Published
Ashton’s seismic design category is D. Here is the official query that answers the question.
A one-question page, with an official answer
Buyers on this corridor ask about snow load — it is the number that shapes a roof here. But there is a second number on the same structural sheet, and almost nobody asks it: what seismic design category does my lot fall in? It is a rare question in this niche because the answer usually looks buried. It is not. The U.S. Geological Survey runs a free public design-values service that answers it for any coordinates in the country, and this page ran it for Ashton’s. The response’s key lines:
“sdcs”: “C”, “sdc1”: “D”, “sdc”: “D” — with “ss”: 0.487, “s1”: 0.176, “sds”: 0.458, “sd1”: 0.264
— USGS ASCE 7-16 Design Web Service, queried for latitude 44.072, longitude -111.448, siteClass D, riskCategory II. Retrieved 11 September 2026
The exact query, which any reader can rerun for free, is earthquake.usgs.gov/ws/building-codes/asce7-16/calculate?latitude=44.072&longitude=-111.448&siteClass=D&riskCategory=II. The service computes design values under ASCE 7-16, the design standard the adopted building-code family references, and returns one governing answer: for Ashton’s coordinates, Seismic Design Category D.
Everything the service returned, in plain language
The response carries more than the category. These are the numbers your structural engineer works from, exactly as returned on 11 September 2026:
| Value | Returned | What it is |
|---|---|---|
| SDC | D | The seismic design category — the governing answer |
| Ss | 0.487 g | Mapped short-period ground-motion parameter for the site |
| S1 | 0.176 g | Mapped one-second ground-motion parameter for the site |
| SDS | 0.458 g | Short-period design spectral acceleration |
| SD1 | 0.264 g | One-second design spectral acceleration |
| SDCS / SDC1 | C / D | Category from the short-period value / from the one-second value |
| PGA / PGAM | 0.207 g / 0.288 g | Peak ground acceleration, mapped and site-modified |
Why the answer is D when one of the two lines says C: ASCE 7-16 determines the category twice — once from the short-period design value (which lands at C here) and once from the one-second design value (which lands at D) — and the more severe of the two governs the design. That is the method the USGS service implements, and it is why the response carries all three fields.
The two inputs you are allowed to disagree with
The query is honest about its assumptions: both are printed in the request line, and both belong to your project, not to the town.
- Site Class D. The query assumes the default soil condition — Site Class D, the stiff-soil class. A geotechnical report for your actual lot can establish a different site class, and with it different mapped-to-design multipliers (this response applied a short-period factor of 1.41 and a long-period factor of 2.248) and potentially a different category. The lot-level version of that question — what the ground under a specific parcel does — is the same one that makes lot-to-lot conditions differ all over this corridor.
- Risk Category II. The query is for ordinary buildings — the risk category that covers houses and most commercial construction. Facilities with higher occupancy or critical functions map to stricter categories and can land in a more severe design category at the same coordinates.
Where this sits on the structural sheet
None of this replaces the numbers this site spends most of its time on. Snow load governs the gravity design of a corridor roof — the county’s 90 PSF Ashton district figure and the university study that says 103 — and frost depth governs the footing below it, 32 inches countywide. The seismic category is the third leg: it governs how the structure is tied together against lateral motion, and it is decided by mapped values the engineer does not choose. A bid set that pins the snow load but leaves the seismic values implicit is an incomplete sheet; the same discipline that makes two snow-load bids comparable applies to all three numbers at once.
And when the design turns into a permit and the permit into inspections, the 2025 inspection-timeliness rules now on the books are worth knowing in advance — see the 48-business-hour clock and the refund rules it created.
Getting to your own answer
- Pull the coordinates for your actual parcel off the survey or site plan — not the town’s. The query here used 44.072, -111.448 for Ashton, and the service accepts any coordinates.
- Rerun the query with them, or hand the coordinates to your structural engineer and ask for the printed response. The service is public, free, and returns the values above in seconds.
- Ask whether a geotechnical report would change the site class before the structural set is drawn — the D in this answer is an assumption the ground under your lot is allowed to correct.
- Confirm the category that appears in the bid documents matches the one your engineer designed to — the numbers and the drawings should tell one story.
- Then ask the question that puts it to work: “What seismic design category is this frame designed to, and where does it show in the drawings?” If the answer is a shrug, the sheet is thinner than it looks — this site takes that kind of question at (208) 656-1516.
Common questions
What seismic design category is Ashton, Idaho?
D. The USGS ASCE 7-16 design service, queried 11 September 2026 for latitude 44.072, longitude -111.448 with Site Class D and Risk Category II, returns Seismic Design Category D, with SDS 0.458 g and SD1 0.264 g.
What does Seismic Design Category D mean?
It is the code's shorthand for how demanding the seismic design requirements are at a site. ASCE 7-16 determines a category twice — from the short-period design value and from the one-second design value — and the more severe governs. For Ashton the short-period value lands at C and the one-second value at D, so D governs. Your engineer translates the category into specific detailing and analysis decisions.
What is Site Class D?
The stiff-soil site class — the assumption this query makes about ground conditions. The USGS response applied a short-period site coefficient of 1.41 and a long-period coefficient of 2.248 on top of the mapped values. A geotechnical report for your specific lot can establish a different site class, which changes the coefficients and potentially the category.
What is Risk Category II?
The ordinary-buildings risk category — the one that covers houses and most commercial construction. The category reflects the consequence of failure, not the building's size; hospitals, fire stations and similar facilities fall in stricter categories and can receive a more severe design category at the same coordinates.
Do these exact numbers apply to my lot?
They are the values for the coordinates queried — 44.072, -111.448. Mapped ground-motion values change gradually with distance, but the honest answer for a specific parcel is to rerun the query with the parcel's own coordinates and, better, its own established site class. The service accepts any coordinates and is free.
Does Seismic Design Category D affect my permit?
The category itself is an input to design, not a permit fee or a form. What the building department reviews is whether the submitted structural design demonstrates compliance with the adopted codes — and the seismic values are part of that structural documentation. Your engineer's drawings carry them.
Does category D mean a big earthquake is expected in Ashton?
No — that is not what the category says. The service does not predict earthquakes. It converts national seismic hazard mapping into design values for a location; a category D answer means the mapped ground-shaking values at these coordinates correspond to that design tier under ASCE 7-16, nothing more. Hazard and probability questions belong to USGS hazard products, not to this design query.
Who can run this query?
Anyone. It is a free public web service of the U.S. Geological Survey. The query used for this page: earthquake.usgs.gov/ws/building-codes/asce7-16/calculate?latitude=44.072&longitude=-111.448&siteClass=D&riskCategory=II
Which code uses these values?
ASCE 7-16 — the design standard the USGS service is built around and that modern adopted building codes reference for seismic design. The adopted-code editions in force on a Fremont County permit are covered on this site's permit and energy-code pages.