58% of housing stock in high seismic-risk counties predates 1980, vs. 51% nationally

Summary

Across the 48 US counties FEMA's National Risk Index rates "Relatively High" or "Very High" for earthquake risk, about 58.1% of the housing stock (12.6M of 21.7M units) was built before 1980 — roughly 7.6 percentage points above the 50.5% national average (145.4M of 287.8M units, all ~3,142 counties). 1980 is used as a proxy threshold for "predates modern seismic code": it is the nearest Census decade-bracket boundary to the 1976 Uniform Building Code overhaul, the first major nationwide seismic-design tightening, adopted after the 1971 San Fernando earthquake exposed serious design gaps. The gap is real but modest in aggregate, and it masks very large spread: Los Angeles County (California's largest, rated "Very High") is 71.1% pre-1980, while fast-growing Sun Belt seismic counties like Clark County NV (Las Vegas) are only 17.8% pre-1980 because most of their housing was built after the threshold.

How "high-seismic-risk county" and "modern building code" were defined

High-seismic-risk counties: FEMA's National Risk Index (NRI) county-level earthquake_risk_rating column, filtered to 'Relatively High' (47 counties) and 'Very High' (1 county, Los Angeles). This was the only county-grain seismic measure found in this corpus (search_catalog returned no table containing the word 'seismic'; NRI's earthquake block was the closest match). Important limitation: NRI's earthquake risk rating is a composite of hazard, building-value/population exposure, and social vulnerability — not pure ground-shaking hazard. This is visible in the results: St. Louis MO, and New York City's four boroughs, rank among the very highest 'risk' counties in this list not because they sit on major active faults but because of the New Madrid seismic zone's historical hazard combined with (for St. Louis) or a smaller, more diffuse hazard combined with enormous building-value exposure (for NYC). A purer ground-motion-hazard cut (e.g., USGS peak ground acceleration) would likely rank West Coast and Pacific Northwest counties (LA, SF Bay Area, Cascadia subduction zone counties in OR/WA) more heavily relative to high-exposure/moderate-hazard metros. No PGA or seismic-design-category column exists in this warehouse to run that alternate cut directly.

"Modern building codes" threshold: There is no single national date at which seismic codes became "modern" — code history is regional and incremental. Key milestones, per structural-engineering literature: the 1976 UBC substantially raised seismic design force levels following observations from the 1971 San Fernando earthquake, and is widely treated as the first major modern-era threshold; the 1994 Northridge earthquake then exposed steel moment-frame connection deficiencies even in 1994-and-earlier code editions, driving further reforms adopted into the 1997 UBC (considered the modern 'benchmark' code for several building types); and a nationally uniform code (the International Building Code) did not fully supersede regional codes like the UBC until around 2000. Census's ACS housing-age brackets are only available in decade buckets, so 1980 (the nearest decade boundary to 1976) is used here as the working proxy for "pre-modern-code." This likely UNDERSTATES the true pre-modern-code share, since a stricter 1994/1997 Northridge-era or 2000 IBC threshold would move a large share of 1980s-1990s construction into the "predates modern code" bucket too — a secondary pre-2000 figure is included in the underlying data for reference: pre-2000 shares run 57%-93% across the same 48 counties, well above the pre-1980 figures shown.

What the data shows, by county

The full 48-county list, sorted by percent of housing built before 1980, was computed as (built_1939_or_earlier + ... + built_1970_to_1979) / total_units from Census ACS 2023 5-year table B25034, joined to FEMA NRI earthquake_risk_rating on county_fips. Highest: St. Louis MO (85.9%), Queens NY (83.6%), Kings/Brooklyn NY (81.0%), Bronx NY (79.7%), New York/Manhattan NY (78.7%), San Francisco CA (77.2%), San Mateo CA (73.3%), Los Angeles CA (71.1%). Lowest: Clark NV/Las Vegas (17.8%), Yuma AZ (26.0%), Riverside CA (30.2%), Charleston SC (35.3%), Hawaii County HI (35.5%), Washoe NV/Reno (35.9%). The spread (18%-86%) is far wider than the difference between the high-risk-county average and the national average, so a single national number understates how differently exposed individual high-risk metros are.

Every query behind this report

3 warehouse calls ran in this session, in order. Each is reproducible against the same snapshot.

SELECT earthquake_risk_rating, count(*) FROM disasters.fema_nri_earthquake_by_county GROUP BY earthquake_risk_rating ORDER BY 2 DESC

WITH hi AS (
  SELECT county_fips, county_name, state_abbr, earthquake_risk_rating, population, building_value_usd
  FROM disasters.fema_nri_earthquake_by_county
  WHERE earthquake_risk_rating IN ('Very High','Relatively High')
),
h AS (
  SELECT county_fips, total_units,
    built_1939_or_earlier + built_1940_to_1949 + built_1950_to_1959 + built_1960_to_1969 + built_1970_to_1979 AS pre1980_units,
    built_1939_or_earlier + built_1940_to_1949 + built_1950_to_1959 + built_1960_to_1969 + built_1970_to_1979 + built_1980_to_1989 + built_1990_to_1999 AS pre2000_units
  FROM census.acs_housing_year_built_detail
  WHERE year = '2023'
)
SELECT hi.county_name, hi.state_abbr, hi.earthquake_risk_rating, h.total_units, h.pre1980_units,
  round(100.0*h.pre1980_units/h.total_units,1) AS pct_pre1980,
  round(100.0*h.pre2000_units/h.total_units,1) AS pct_pre2000
FROM hi JOIN h ON hi.county_fips = h.county_fips
ORDER BY pct_pre1980 DESC

WITH hi AS (
  SELECT county_fips FROM disasters.fema_nri_earthquake_by_county
  WHERE earthquake_risk_rating IN ('Very High','Relatively High')
),
h AS (
  SELECT county_fips, total_units,
    built_1939_or_earlier + built_1940_to_1949 + built_1950_to_1959 + built_1960_to_1969 + built_1970_to_1979 AS pre1980_units
  FROM census.acs_housing_year_built_detail WHERE year='2023'
)
SELECT
  (SELECT sum(total_units) FROM h JOIN hi USING(county_fips)) AS hi_total,
  (SELECT sum(pre1980_units) FROM h JOIN hi USING(county_fips)) AS hi_pre1980,
  (SELECT sum(total_units) FROM h) AS us_total,
  (SELECT sum(pre1980_units) FROM h) AS us_pre1980