建物はすべての地震でパンケーキのように潰れるか?

The Myth: Buildings Always Pancake in Earthquakes

Post-earthquake images of collapsed structures — floors stacked on top of each other like pancakes, buildings reduced to rubble — are among the most visceral and memorable in disaster coverage. These images have created a widespread belief that "earthquake = building collapse" and that surviving an earthquake in a building is largely a matter of luck. This myth is harmful in two opposite ways: it either paralyzes people into fatalistic acceptance ("nothing I do matters if the building falls") or causes dangerous behavior like running outside during shaking. The reality of how buildings perform in earthquakes is far more nuanced and, in many respects, considerably more reassuring — while also highlighting genuine ongoing vulnerabilities.

What Structural Pancake Collapse Actually Requires

The "pancake collapse" or "progressive floor collapse" failure mode occurs when vertical load-carrying columns or walls fail, allowing upper floor slabs to fall onto lower slabs. It is the most lethal failure mode because it leaves no survival space. Pancake collapse is typically associated with specific structural deficiencies: non-ductile concrete frames in which columns are not reinforced to bend without shattering, a ソフトストーリー（弱層）駐車場や店舗などの大きな開口部が原因で、上階に比べて著しく弱くなっている建物の階（通常は1階）。ソフトストーリーは最も一般的な倒壊メカニズムである。 ground floor (open plan with minimal shear walls), or inadequate connections between floor slabs and supporting elements. It is not a general outcome of all earthquakes or all buildings.

Modern 耐震設計地震力に耐えられるよう構造物を設計する手法。現代の耐震設計は、大地震における構造被害はある程度許容しつつ、倒壊の防止と人命の保護を目指す。 codes for reinforced concrete and steel construction specifically aim to prevent brittle column failure and progressive collapse. The philosophy of current codes is "life safety" — ensuring that buildings deform substantially, absorb energy, and may be damaged beyond economic repair, but do not collapse on occupants. Well-engineered modern buildings achieving code compliance are designed to survive the design-level earthquake (typically 10% probability of exceedance in 50 years) with occupant safety preserved.

Real-World Building Performance in Major Earthquakes

The evidence from instrumentally recorded, well-documented earthquakes consistently shows that pancake collapse of modern code-compliant buildings is rare. In the 1994 Northridge earthquake (M6.7, Los Angeles), about 25,000 buildings were inspected post-earthquake. The vast majority were habitable, a significant fraction sustained moderate to severe damage, and a small number collapsed or were condemned — but the collapses occurred overwhelmingly in pre-code structures, especially 無補強組積造（URM）鉄筋補強のないレンガまたはブロック造の建物で、地震の揺れに極めて脆弱である。URM建物は、世界の地震死者数の大部分を占める。 buildings, non-ductile concrete frames built before 1970s code improvements, and the infamous ソフトストーリー（弱層）駐車場や店舗などの大きな開口部が原因で、上階に比べて著しく弱くなっている建物の階（通常は1階）。ソフトストーリーは最も一般的な倒壊メカニズムである。 apartment buildings of which Los Angeles had thousands.

The 1995 Kobe earthquake (M6.9, Japan) killed about 6,400 people, largely in structures built before Japan's 1981 seismic code revision. Buildings constructed after 1981 performed significantly better; buildings constructed after the 2000 revision performed better still. The death toll was concentrated in older structural types and in fires that spread through wooden building districts — not in modern engineered construction.

The ソフトストーリー（弱層）駐車場や店舗などの大きな開口部が原因で、上階に比べて著しく弱くなっている建物の階（通常は1階）。ソフトストーリーは最も一般的な倒壊メカニズムである。 Problem Is Real and Specific

While general building pancake is a myth, specific structural vulnerabilities are real. [[Soft-story]] buildings — typically multi-story residential buildings with an open, insufficiently braced ground floor designed for parking or retail — have a well-documented failure mode: the flexible first story concentrates deformation, columns lose lateral capacity, and the upper floors collapse onto the ground floor while remaining relatively intact as a block. This creates a partial pancake with potentially very high casualties in the occupied soft story.

Los Angeles has estimated 13,500 wood-frame soft-story buildings with over 1.1 million residents. A mandatory retrofit program, passed in 2015, requires owners to strengthen these structures over a phased timeline. San Francisco has similar programs. The retrofit cost per building is typically $60,000-$150,000 — significant but a fraction of replacement cost, and a known intervention that prevents the most predictable failure mode.

無補強組積造（URM）鉄筋補強のないレンガまたはブロック造の建物で、地震の揺れに極めて脆弱である。URM建物は、世界の地震死者数の大部分を占める。 and Non-Ductile Concrete Risks

[[Unreinforced-masonry]] buildings — brick or concrete block construction without steel reinforcing — account for a disproportionate share of earthquake casualties worldwide. They fail suddenly and completely without warning. Many historic commercial buildings, older schools, and residences in older US cities retain URM construction. Retrofit programs involving steel moment frames, reinforced concrete shear walls, or base isolation are technically feasible but expensive, and implementation lags behind the vulnerability.

Non-ductile concrete frame buildings — constructed before approximately 1970 in California and before corresponding code reforms elsewhere — lack the column ties and strong-column-weak-beam design that prevents brittle column failure. These buildings can pancake. California's mandatory retrofit law (SB 1473) mandates strengthening or demolition of identified non-ductile concrete buildings, but the inventory is large and progress is gradual.

What モーメント抵抗フレーム（ラーメン構造）梁と柱を剛接合し、曲げによって地震の水平力に抵抗する構造システム。優れた靭性を持つが、他の構造システムより費用がかかる。 Construction Actually Does

A モーメント抵抗フレーム（ラーメン構造）梁と柱を剛接合し、曲げによって地震の水平力に抵抗する構造システム。優れた靭性を持つが、他の構造システムより費用がかかる。 structural system — in which beams and columns are rigidly connected to transfer bending moments — is one of the primary strategies for seismic resistance in steel and concrete buildings. Moment frames are designed to deflect significantly under seismic loading, absorbing energy through plastic deformation in deliberately designated "ductile zones" while maintaining structural integrity. The system allows buildings to sway without collapsing — the opposite of pancake behavior.

The 1994 Northridge earthquake revealed unexpected weld fractures in steel moment frames that had been considered highly reliable, prompting significant research and updated welding standards. This is how engineering science advances: failures inform improvements, and codes are revised accordingly. Use the Building Safety Checker to understand what structural type your building likely uses and what that implies for earthquake performance.

The 免震基礎部分に柔軟な支承を用いることで、建物と地面の揺れを切り離す耐震工学技術。構造物に伝わる力を75〜90%低減する。 Revolution

[[Base-isolation]] — inserting flexible bearing systems between a building's foundation and its superstructure — represents the most advanced seismic protection strategy. By isolating the building from ground motion, these systems dramatically reduce the forces transmitted to the structure, often allowing buildings to remain fully operational after major earthquakes. Isolated buildings including hospitals, government facilities, and some residential towers have been built throughout California, Japan, and New Zealand. They perform exceptionally well in earthquakes and represent the frontier of 性能設計（性能規定型耐震設計）画一的な基準要件ではなく、地震の強さごとに異なる目標性能レベル（供用継続・人命安全・倒壊防止）を設定する高度な設計手法。 thinking.

Practical Implications

The pancake myth should neither paralyze nor reassure inappropriately. The honest picture: most people in developed countries with active building code enforcement and seismic design requirements face much lower building collapse risk than images of pancaked structures suggest. Specific vulnerable building types — soft-story wood frame, unreinforced masonry, non-ductile concrete frame — present elevated and addressable risk. Checking whether your building is on a city's soft-story or URM inventory, supporting and pursuing retrofit programs, and understanding your building's construction type are more productive responses than either fatalism or indifference.