Earthquakes don't kill people — buildings do. That blunt reality drives the entire field of earthquake engineering. According to the USGS, the vast majority of earthquake fatalities result from building collapse rather than ground shaking itself. The 2010 Haiti earthquake (M7.0) killed an estimated 100,000 to 316,000 people largely because Port-au-Prince's building stock was constructed with little to no seismic consideration. By contrast, Chile's M8.8 earthquake just weeks later — releasing roughly 500 times more energy — killed 525 people, in large part because Chile enforces rigorous seismic building codes.
The phrase "earthquake-proof" is technically a misnomer that engineers avoid. No structure can be made completely immune to seismic forces, particularly when a fault ruptures directly beneath it. The realistic goal is earthquake resistance: designing buildings that protect human life during the expected level of shaking in their region, even if the structure itself is damaged beyond repair. Some critical facilities — hospitals, fire stations, emergency operations centers — are designed to remain functional after an earthquake, but achieving that standard for every building would be prohibitively expensive.
This article covers how buildings fail during earthquakes, the engineering strategies used to keep them standing, the evolution of seismic building codes worldwide, and the challenge of retrofitting the millions of vulnerable structures that predate modern standards.
Why Buildings Fail During Earthquakes
Understanding how structures collapse is the first step toward preventing it. Earthquakes subject buildings to lateral (horizontal) forces that most structures are not naturally designed to resist — gravity pulls downward, and buildings are excellent at handling vertical loads. When the ground moves sideways, buildings must transfer that energy through their structural systems without losing integrity. When they can't, failure follows predictable patterns.
Soft-Story Collapse
Soft-story collapse is one of the most common and deadly failure modes in earthquakes. It occurs when one floor of a building — typically the ground floor — is significantly weaker or more flexible than the floors above it. The classic example is an apartment building with an open parking garage or retail space at street level. The ground floor has large openings, fewer walls, and less lateral bracing, while the upper floors are stiffened by interior walls. During an earthquake, the weak ground floor absorbs a disproportionate amount of lateral displacement and collapses, pancaking the floors above downward.
The 1994 Northridge earthquake (M6.7) in Los Angeles destroyed or severely damaged approximately 200 soft-story apartment buildings. The Northridge Meadows apartment complex collapse killed 16 people — the single deadliest structural failure of that earthquake. The building's ground-floor parking structure, supported by slender columns with minimal bracing, gave way within seconds.
Unreinforced Masonry (URM) Failure
Unreinforced masonry buildings — constructed from brick, stone, or concrete block without steel reinforcement — are among the most vulnerable structures in any earthquake. The mortar joints between masonry units have very little tensile strength, meaning they crack and separate under lateral loading. URM buildings can shed their walls outward, collapse parapets onto sidewalks, or lose entire facades. According to FEMA, unreinforced masonry buildings account for a disproportionate share of earthquake casualties in the United States, particularly in the central and eastern states where older brick construction is common but earthquake awareness is lower.
Pancake Collapse
Pancake collapse occurs when floor slabs detach from supporting columns and fall flat, stacking on top of each other. This failure mode is particularly common in reinforced concrete buildings where the connections between columns and floor slabs lack adequate ductility. The 1999 İzmit earthquake (M7.6) in Turkey produced widespread pancake collapses in mid-rise concrete frame buildings, contributing to a death toll exceeding 17,000. Many of the collapsed structures had been built using the "flat-slab" construction method, where floors rest directly on columns without beams — a design highly susceptible to punching shear failure during lateral loading.
Torsional Failure
When a building's center of stiffness does not coincide with its center of mass, earthquake forces cause the structure to twist rather than sway uniformly. This torsional response concentrates stress at the building's corners and on whichever side is farthest from the center of stiffness. L-shaped, T-shaped, and irregularly-planned buildings are particularly susceptible. Buildings with stiff shear walls on only one side and flexible frames on the other can experience severe torsional effects even from moderate earthquakes.
Foundation Failure
Even a well-designed superstructure can fail if its foundation cannot maintain contact with the ground or transfers seismic loads unevenly. Soil liquefaction — where saturated, loose soil temporarily loses its strength and behaves like a liquid — can cause foundations to sink, tilt, or shift laterally. The 2011 Christchurch earthquake (M6.2) in New Zealand caused widespread liquefaction across the city's eastern suburbs, undermining foundations and rendering thousands of homes uninhabitable regardless of the structural quality of the buildings themselves.
Performance-Based Seismic Design
Modern earthquake engineering has largely moved beyond the binary question of "will this building collapse?" toward a more nuanced framework called performance-based seismic design (PBSD). Rather than designing to a single threshold, engineers define multiple performance objectives corresponding to different levels of earthquake intensity.
The three primary performance levels, as defined by ASCE 41 (Seismic Evaluation and Retrofit of Existing Buildings), are:
Life Safety is the minimum standard required by most building codes. The building may sustain significant structural damage — cracked walls, permanent deformation, broken windows — but the structural system retains enough integrity to prevent collapse. Occupants can evacuate safely. The building may be a total economic loss.
Immediate Occupancy means the building sustains only minor damage. The structural system retains nearly all of its pre-earthquake strength. Doors and elevators function. The building can be occupied immediately after the earthquake with only minor repairs needed. This performance level is typically specified for essential facilities like hospitals and fire stations.
Operational is the highest performance level. Not only is the structure undamaged, but all building systems — mechanical, electrical, plumbing, IT — remain functional. The building continues operating without interruption. Data centers, emergency operations centers, and certain military facilities may be designed to this standard.
This framework allows building owners and communities to make informed decisions about how much seismic protection they're willing to pay for. A warehouse may only need life safety performance, while a hospital needs immediate occupancy, and a 911 dispatch center needs operational performance.
Engineering Strategies for Earthquake Resistance
Base Isolation
Base isolation is one of the most effective and elegant strategies in earthquake engineering. The concept is simple: decouple the building from the ground so that when the earth shakes, the building doesn't move as much. In practice, this is achieved by placing flexible bearings — usually lead-rubber bearings or friction pendulum bearings — between the building's foundation and its superstructure.
Lead-rubber bearings consist of layers of rubber bonded to steel plates with a central lead core. The rubber layers provide flexibility, allowing the bearing to deform laterally during an earthquake. The lead core provides damping, absorbing energy and limiting how far the building moves. Friction pendulum bearings use a curved sliding surface; the building slides along the curve during shaking and gravity returns it to center afterward.
Base isolation reduces the acceleration experienced by the building by a factor of 3 to 10 compared to a fixed-base structure, depending on the specific system and earthquake characteristics. It is most effective for stiff, low-to-mid-rise buildings where the natural period of the structure can be shifted well below the dominant frequency of expected ground motion.
Notable base-isolated buildings include:
- San Francisco City Hall — Retrofitted with 530 lead-rubber isolators after the 1989 Loma Prieta earthquake exposed vulnerabilities. The building can now move up to 26 inches laterally during an earthquake while the superstructure remains essentially undamaged.
- Los Angeles City Hall — Retrofitted in the late 1990s with base isolation and viscous dampers. The 1928 building, a Los Angeles landmark, sits atop 416 isolators.
- Utah State Capitol — Completed a $260 million seismic retrofit in 2008 that included base isolation, making it one of the most seismically resilient government buildings in the United States.
- Japan's National Museum of Western Art (Tokyo) — A UNESCO World Heritage site retrofitted with base isolation to protect both the Le Corbusier-designed building and its irreplaceable art collection.
Tuned Mass Dampers
A tuned mass damper (TMD) is a heavy mass mounted inside a building, typically near the top, designed to swing in opposition to the building's natural sway. When the building moves in one direction, the damper moves in the opposite direction, counteracting the motion and reducing peak displacement and acceleration.
The most famous example is Taipei 101's 730-metric-ton (approximately 1.6 million pounds) steel pendulum, suspended between the 87th and 92nd floors. The sphere — 5.5 meters in diameter — is the world's largest and heaviest tuned mass damper. During Typhoon Soudelor in 2015, the damper was recorded swinging over one meter in each direction, visibly counteracting the building's motion. The same system provides seismic protection for the 508-meter supertall tower, which sits in an active seismic zone near the Manila Trench subduction system.
Other buildings employing tuned mass dampers include One Wall Centre in Vancouver, Citicorp Center (now Citigroup Center) in New York, and numerous high-rise buildings in Japan. Some structures use active mass dampers, which employ sensors and actuators to move the mass in real time based on measured building response rather than relying on passive pendulum motion.
Viscous Dampers, Shear Walls, and Braced Frames
Viscous dampers function like large shock absorbers. Consisting of a piston within a cylinder filled with silicone fluid, they convert kinetic energy from building motion into heat. Viscous dampers are commonly installed at the ends of diagonal braces or between adjacent floors. They are particularly effective for retrofits because they can be added to existing structural systems without fundamentally altering the building's load path.
Shear walls are vertical walls made of reinforced concrete or plywood-sheathed wood framing that resist lateral forces through in-plane rigidity. In residential construction, plywood shear walls are one of the most cost-effective seismic resistance measures available. In high-rise construction, reinforced concrete shear walls (often combined into a central core around elevator shafts) provide the primary lateral force-resisting system.
Moment-resisting frames use rigid beam-column connections that resist lateral forces through the bending (moment) capacity of the joints. Steel moment frames were long considered the gold standard for seismic design, but the 1994 Northridge earthquake revealed that many welded beam-column connections were fracturing in a brittle manner rather than deforming ductilely. Post-Northridge research led to improved connection details, including reduced beam sections (the "dogbone" connection) that force yielding into the beam away from the vulnerable weld.
Braced frames use diagonal members to form triangulated structures that resist lateral forces through axial tension and compression in the braces. Concentrically braced frames (CBFs) are stiff and economical but can lose strength after brace buckling. Buckling-restrained braced frames (BRBFs) encase the steel brace core in a concrete-filled tube that prevents buckling, allowing the brace to yield in both tension and compression — significantly improving ductility and energy dissipation.
Cross-Laminated Timber (CLT)
Cross-laminated timber is an emerging structural material that has shown promising seismic performance in testing. CLT panels consist of layers of dimension lumber glued together with alternating grain directions, creating large, stiff panels that can serve as walls, floors, and roofs.
In shake-table testing, including a 2009 test at Japan's E-Defense facility, a seven-story CLT building survived 14 consecutive earthquake simulations — including shaking comparable to the 1995 Kobe earthquake — with minimal damage. CLT's favorable strength-to-weight ratio means that a CLT building weighs significantly less than an equivalent reinforced concrete structure, generating lower seismic forces. The material also offers carbon sequestration benefits that have made it attractive from a sustainability perspective.
Several earthquake-prone regions have updated building codes to accommodate tall timber construction, including British Columbia, Oregon, and Washington in the Pacific Northwest, and multiple European countries under Eurocode provisions.
Seismic Building Codes: A Global Overview
Seismic building codes are the regulatory backbone of earthquake safety. They establish minimum standards for new construction based on the expected level of seismic hazard in a given region. The development of these codes has been driven almost entirely by earthquake disasters — each major earthquake exposes vulnerabilities that codes are then updated to address.
United States
In the United States, the first seismic building code provisions were adopted by the city of Long Beach, California, following the 1933 Long Beach earthquake (M6.4), which destroyed numerous unreinforced masonry school buildings. The Field Act, also passed in 1933, established seismic design requirements specifically for California public schools.
The Uniform Building Code (UBC), first published in 1927, incorporated increasingly detailed seismic provisions through successive editions. The UBC was replaced in 2000 by the International Building Code (IBC), which is now the model code adopted (with modifications) by most U.S. jurisdictions. The IBC references ASCE 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) for its seismic design provisions, and ASCE 7 in turn uses the USGS National Seismic Hazard Maps to define design ground motions.
Key milestones in U.S. seismic codes include the post-1971 San Fernando earthquake requirement for ductile detailing in concrete frames, the post-1994 Northridge earthquake revisions to steel moment frame connection design, and the introduction of probabilistic seismic hazard analysis as the basis for design ground motions.
Japan
Japan has the world's most advanced seismic building code, refined through centuries of devastating earthquakes. The Building Standard Law of Japan was first enacted in 1950, heavily influenced by the 1923 Great Kanto earthquake (M7.9, approximately 105,000 deaths). The law underwent a major revision in 1981, introducing the "new seismic design method" that requires buildings to remain elastic during moderate earthquakes and prevent collapse during severe earthquakes — a two-level design philosophy.
After the 1995 Kobe earthquake (M6.9, 6,434 deaths), which revealed that many pre-1981 buildings performed poorly, Japan launched a massive national retrofit program targeting older buildings. The 2011 Tohoku earthquake (M9.1) — while devastating due to the tsunami — largely validated the post-1981 building code, as structural collapse of modern buildings due to shaking was rare despite the extreme ground motion.
Europe
Eurocode 8 (EN 1998) governs seismic design across European Union member states. Adopted starting in the 2000s, it establishes common principles but allows each country to define its own seismic hazard zones and importance factors through National Annexes. Countries like Italy, Greece, and Turkey have significant seismic hazard zones, while countries like the United Kingdom and Scandinavia have minimal seismic design requirements.
Turkey's seismic code has undergone urgent revisions following the 1999 İzmit earthquake and again after the catastrophic February 2023 earthquakes in southeastern Turkey (M7.8 and M7.5), which killed over 50,000 people and exposed widespread failures in code enforcement and construction quality.
| Year | Country/Region | Code Milestone | Triggered By |
|---|---|---|---|
| 1933 | United States (California) | Field Act — seismic standards for schools | 1933 Long Beach M6.4 |
| 1950 | Japan | Building Standard Law enacted | 1923 Great Kanto M7.9 aftermath |
| 1971 | United States | Ductile concrete detailing required | 1971 San Fernando M6.6 |
| 1981 | Japan | New Seismic Design Method (two-level) | Cumulative research and policy |
| 2000 | United States | IBC replaces UBC as model code | Consolidation of model codes |
| 2004 | European Union | Eurocode 8 published | EU standardization effort |
| 2018 | Turkey | Revised Turkish seismic code (TBEC) | 1999 İzmit M7.6 lessons |
| 2023 | Turkey | Enforcement review and code revision initiated | Feb 2023 M7.8 Kahramanmaraş |
Engineering Techniques Comparison
| Technique | Mechanism | Approximate Cost Range | Best Suited For |
|---|---|---|---|
| Base isolation (lead-rubber bearings) | Decouples building from ground motion using flexible bearings | $50–$150 per sq ft (new); more for retrofit | Low-to-mid-rise stiff buildings, hospitals, museums, government buildings |
| Tuned mass damper | Heavy pendulum counteracts building sway at resonant frequency | $1M–$10M+ depending on size | Tall and supertall buildings (30+ stories) |
| Viscous dampers | Piston-in-cylinder shock absorbers convert motion to heat | $10–$40 per sq ft | Retrofits, braced frame systems, bridge structures |
| Buckling-restrained braced frames | Steel core braces in concrete-filled tubes yield without buckling | $15–$50 per sq ft | Mid-rise steel and concrete buildings |
| Reinforced concrete shear walls | Rigid vertical walls resist lateral forces in-plane | $8–$25 per sq ft | All building heights; core walls for high-rises |
| Moment-resisting steel frames | Rigid beam-column joints flex to absorb lateral forces | $12–$35 per sq ft | Commercial and institutional buildings |
| Cross-laminated timber (CLT) panels | Lightweight, stiff wood panels with alternating grain layers | $20–$40 per sq ft | Low-to-mid-rise residential and commercial (up to ~18 stories) |
Bar or comparison diagram — Base-Isolated vs. Fixed-Base Building Response During a M7.0 Earthquake. Show peak floor acceleration at each story level for both building types. A fixed-base 5-story building might experience 0.4g at the base amplifying to 1.0g+ at the roof, while the same building with base isolation might experience 0.1–0.2g throughout. Data source: General seismic engineering research; conceptual representation based on shake-table test data from E-Defense facility and published engineering studies.
Retrofitting Vulnerable Buildings
The Retrofit Challenge
The seismic vulnerability of existing buildings vastly exceeds that of new construction. According to FEMA, there are an estimated 90 million buildings in the United States, the majority of which were built before modern seismic code provisions were adopted in their region. In California alone, the California Seismic Safety Commission has estimated that hundreds of thousands of buildings may require seismic upgrades.
Retrofitting is more complex and often more expensive per square foot than designing seismic resistance into a new building. The engineer must work within the constraints of an existing structure, often without complete documentation of how it was built, while accommodating ongoing building occupancy and use.
Common Retrofit Approaches
Steel bracing involves adding diagonal steel braces to existing frames, typically on the building's exterior or within selected bays of the interior. This increases the building's lateral stiffness and strength. Chevron braces and X-braces are common configurations.
Concrete jacketing adds a new layer of reinforced concrete around existing columns to increase their strength, stiffness, and ductility. The existing column is essentially encased in a new, larger column. This technique is widely used for deficient reinforced concrete frames but adds significant weight to the structure.
Fiber-reinforced polymer (FRP) wrapping applies sheets of carbon fiber or glass fiber composite material around concrete columns. The FRP wrap confines the concrete, dramatically increasing its ductility and shear capacity without significantly increasing the column's dimensions or weight. FRP wrapping has become increasingly popular because it is fast to install, minimally disruptive, and highly effective.
Foundation bolting secures a wood-frame house to its concrete foundation using anchor bolts and steel plates. Many older homes in California and the Pacific Northwest were simply resting on their foundations by gravity, with no positive connection. During an earthquake, these houses can slide off their foundations entirely.
Cripple wall bracing strengthens the short wood-framed walls between the foundation and the first floor in older raised-foundation homes. These cripple walls — often only 2 to 4 feet tall — are typically unbraced and collapse readily during earthquakes, dropping the entire house to the ground. Adding plywood sheathing to cripple walls is one of the simplest and most cost-effective seismic retrofits available for residential structures.
California's Mandatory Retrofit Programs
California leads the nation in mandatory seismic retrofit programs, driven by the state's high seismic hazard and its history of earthquake losses.
Los Angeles Soft-Story Retrofit Ordinance (2015): The city of Los Angeles passed an ordinance requiring approximately 13,500 wood-frame soft-story buildings to be retrofitted within several years. These buildings — typically apartment buildings with ground-floor parking — were identified as among the most dangerous building types in Los Angeles. Property owners are responsible for the cost, though the expense may be passed to tenants. As of recent reporting, the vast majority of the identified buildings have either completed retrofits or have work in progress.
Los Angeles Concrete Building Ordinance (2015): In conjunction with the soft-story ordinance, Los Angeles also mandated the evaluation and retrofit of approximately 1,500 non-ductile concrete buildings. These structures, built before 1976, lack the ductile reinforcing details required by modern codes and are susceptible to brittle collapse. The concrete ordinance provides a longer compliance timeline (up to 25 years) given the greater complexity and cost involved.
San Francisco Soft-Story Program: San Francisco's Mandatory Seismic Retrofit Program targets wood-frame buildings with five or more residential units and a ground floor that is potentially weak or soft. The program was implemented in phases, with the first compliance deadlines beginning in 2017. The program covers approximately 6,000 buildings across the city.
Learn more about seismic retrofitting Understanding retrofit costs Soft-story retrofit requirements How to earthquake-proof your home California earthquake insurance options
The Cost Problem
The fundamental challenge of seismic retrofitting is economic. Retrofitting a soft-story apartment building in Los Angeles typically costs $60,000 to $200,000 or more, depending on the building's size and configuration. Non-ductile concrete building retrofits can run into the millions. For unreinforced masonry buildings, costs can range from $20 to $100+ per square foot.
Who pays for seismic retrofits is a contentious policy question. In rental housing, the cost typically falls on property owners, but many jurisdictions allow some or all of the expense to be passed through to tenants via rent increases. In Los Angeles, the soft-story ordinance permits owners to pass through up to $38 per month to tenants for a defined period. This creates an equity issue: the tenants who can least afford rent increases are often those living in the most seismically vulnerable buildings.
For historic buildings — churches, theaters, civic buildings — the cost of seismic retrofitting can rival the building's entire assessed value. Without public funding or tax incentives, owners of historic unreinforced masonry buildings sometimes face a choice between demolition and a retrofit that costs more than the building is worth.
FEMA provides grants through its Hazard Mitigation Grant Program (HMGP) and Building Resilient Infrastructure and Communities (BRIC) program, but demand far exceeds available funding. Some states, including California and Oregon, offer tax incentives or reduced-interest loan programs for seismic retrofits.
FEMA Earthquake Risk Reduction Resources California Seismic Safety Commission
The Future of Earthquake-Resistant Design
Several emerging trends are reshaping earthquake engineering:
Resilience-based design extends performance-based design to consider not just the building's physical performance but the time and cost required to restore it to full function. The REDi (Resilience-Based Earthquake Design Initiative) rating system, developed by Arup, assigns ratings (Platinum, Gold, Silver) based on how quickly a building can return to service after a design-level earthquake.
Self-centering systems use post-tensioned tendons or shape-memory alloys that allow a building to deform during an earthquake but return to its original position afterward, like a rubber band. Conventional seismic systems accept permanent deformation; self-centering systems aim to eliminate it.
Real-time structural health monitoring uses sensors throughout a building to detect damage immediately after an earthquake, providing building owners and first responders with rapid assessments of whether a building is safe to occupy. Several commercial systems are now available, and instrumented buildings have provided valuable data during recent earthquakes in Japan and California.
3D-printed and modular construction techniques allow for precise, repeatable fabrication of structural elements with optimized seismic details. While still in early stages, these methods have the potential to reduce construction costs and improve quality control, particularly in developing countries where construction practices often fall short of code requirements.