae multiplies the backfill weight above the wall and adds to the static Rankine or Coulomb thrust. Required checks per ASCE 7-22 and AASHTO LRFD Section 11: FSoverturning ≥ 1.5 static / 1.1 seismic, FSsliding ≥ 1.5 static / 1.1 seismic. The surface acceleration amax = 0.84 g feeds the ASCE 7-22 design spectrum for the wall structure itself (not the soil thrust analysis).
Result: Very high hazard · PGA = 0.60 g · kh = 0.30 · amax = 0.84 g · apply in Mononobe-Okabe with AASHTO LRFD checks
Regional hazard table (USGS + ASCE 7-22)
| Level | PGA | Representative regions |
|---|---|---|
| Very high | ≥ 0.50 g | San Francisco Bay Area, Los Angeles basin, Puget Sound (Seattle, Tacoma), coastal Oregon, parts of Alaska (Anchorage), Hawaii (Big Island) |
| High | 0.25-0.50 g | Reno NV, Salt Lake City UT (Wasatch Fault), inland California, Memphis TN (New Madrid seismic zone), coastal Washington |
| Moderate | 0.10-0.25 g | Boston and New England, Charleston SC (1886 earthquake), central Oklahoma (induced seismicity), St. Louis MO, northern New Jersey |
| Low | ≤ 0.10 g | Most of Texas, Great Plains, upper Midwest, Florida, south Georgia, much of the Gulf Coast inland |
Interpretation of results
U.S. seismic hazard is highly non-uniform. The Pacific Coast sits on the Cascadia Subduction Zone and the San Andreas transform system, producing PGA values above 0.50 g over broad areas. Intermountain regions such as the Wasatch Front in Utah and the Basin and Range in Nevada carry normal-fault hazard in the 0.30-0.50 g range. The New Madrid seismic zone (Missouri-Tennessee-Arkansas) is the main intraplate hazard in the central U.S., with PGA up to 0.40 g despite being far from a plate boundary. For geotechnical work, projects with PGA ≥ 0.30 g require specific liquefaction triggering analysis in saturated sands (Youd et al. 2001, Boulanger & Idriss 2014), Mononobe-Okabe or Wood method for walls, and Newmark displacement analysis for slopes. Dams under FEMA P-65 and Corps of Engineers EM 1110-2-6053 require site-specific PSHA and nonlinear dynamic analysis.
Reference standards
- ASCE 7-22 — Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Chapter 11
- IBC 2021 — International Building Code, Section 1613 Earthquake Loads
- USGS Open-File Report 2023 — National Seismic Hazard Model update
- AASHTO LRFD Bridge Design Specifications 9th ed., Section 11 (earth pressures)
- FEMA P-1050 / P-2082 — NEHRP Recommended Seismic Provisions commentary
- Hynes-Griffin & Franklin (1984) — Rationalizing the seismic coefficient method
- Boulanger & Idriss (2014) — CPT and SPT based liquefaction triggering procedures
Frequently asked questions
What is the difference between PGA, SS and S1?
PGA is the peak ground acceleration (zero-period spectral response), used for geotechnical pseudo-static analysis. SS is the mapped 5%-damped spectral response acceleration at 0.2 s (short period), and S1 is the same quantity at 1.0 s. Buildings are designed from SDS and SD1, which are SMS and SM1 times 2/3. For retaining walls, slopes and foundations the pseudo-static coefficient is built from PGAM, not from SS.
Where does the USGS get these hazard values?
The USGS National Seismic Hazard Model combines historical earthquake catalogs (since 1568 for some states), GPS crustal strain data, fault slip rates, and ground-motion prediction equations calibrated against instrumental records. The 2023 update incorporated the NGA-West2 and NGA-East GMMs. Site-specific PGA is retrieved from the USGS Design Maps web service or the ATC Hazards by Location tool using latitude and longitude.
Does the New Madrid zone really matter for design?
Yes. Although strain rates are low, the 1811-1812 sequence produced three M7.5-7.7 events in five months, and paleoseismology shows recurrence of large events every 500-1,000 years. Memphis, TN and Paducah, KY sit on thick Mississippi embayment soils that amplify long-period motion significantly. ASCE 7-22 maps yield PGA of 0.30-0.50 g in the epicentral region and design is mandatory for Risk Category III and IV structures.
Is ASCE 7-22 enough for geotechnical design?
ASCE 7-22 gives PGAM and the Site Class framework. Geotechnical design complements it with ASCE 7-22 Chapter 11.8 for liquefaction assessment, AASHTO LRFD Section 11 for retaining walls and abutments, and project-specific procedures: Mononobe-Okabe for walls, Boulanger-Idriss for liquefaction, Newmark for slope displacement, and nonlinear FEM for dams and waterfront structures. For standard projects with acceptable deformation, kh = 0.5·PGA is sufficient.