Published
What seismic design category is St. Anthony? B — on Site Class B soil.
A question with a computable answer
Most of what this site publishes is a question with a document behind it — a statute, a checklist, a form. This one is better: it is a question you can put to a federal web service and get a dated, reproducible answer from. The USGS maintains a free design service that returns seismic design values for any coordinate in the country under the method of ASCE 7-16, the standard the service is named for. Give it a latitude, a longitude, a site class and a risk category, and it answers.
Run it for St. Anthony — latitude 43.966, longitude −111.682, Risk Category II — and the heart of the response reads:
“ss”: 0.394 · “s1”: 0.151 · “pga”: 0.167 — and, for Site Class B soil: “fa”: 0.9, “fv”: 0.8, “sds”: 0.237, “sd1”: 0.081, “sdc”: “B”
— USGS ASCE 7-16 Design Web Service, response for 43.966, −111.682, Site Class B, Risk Category II. Queried 11 September 2026 (service timestamp 2026-09-11T18:32:01Z)
So the headline answer: Seismic Design Category B, on Site Class B soil, at these coordinates, for Risk Category II. The category is the classification that scales how much seismic detailing a structure’s design has to carry — and B sits at the quiet end of what this valley’s geology produces here. Every field in that response, decoded — all values as returned on 11 September 2026, Site Class B:
| Field | What it is | Returned here |
|---|---|---|
| ss | The mapped short-period spectral acceleration | 0.394 |
| s1 | The mapped one-second-period spectral acceleration | 0.151 |
| pga | Peak ground acceleration | 0.167 |
| fa / fv | Site coefficients, set by the site class | 0.9 / 0.8 |
| sds / sd1 | Short- and long-period design spectral accelerations | 0.237 / 0.081 |
| sdc | The seismic design category itself | B |
But that headline sentence carries two inputs most readers have never been asked for, and one of them changes everything.
The input that changes the answer: your soil
The service does not return one category for a town. It returns one per site class — the soil under the building. The mapped ground-motion values (Ss 0.394, S1 0.151) are fixed by location, but the site coefficients and the category that follows them are computed from the class you supply. Same coordinates, four queries, run on the same morning:
| Query input: site class | What the service returned at 43.966, −111.682 |
|---|---|
| B | Site coefficients Fa 0.9, Fv 0.8 → SDS 0.237, SD1 0.081 → Seismic Design Category B |
| BC (the boundary class — and the service’s default if you leave the field blank) | The mapped values only — no site coefficients and no category at all; every derived field returns null |
| C | Site coefficients Fa 1.3, Fv 1.5 → SDS 0.342, SD1 0.151 → Seismic Design Category C |
Two readings of that table matter. The first: the category follows the ground. Move from class B soil to class C soil on the same lot’s coordinates and the answer steps up from B to C; each softer lettered class raises it again. That is why the number that belongs on your plans is not “what the town is” but “what the soil under this house is” — a question a soil investigation answers, not a map. The second reading is the trap.
The default query returns no answer at all
Leave the site-class field off — which is exactly what most people do the first time they run the service — and it fills in its own default: BC, the boundary class between B and C. And for BC, the service computes nothing beyond the mapped values: no coefficients, no SDS, no SD1, and no category. A builder who runs the naive query gets a page of numbers with the one answer missing, and may conclude the tool is broken. It is not. It is waiting for the one input only the soil under your lot can supply.
What this looks like in practice, as a short list:
- The coordinates are not the input that matters most. They are fixed — 43.966, −111.682 gives Ss 0.394 and S1 0.151 whether you like it or not.
- The site class is. B, C and the boundary BC all sit inside one town; which one your lot sits on is a soil question.
- The default answers nothing. BC — the blank-field default — returns nulls for the category; you must ask for a lettered class to get one.
- The risk category is the other quiet input. This page’s numbers are Risk Category II; a designer chooses it to match the building, and the service computes per what you give it.
Where the number goes
A seismic design category is not a permit-desk question — no form at the city or the county asks you to write “B” in a box. It is a design input, and it reaches your project through three doors:
- The code editions underneath. The designs are drawn under the editions the state adopts, and which code editions apply to a permit here is the previous day’s page on this site.
- Who holds the pen. Whether your structure’s design needs a licensed hand behind it is a question this valley answers with rules of its own: who has to stamp your plans around St. Anthony.
- The snow load beside it. Seismic is only half of what the ground asks of a roof here; the vertical half is the snow, set by district — the 50 PSF St. Anthony district — and a designer needs both numbers before anything is drawn.
Run the query yourself — and record it
- Open the USGS design service for ASCE 7-16 and enter your lot’s coordinates to three decimals; 43.966 and −111.682 are the ones behind this page.
- Do not leave site class blank — the BC default returns no category; put in the lettered class your soil information supports.
- Set the risk category to match the structure, as your designer specifies it.
- Read the response for sdc — that single letter is the answer — and note the date on the response.
- File the whole response with your project papers; a dated service answer is a better record than a remembered letter.
- Put the category, the site class behind it, and the snow district in front of whoever draws the structure — together they size the bracing and the roof.
- Then call (208) 656-1745 with the one question your build actually turns on — which soil, which letter, which desk.
Common questions
What seismic design category is St. Anthony, Idaho?
Seismic Design Category B - returned by the USGS ASCE 7-16 design service for the town's coordinates (43.966, -111.682) at Site Class B and Risk Category II, queried 11 September 2026. The category changes with the soil, so the answer for a specific lot follows its site class.
What does 'seismic design category' mean?
A classification, produced under the ASCE 7-16 method the USGS service implements, that scales how much seismic detailing a structure's design has to carry. It is computed from mapped ground-motion values adjusted for the site class and the building's risk category.
Why does the site class matter so much?
Because the service computes a category per soil class, not per town. At St. Anthony's coordinates, Site Class B returned category B and Site Class C returned category C on the same morning's queries - the ground under the lot, not the town's name, picks the answer.
Why did my USGS query come back with no seismic design category?
You probably left site class blank. The service's default is the boundary class BC, and for BC it returns the mapped values but nulls for the site coefficients and the category. Enter a lettered class - B, C, or the class a soil investigation gives you - and the category appears.
What are Ss and S1?
The mapped spectral accelerations the service returns for your coordinates - the short-period value Ss and the one-second-period value S1. For St. Anthony they are 0.394 and 0.151. They are location facts; the site class then adjusts them into design values.
Is St. Anthony a high-seismic town?
The service does not grade towns; it returns values. At these coordinates it returned category B on Site Class B soil - and stepped up to C on softer ground. What counts as demanding is the designer's judgment against the numbers, not a label this page could hand out.
Who decides which site class my lot is?
The class comes from the soil information for the lot - the investigation a designer commissions when the design needs it. The USGS service computes only what you give it, which is why the class, not the coordinates, is the load-bearing input.
Does the seismic category change my permit at the city or the county?
No desk asks for the letter on a form. The category is a design input - it shapes the bracing and detailing in the plans, and the plans are what the permit reviews. Which desk reviews them depends on which side of the city limits the lot is on.
Can I run the USGS query myself?
Yes - the service is free and public: the ASCE 7-16 calculate endpoint at earthquake.usgs.gov, with a latitude, longitude, site class and risk category. Record the response date alongside the answer; this page's numbers were queried 11 September 2026.