1923年関東大地震: 火が東京を破壊したとき

11:58 JST, September 1: Earthquake at Lunchtime

There could scarcely have been a worse time for a catastrophic earthquake to strike the Tokyo-Yokohama metropolitan area. Shortly before noon on September 1, 1923 — a Saturday, when shops and markets were busy — the residents of the Kanto Plain were preparing or eating the midday meal. Charcoal braziers and hibachi cooking fires were lit throughout the wooden residential districts. Fish markets in Tokyo's Fukagawa district were packed with morning shoppers. Restaurant kitchens had active flames. The combination of a moment of maximum fire hazard with a densely populated, overwhelmingly wooden city would produce one of the most catastrophic urban fires in recorded history — not as an alternative to earthquake destruction but in terrible addition to it.

At 11:58 and 44 seconds Japan Standard Time, the ground began to shake. The 震央地下で地震が発生した震源の真上にあたる地表の地点。ニュース報道では地震の発生場所としてよく報じられる。 was located in Sagami Bay, roughly 80 kilometres southwest of central Tokyo, at a depth of approximately 10-15 kilometres. Contemporary 地震計地震波によって生じる地面の揺れを検知し記録する装置。現代のデジタル地震計は、ナノメートル未満の変位も検出できる。 records analysed with modern methods place the マグニチュード地震が放出した総エネルギー量を表す単一の数値。整数値が1増えるごとに、放出エネルギーはおよそ31.6倍になる。 at approximately 7.9, though some assessments reach 8.2 depending on the method used. For the highly urbanised Kanto Plain — encompassing not only Tokyo and Yokohama but the surrounding cities of Kamakura, Odawara, Yokosuka, Chiba, and dozens of smaller communities — the shaking was catastrophic from the first seconds.

The ground motion lasted between three and five minutes in most accounts, considerably longer than the ten or twenty seconds of a typical damaging earthquake. In the soft alluvial sediments of the Kanto Plain — built up by millennia of river deposition from the surrounding mountains and more recent land reclamation for urban development — 地盤増幅（サイト効果）軟弱な土壌や堆積層が地震波を増幅させることによって生じる、揺れの強さの増大。軟弱地盤上の構造物は、基盤岩上の構造物に比べて2〜10倍強い揺れを経験することがある。 dramatically intensified the shaking compared to what would have been experienced on the hard volcanic rock of the surrounding uplands. Buildings constructed of heavy clay tile roofs on wood-frame structures, or of unreinforced masonry, collapsed in enormous numbers. The elegant Western-style buildings that the Meiji era had constructed in stone and brick, once symbols of modernisation, proved particularly dangerous, their brittle masonry walls shattering and their heavy tile roofs crashing inward.

The final death toll is estimated at between 100,000 and 142,000 people. The uncertainty in that range is itself informative: the chaos of the disaster, the destruction of administrative records, and the impossibility of accounting for all the missing in a city of more than two million people made precise enumeration impossible for months. Most of the dead were killed not by falling buildings but by the fires that swept through the ruins in the hours and days following the initial shaking.

The Sagami Trough: Subduction Beneath Tokyo

The 1923 earthquake was generated at the Sagami Trough, where the Philippine Sea Plate subducts beneath the Eurasian and North American plates in the geologically complex environment beneath central Honshu. Japan sits at one of the most seismically and volcanically active locations on Earth — the meeting point of four major プレート（テクトニックプレート）移動し、浮遊し、時に破断する地球のリソスフェアの巨大な区画。7つの主要プレートと約8つの小規模プレートがあり、それらの相互作用がほとんどの地震の原因となる。s, each interacting with the others through a combination of 沈み込み帯1枚のプレートがもう1枚のプレートの下にもぐり込み、マントルへと沈み込む領域。沈み込み帯は世界最大級の地震（M8.5以上）を引き起こし、深い海溝や火山弧を伴う。 dynamics, collision, and transform motion. The result is a density of active faults, volcanic systems, and seismic hazard zones that is unmatched anywhere in the world.

The Sagami Trough is a relatively short 沈み込み帯1枚のプレートがもう1枚のプレートの下にもぐり込み、マントルへと沈み込む領域。沈み込み帯は世界最大級の地震（M8.5以上）を引き起こし、深い海溝や火山弧を伴う。 compared to the Japan Trench to the northeast or the Ryukyu Trench to the southwest, but its proximity to Tokyo gives it an outsized significance in Japan's earthquake risk landscape. The trough dips directly beneath the most densely populated metropolitan area in the world, and the recurrence of great earthquakes from the Sagami Trough — with estimated return intervals of roughly 200 to 400 years based on the historical and paleoseismic record — represents one of the primary earthquake hazards facing modern Tokyo.

The 1923 断層破壊地震時に断層に沿って岩盤が破断し、蓄積された弾性エネルギーが地震波として放出される現象。破壊の長さは、小規模地震では数メートル、大地震では1,000km以上に及ぶこともある。 propagated across an area of approximately 9,000 to 12,000 square kilometres in Sagami Bay, initiating near the Izu Peninsula and propagating northward and eastward beneath the Boso Peninsula. The maximum slip on the fault surface was approximately 7 metres, concentrated in the northern portion of the rupture zone closest to the heavily populated Kanto Plain. Co-seismic deformation was significant and geographically complex: the Boso Peninsula rose by as much as 2 metres along its southern tip, while the Yokohama area and parts of Tokyo Bay subsided. These patterns of uplift and subsidence, reconstructed from historical tide gauge records and re-levelling surveys conducted after the earthquake, have been used to constrain the fault geometry for hazard assessments of future Sagami Trough events.

The complex 収束型境界2枚のプレートが互いに近づき合うプレート境界。海洋プレートと大陸プレートでは沈み込み帯を、大陸プレート同士では造山運動を、海洋プレート同士では深い海溝を生じる。 geometry beneath Tokyo also includes the collision of the Izu-Bonin volcanic arc with central Honshu, which creates additional structural complexity in the fault system and contributes to the variety of earthquake types — shallow crustal events, intermediate-depth intraslab earthquakes, and deep plate-interface events — that affect the region. Understanding which fault system poses the dominant risk for future Tokyo earthquakes, and characterising the rupture scenarios and associated ground motions for each, remains one of the central research challenges of Japanese seismology.

The Firestorms: 447,000 Buildings Burned to Ash

The fires that consumed Tokyo and Yokohama in the aftermath of the September 1 earthquake represent one of the largest urban conflagrations in recorded history — comparable to the Great Fire of London in 1666, the San Francisco fire of 1906, and the incendiary bombing raids of World War II, but exceeded in scale by few events in the pre-nuclear era. Within an hour of the earthquake, dozens of separate fires were burning throughout the metropolitan area, fed by overturned cooking braziers, broken gas mains, severed electrical lines, and the incidental ignition of debris by the various open flames that a traditional Japanese domestic morning required.

Yokohama, 30 kilometres south of Tokyo and directly above the most severely ruptured section of the Sagami Trough fault, was struck by both severe earthquake damage and tsunami waves from Sagami Bay in the minutes immediately following the shaking. The tsunami, reaching perhaps 3-4 metres in the harbour area, flooded the low-lying waterfront and contributed to destruction that was already catastrophic from the shaking alone. Its downtown commercial district, much of it built in Western styles of brick and stone, was destroyed by a combination of earthquake damage and fire. The foreign quarter — home to large communities of British, American, Chinese, and other nationalities — was reduced to ruins. The Grand Hotel, which had survived the earthquake structurally, burned to the ground with guests and staff trapped inside.

In Tokyo itself, the fires that had started in dozens of locations merged progressively into larger fire complexes driven by the dry early-September winds from the northwest. The Sumida River and the city's network of canals offered natural firebreaks in some locations, but also created dead ends that trapped evacuees between water and advancing flames. The flat, densely packed residential districts of eastern Tokyo — the traditional neighbourhoods of Shitamachi — offered almost no resistance to the advancing fires. The wooden machiya townhouses, with their closely spaced facades and narrow alleys, burned from block to block in what survivors described as a solid wall of advancing flame.

After five days, the fires had consumed approximately 447,000 buildings across the greater metropolitan area, in addition to the roughly 128,000 buildings that had been damaged or destroyed by the earthquake itself. Of the approximately 2.1 million residents of Tokyo, roughly half were left homeless. The material destruction was immense: archives, artworks, libraries, and the accumulated records of the Meiji and Taisho eras were lost. The Imperial Palace escaped serious damage because the broad stone grounds of the palace precinct formed an effective firebreak, but the surrounding city was devastated.

The Rikugun Honjo Fire Tornado: 38,000 Dead in One Night

Among all the horrors of the Great Kanto Earthquake and the fires that followed, the disaster that occurred at the former Army Clothing Depot in the Honjo district of eastern Tokyo stands as perhaps the single most terrible incident in the entire catastrophe, and one of the deadliest single events in any natural disaster in recorded history.

The open space of the abandoned military clothing depot — roughly 360 metres by 200 metres, cleared of its former buildings — appeared to offer precisely the kind of refuge that the burning city desperately lacked: open ground, away from collapsing buildings, without any overhanging structures to fall on the people crowding into it. Families fleeing from the burning districts of Honjo and Fukagawa converged on the depot grounds throughout the afternoon of September 1, carrying their belongings wrapped in bundles and baskets. By late afternoon, an estimated 38,000 people had packed themselves into the open space, waiting for the fires to burn past.

Instead, the fires surrounding the open space — burning in at least three directions — created the conditions for a fire whirl or fire tornado: a rotating column of superheated gases drawn upward by the intense differential heating of the fires around the clearing. Wind speeds within such a vortex can reach hurricane force; temperatures can reach hundreds of degrees. Witnesses who survived described a rotating column of fire sweeping across the depot grounds in the early evening. The 38,000 people sheltering there had no avenue of escape — they were surrounded by fire on multiple sides, with the river behind them accessible only by fighting through crowds. Within an hour, almost all of them were dead.

A single incident — the product of the specific geometry of the fires, the specific meteorological conditions, and the tragic human decision to concentrate vulnerable people in a particular open space — accounted for more deaths than many entire major earthquakes. It was a reminder that 二次的地震災害揺れそのものではなく、揺れによって引き起こされる災害——津波・地すべり・液状化・火災・ダム決壊・化学物質の流出などを指す。揺れそのものより大きな被害をもたらすことが多い。 following a great earthquake are not secondary in their lethality, and that the choices made by survivors in the chaotic hours following an earthquake can determine their fate as surely as the earthquake itself.

Social Aftermath: Martial Law and Vigilante Violence

The human catastrophe of the Great Kanto Earthquake was compounded in its immediate aftermath by episodes of ethnic violence that stand as among the most disturbing chapters in modern Japanese history and remain a source of tension in Japanese-Korean relations today. Within hours of the earthquake, rumours spread through the traumatised and confused population of Tokyo and Yokohama that Korean residents were engaged in deliberate acts of sabotage: poisoning wells, looting stores, and committing violence against Japanese residents. These rumours were entirely fabricated and completely false, but in the chaos and fear of the post-earthquake environment, they spread rapidly through verbal communication and were acted upon by vigilante groups who had no means of verifying them.

Neighbourhood self-defence associations — jishukai — that had formed spontaneously in the hours following the earthquake to protect property and maintain local order, began to interrogate Koreans and attack those they identified as suspicious. The killings spread rapidly across the eastern districts of Tokyo and into surrounding prefectures. The number killed in these massacres is estimated to range from 3,000 to 6,000 — a significant fraction of the entire Korean population living in the Tokyo metropolitan area. Some Chinese residents and Okinawans were also killed in cases of mistaken identity. The massacres occurred over a period of several days, in some cases directly in front of police who did not intervene and in a few cases with the participation of police officers.

The government declared martial law on September 2, ostensibly to maintain order, but the martial law framework also restricted press coverage and provided a legal basis for subsequent efforts to suppress investigation of the massacres. The military, when it finally intervened, arrested some Koreans for their protection as well as some of the most visible perpetrators of violence. But prosecutions were few and sentences light; the violence was generally treated by authorities as an understandable, if regrettable, response to the social stress of the disaster.

The social dimensions of the 1923 disaster — the ethnic violence, the martial law, the censorship of subsequent reporting — remind us with force that earthquakes do not strike politically neutral societies. They strike societies with pre-existing tensions, hierarchies, and vulnerabilities. The Korean massacre of 1923 was not caused by the earthquake; it was enabled by the conditions of chaos and fear that the earthquake created, combined with pre-existing prejudice and the absence of functioning social institutions capable of restraining mob violence.

Frank Lloyd Wright's Imperial Hotel: A Survivor's Story

Among the stories of destruction that dominated accounts of the 1923 earthquake, one story of structural survival became internationally famous and contributed directly to the development of seismic engineering practice: the performance of the Imperial Hotel in central Tokyo, designed by the American architect Frank Lloyd Wright.

The Imperial Hotel, completed in 1922 after years of construction delays and budget disputes, stood on the particularly soft alluvial soils of central Tokyo — precisely the soils that produced the most severe shaking amplification and the greatest structural failures during the earthquake. Wright had anticipated this challenge and designed accordingly. Rather than using conventional deep pile foundations driven to bedrock — the standard approach for heavy buildings on soft ground — Wright designed a relatively shallow foundation system bearing on a more competent layer of ground a few metres below the surface. He described this as a 'battleship hull' principle: a floating foundation that would ride the earthquake rather than resist it rigidly. He also designed the building's structure with extensive internal flexibility: cantilevered floor and roof elements that could accommodate differential settlement, a relatively low profile, and heavy materials used judiciously rather than uniformly.

The Imperial Hotel survived the earthquake with minimal structural damage and minimal loss of life among its occupants. Its water supply, fed from a large ornamental garden pool, remained functional, and the hotel served as a refuge, emergency hospital, and communications centre for international residents of Tokyo in the days following the disaster. Wright, then in the United States, received the famous telegram from the hotel manager Inumaru that began: 'Hotel stands undamaged as monument of your genius hundreds of homeless provided for by perfectly maintained service.'

The hotel's survival became a proof of concept. In subsequent decades, the Imperial Hotel was studied by engineers seeking to understand what design principles had made the difference. The combination of the floating foundation system, the flexible structural framing, and the building's relatively modest height were all identified as contributing factors. The hotel was eventually demolished in 1968 — not by earthquake or fire but by commercial decision to build a larger, more modern facility — and its famous lobby was preserved and re-erected at the Meiji Mura open-air architectural museum in Inuyama, where it can be visited today.

September 1 as Disaster Prevention Day: Kanto's Living Legacy

In 1960, exactly 37 years after the Great Kanto Earthquake, the Japanese government designated September 1 as National Disaster Prevention Day (Bosai no Hi). Every year since, Japan conducts its most extensive earthquake and disaster preparedness exercises of the year on that date. Schools, government agencies, corporations, neighbourhood associations, and community organisations across the country participate in drills that reinforce the foundational behaviours of earthquake survival and post-earthquake management: drop, cover, and protect yourself during shaking; evacuate carefully to designated safe areas; account for all members of your community; know where emergency shelters are located; check on neighbours who may be elderly, disabled, or otherwise in need of assistance.

The annual September 1 national drill is itself a remarkable institution. It requires every level of Japanese society — from elementary school children to corporate executives, from rural farmers to the Prime Minister — to stop and practise preparedness behaviours that, if they become habitual, can save thousands of lives when the next great earthquake strikes. The ritual acknowledgment that a great earthquake will come again, and that preparation is both possible and obligatory, is a cultural response to the 1923 experience that distinguishes Japan from most countries in its relationship with seismic risk.

The Tokyo metropolitan government publishes revised earthquake damage estimates every several years for a scenario earthquake similar in character and magnitude to 1923. These estimates — which project potential deaths in the tens of thousands if certain conditions of time of day, wind speed, and building vulnerability are assumed — drive investment decisions about which older wooden districts require prioritised seismic retrofit and fire resistance improvement, where firebreak open spaces should be created and maintained in the urban fabric, and how evacuation routes should be designed to allow populations to move safely from high-density urban areas to designated refuge spaces. The 1923 earthquake thus continues to shape Tokyo's urban form and disaster risk governance nearly a century after it occurred, embedded in the city's planning codes, its landscape, and the annual ritual of September 1.