Stable Ground, Shifting Risk: How America's Interior Is Confronting an Unexpected Seismic Reality

The Midwest Was Not Supposed to Shake

Oklahoma recorded more magnitude 3.0 or greater earthquakes in 2015 than California. That single data point, when it emerged from USGS monitoring reports, sent a signal through the structural engineering community that the country's seismic risk map was no longer a reliable guide to where ground motion could occur. What had changed was not the geology of the Great Plains—it was what humans were doing to it.

The proliferation of wastewater injection wells associated with oil and gas extraction, particularly from hydraulic fracturing operations, had altered subsurface pressure conditions along ancient fault systems that geologists had considered dormant for millennia. The result was a dramatic increase in seismic activity across states that most structural codes had historically treated as low-hazard zones. Oklahoma, Kansas, Arkansas, Ohio, and parts of Colorado and Texas all experienced elevated seismic frequency during the peak injection period of the 2010s.

This phenomenon—induced seismicity—represents only one dimension of a broader challenge now confronting engineers and code officials across the interior United States. The convergence of industrial ground disturbance, climate-related hydrological changes, and the sheer age of much of the region's built infrastructure has created a structural risk profile that demands a more sophisticated response than existing standards in many jurisdictions currently require.

A Code Landscape Struggling to Keep Pace

The International Building Code (IBC) and ASCE 7, the primary standards governing seismic design in the US, are updated on regular cycles. Both documents have progressively expanded seismic design requirements into regions that earlier editions treated as negligible-risk zones. The 2022 edition of ASCE 7 incorporated updated seismic hazard maps reflecting more recent ground motion data, including adjustments that affected portions of the central and eastern United States.

However, code adoption in the US is not uniform. Individual states and municipalities adopt model codes on their own schedules, and many jurisdictions in the interior remain on older editions that predate the most current hazard assessments. The result is a patchwork regulatory environment in which the structural standards applicable to a given project may reflect seismic risk data that is a decade or more out of date.

For engineers operating in this environment, the question is not simply what the code requires—it is what the actual risk profile of a site demands. Forward-thinking firms are increasingly framing this as a professional responsibility question rather than a minimum-compliance exercise. When the gap between what a code mandates and what current science indicates is significant, designing only to the code floor may not constitute adequate professional practice.

What Structural Adaptation Actually Looks Like

Applying seismic design principles in regions where they have not historically been standard practice does not necessarily mean retrofitting every structure in the central US to California standards. It does mean incorporating a risk-informed mindset into design decisions that would previously have been made without seismic considerations.

For new commercial and industrial construction in affected zones, this can involve relatively modest interventions: specifying ductile detailing in concrete and steel connections, designing diaphragms with sufficient in-plane stiffness to distribute lateral forces, and ensuring that nonstructural components—mechanical systems, cladding, and ceiling assemblies—are anchored to accommodate dynamic loading. These measures add marginal cost when incorporated at the design stage. They add substantial cost when retrofitted after the fact.

For infrastructure—bridges, water treatment facilities, pipelines, and utility substations—the calculus is more complex. Much of the interior US's critical infrastructure was designed and constructed under standards that assigned these regions a seismic hazard of essentially zero. Structural assessments of older bridges in states like Missouri and Tennessee, which sit near the New Madrid Seismic Zone—one of the most historically active fault systems in North America—have identified significant vulnerabilities in the event of a major seismic event. The New Madrid zone produced a sequence of catastrophic earthquakes in 1811 and 1812 that are estimated to have exceeded magnitude 7.0. A comparable event today would affect infrastructure that was built with no consideration of that possibility.

Climate Change as a Compounding Variable

Induced seismicity is not the only driver of elevated ground motion risk in historically stable regions. Climate scientists and geotechnical engineers have begun examining the relationship between large-scale hydrological changes and subsurface stress conditions. Extended drought cycles alter the moisture content of deep soil and rock formations, affecting the pore pressure conditions that influence fault stability. Conversely, extreme precipitation events and the accelerated drawdown of aquifers—both of which are becoming more frequent across the central US—create subsurface disturbance patterns that are not well represented in historical seismic hazard models.

This intersection of climate science and structural engineering is still an emerging area of research, but its practical implications are directionally clear: the subsurface conditions that informed twentieth-century seismic hazard assessments are not necessarily stable parameters for twenty-first-century design.

The Business Case for Proactive Resilience

Beyond the technical and ethical arguments, there is a straightforward economic case for incorporating seismic resilience into structures in regions where it is not yet mandated. Insurance markets have begun to price seismic risk in the central US more aggressively as loss data has accumulated. Institutional investors and commercial tenants in affected markets are increasingly asking questions about structural performance during ground motion events. And the liability exposure associated with a structure that sustains preventable damage during a seismic event—when the risk was known and the engineering response was to do the minimum the code required—is a scenario that no firm should be comfortable accepting.

Resilience, properly understood, is not an amenity. It is a measure of a structure's capacity to protect its occupants, preserve its functional integrity, and minimize economic loss under conditions that fall outside the normal operating envelope. In a risk environment that is demonstrably shifting, designing to historical norms is not conservative—it is complacent.

Engineering to What the Future Requires

At ADD Structures, our approach to structural design is grounded in the principle that precision requires foresight. The seismic risk profile of the American interior is not what it was fifty years ago, and the standards applied to structures built today must reflect what we now know—not the assumptions that prevailed when the codes were last revised.

Building resilience as a standard practice, rather than as an exception triggered only by code mandate, is not an ideological position. It is a professional one. The structures we design and build today will stand for decades. The ground beneath them is not as stable as we once assumed, and the engineering response to that reality should be proportionate to the risk—not to the minimum line printed in a code that may already be obsolete.