Earthquake Risk in the US

The West Coast: Highest Ongoing Risk

The western United States sits along the Pacific Ring of Fire, where tectonic plates are actively colliding, grinding, and subducting. Three major fault systems define western seismicity:

• San Andreas Fault System (California): A 1,300-km right-lateral strike-slip fault running through most of California. The San Andreas and its many associated faults (Hayward, Calaveras, San Jacinto, Elsinore) produce thousands of earthquakes annually. The southern section, from Parkfield to the Salton Sea, is considered overdue for a major rupture. Scientists estimate an approximately 60% probability of an M6.7+ earthquake in the San Francisco Bay Area within the next 30 years.
• Cascadia Subduction Zone (Pacific Northwest): The Juan de Fuca Plate is actively diving beneath North America along this offshore megafault. Unlike the San Andreas, which produces frequent moderate earthquakes, the Cascadia zone "locks" for centuries and then releases enormous energy in megathrust events. The last full-margin rupture was January 26, 1700, known from Japanese tsunami records and Pacific Northwest coastal geology.
• Alaska Aleutian Arc: Alaska sits at the convergence of the Pacific and North American plates and experiences more large earthquakes than any other US state. The 1964 Good Friday earthquake (M9.2) remains the second-largest ever instrumentally recorded.

The Intermountain West

The Basin and Range Province, encompassing Nevada, Utah, western Arizona, and parts of Idaho, is one of the most seismically active regions in the US outside of California and Alaska. The region is actively stretching, creating a series of normal faults (where one side drops relative to the other). The Wasatch Front in Utah, running through Salt Lake City, is considered capable of producing an M7+ earthquake. Nevada records more M4+ earthquakes per square mile than California in some years.

Washington and Oregon, beyond the Cascadia offshore threat, also have active crustal faults capable of M6–7 events. The 2001 Nisqually earthquake (M6.8) near Olympia, Washington caused over $2 billion in damage and is a reminder that shallow crustal earthquakes remain a serious hazard in the Pacific Northwest independent of the megathrust scenario.

Central US: New Madrid and Induced Seismicity

The New Madrid Seismic Zone (NMSZ) cuts through the central Mississippi Valley, Missouri, Arkansas, Tennessee, Kentucky, and Illinois. The 1811–1812 New Madrid earthquakes were among the largest in US history, estimated at M7.5–8.0+. Ground effects included sand blows (liquefaction features), river bank collapses, and temporary reversal of the Mississippi River's flow in some locations. These events were so powerful they rang church bells in Boston and were felt across much of the eastern US.

More recently, the central US experienced a dramatic surge in seismicity from induced earthquakes. Oklahoma averaged fewer than 2 M3+ earthquakes per year before 2009. By 2015, it averaged over 900 per year, briefly making it the most seismically active state in the contiguous US. The cause: mass disposal of oil and gas extraction wastewater into deep injection wells, increasing pore pressure along dormant faults. After regulators reduced injection volumes, Oklahoma's earthquake rate fell substantially.

Browse US state earthquake data to see how each state's seismic activity has changed over the 2005–2025 USGS record.

Eastern US: Lower Frequency, Wide Impact

The eastern United States is geologically old and stable, ancient Precambrian rock that doesn't produce frequent earthquakes. But when earthquakes do occur, the dense, cold rock transmits seismic waves far more efficiently than the fractured western rock. The 2011 Virginia earthquake (M5.8) was felt by an estimated 1/3 of the US population, far more than a similar-magnitude California earthquake would affect. Eastern building stock, largely designed without seismic codes, adds to the vulnerability.

Key eastern seismic zones include the Wabash Valley (Illinois/Indiana border), the Charlevoix Seismic Zone (Quebec), and various historical fault zones in New England. Charleston, South Carolina experienced an M7.3 earthquake in 1886 that killed at least 100 people. The USGS National Seismic Hazard Model incorporates these zones into its assessment of eastern US risk.

Regional Hazard Comparison Table

The USGS National Seismic Hazard Map 2023 quantifies the regional differences described above as Peak Ground Acceleration (PGA) values mapped to FEMA Seismic Design Categories used in the International Building Code:

Why coastal California ranks SDC E

The San Andreas plate boundary slips at roughly 35 mm/yr through the Bay Area and southern California. Combined with intermediate thrust faults (Transverse Ranges) and basin enhancement (Los Angeles, Santa Clara Valley), design PGA frequently exceeds 0.50g, placing nearly every major coastal-California metro in SDC E for new construction. The 2008 USGS-FEMA HayWired scenario projected $112B direct loss from a Mw 7.0 Hayward Fault rupture and 800 fatalities.

Why the Pacific Northwest is reclassified upward

USGS NSHM 2023 increased Cascadia subduction-zone hazard estimates after new paleoseismic trenching at Willapa Bay and Discovery Bay confirmed at least seven Mw 8.7+ ruptures in the past 3,500 years. The mean recurrence interval is now estimated at 250 years; the most recent full-margin event was January 26, 1700. This drives Portland and Seattle metro design values from earlier 0.20g into the 0.30-0.45g range.

Worked example: a hospital relocation tradeoff

A 200-bed regional hospital developer compares two sites. Site A in Salt Lake City sits at PGA 0.35g (SDC D) on Class C soil, Risk Category IV requires special concentrically braced frames and base isolation, adding $4.5M to the structural budget vs an SDC C baseline at $2.1M. Site B in Indianapolis sits at PGA 0.08g (SDC C) on Class D soil, basic intermediate moment frame at $1.8M structural cost. Annual earthquake-property insurance premiums: Site A averages 38% loaded vs Site B's 8% loaded for the same coverage limit. Lifecycle cost-of-resilience ratio: roughly 4.2x higher in Salt Lake. Decision often comes down to whether trauma-center catchment and Medicare patient volume (3.2x higher in SLC for the modeled service area) justify the seismic premium.

When central-US moderate-hazard becomes high-risk

The New Madrid Seismic Zone delivers a sobering complication: 75% of buildings in the Memphis metro predate the 1996 IBC adoption, and 25% of those are unreinforced masonry, the most fragile typology under any ground motion above 0.10g. A repeat of the 1811-1812 sequence (estimated Mw 7.3-7.5) in modern St. Louis or Memphis has been modeled by FEMA HAZUS at $300B+ direct loss with 4,500 fatalities, despite only moderate-hazard PGA design values. This is the textbook case for vulnerability dominating the risk equation.

What USGS Data Reveals

The 20-year PlainQuake dataset (2005–2025) provides a clear picture of US seismic frequency. Alaska dominates in both event count and maximum magnitudes. California shows consistent high-frequency activity across its complex fault system. The central US spike from 2013–2016 is visible in state-level data, particularly Oklahoma, Kansas, and Texas.

The USGS National Seismic Hazard Model, updated regularly, estimates that about 42 of the 50 US states have a reasonable chance of experiencing damaging ground motion in the next 50 years. Explore state-level statistics and year-by-year trends to see how seismic activity has evolved over the 20-year USGS record.

Frequently Asked Questions