1985年メキシコシティ地震: 距離を無視した共鳴災害

07:17 CDT: Destruction 350 Kilometres From the Epicenter

On the morning of September 19, 1985, an earthquake of 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。 8.1 struck off the Pacific coast of Mexico in the state of Michoacan. 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 approximately 350 kilometres from Mexico City, beneath the Pacific Ocean near the convergence of the Cocos and North American plates. By any simple calculation of distance attenuation, an earthquake of that magnitude at that distance should have produced moderate shaking in the capital — enough to rattle windows, frighten residents, and topple a few vulnerable chimneys, but not enough to collapse modern buildings or produce mass casualties.

Instead, the 1985 Mexico City earthquake killed between 9,500 and 40,000 people — a range that reflects the genuine uncertainty in counting deaths in a disaster that affected millions of people across a metropolitan area, with many missing persons never formally accounted for. Most of them died not near the coast where the fault ruptured, but in the heart of a metropolis more than 300 kilometres away. Specific city blocks experienced total structural destruction while adjacent blocks were essentially undamaged. Certain heights of buildings — primarily those between 6 and 15 stories — collapsed in clusters while shorter and taller buildings nearby remained standing.

The explanation for these paradoxes lies in one of the most extraordinary examples of 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。 and 構造物の共振地震波の周波数が建物の固有振動数と一致したときに生じる、建物の揺れの増幅現象。低層建物は高周波数の波と、高層建物は低周波数の波と共振しやすい。 ever documented in earthquake engineering history. Mexico City sits on the lakebed of the former Lake Texcoco, a shallow basin that was progressively drained from the sixteenth century onward. Beneath the city's streets lies a layer of extremely soft, water-saturated lacustrine clays that act as a seismic amplifier of extraordinary power — multiplying the intensity of incoming seismic waves by factors of 10 to 50 relative to what would be experienced on nearby rock, and selectively amplifying waves of approximately 2 seconds period, the same natural frequency as many of the mid-rise buildings that dominated Mexico City's residential landscape in 1985.

The earthquake thus produced a perfect resonance catastrophe: long-period 表面波地球内部ではなく地表に沿って伝わる地震波。実体波より速度は遅いが、振幅が大きく継続時間が長いため、被害が大きくなる傾向がある。s from the Cocos 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。, after traveling 350 kilometres across the Pacific coast and the Trans-Mexican Volcanic Belt, arrived at the ancient lakebed clays with the precise frequency needed to excite maximum amplification. The amplified waves then resonated with the natural frequencies of 8- to 15-storey apartment buildings. Buildings that might have survived different ground motion of similar peak amplitude collapsed because the resonant excitation drove them far beyond their elastic capacity.

The Cocos Subduction Zone: Energy Source for Mexican Earthquakes

The Cocos Plate is one of the smaller and most rapidly moving oceanic plates in the global tectonic system. Formed at the East Pacific Rise and the Cocos-Nazca spreading centre in the eastern Pacific, it moves northeastward at approximately 5-7 centimetres per year, subducting beneath the North American and Caribbean plates along the Middle America Trench that runs parallel to the Pacific coast of Mexico from Guatemala to the Gulf of Tehuantepec.

This rapid 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。 convergence generates a continuous train of large earthquakes along the Middle America subduction zone. Mexico's Pacific coastal states — Guerrero, Michoacan, Oaxaca, Chiapas — experience large subduction earthquakes frequently, with events above magnitude 7 occurring every few years along different segments of the trench. The 1985 earthquake ruptured the Michoacan segment, a section of the plate interface that had last produced a great earthquake in 1932. The rupture area was approximately 170 kilometres long and 60 kilometres wide, with maximum fault slip reaching approximately 2.5 metres.

The 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。 geometry beneath central Mexico is somewhat unusual compared to typical subduction systems. In parts of central Mexico, the Cocos Plate subducts at a shallow dip angle — sometimes nearly horizontal for tens of kilometres before steepening. This flat slab geometry has two important consequences for Mexico City's earthquake hazard. First, the locked portion of the plate interface extends further inland than in a more steeply dipping system, so the rupture area of a great Mexican earthquake can extend much further from the trench. Second, the relatively shallow dip means that the subducted slab does not heat up and dewater as rapidly as it would in a steeper system, maintaining its mechanical coupling with the overlying plate over a broader area.

For Mexico City, the critical characteristic of the 1985 source was not the maximum ground acceleration it generated at the coast — which was very high, demolishing many coastal communities in Michoacan — but the specific character of the long-period 地震波地震や爆発によって発生し、地球内部を伝播する弾性波。地震波は、震源で放出されたエネルギーを遠方の地点まで運ぶ。s that traveled 350 kilometres to reach the capital. At that distance, short-period energy had been substantially dissipated, but long-period surface waves retained their amplitude. The waves that arrived in Mexico City had periods of approximately 1.5-2.5 seconds — exactly the range that would prove resonant with both the lakebed soils and the mid-rise building stock.

Lake Texcoco Effect: Why Mexico City Shakes More Than Anywhere

The Valley of Mexico was, until the Spanish conquest of Tenochtitlan in 1521, the site of an interconnected lake system covering much of the basin. Lake Texcoco was the largest, a shallow, slightly saline body of water surrounded by smaller freshwater lakes. Tenochtitlan, the Aztec capital, was built on an island in Lake Texcoco and connected to the lakeshore by great causeways. The Spanish demolished Tenochtitlan and began the systematic drainage of the lake system, a process driven by chronic flooding problems in the colonial city that continued through the nineteenth century. By the twentieth century, most of the lake had been drained, and Mexico City had been built across the former lakebed.

What remains beneath the central and eastern portions of Mexico City is one of the most seismically hazardous soil conditions known to geotechnical science. The lacustrine clays deposited in the lakebed are extraordinarily soft and compressible — water contents sometimes exceeding 400 percent by weight, shear wave velocities as low as 50-75 metres per second. For comparison, rock has shear wave velocities of 1,500 metres per second or more. The ratio between rock velocity and clay velocity — sometimes called the impedance contrast — is what drives 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。. When a 地震波地震や爆発によって発生し、地球内部を伝播する弾性波。地震波は、震源で放出されたエネルギーを遠方の地点まで運ぶ。 crosses from fast rock into slow clay, conservation of energy requires that the wave amplitude increase proportionally to compensate for the reduction in wave speed. The 20:1 velocity ratio between rock and Mexico City clay produces theoretical amplification factors of 20 or more, and measured amplification in the lakebed zone during the 1985 earthquake reached factors of 50 at the resonant period of 2 seconds.

The resonant period of approximately 2 seconds is a property of the lakebed clay layer itself — determined by the thickness of the clay and its wave velocity, much as the pitch of a tuning fork is determined by the length and material of its tines. Mexican geotechnical engineers had measured this resonant period before 1985, and it was understood in principle that it could amplify earthquakes from the Cocos subduction zone. What was not fully appreciated was the degree to which the resonant period of the clay would coincide with the natural periods of the mid-rise concrete buildings that constituted most of Mexico City's residential inventory, or the extent to which that coincidence would produce catastrophic building resonance rather than merely intensified shaking.

The three seismic zones of Mexico City — Zone I (rock), Zone II (transition), and Zone III (lakebed) — have dramatically different seismic characteristics. A recording station in Zone I might measure a peak ground acceleration of 0.04g from a major Cocos subduction zone earthquake. A recording station in Zone III, at the same epicentral distance, might measure 0.20g or more at the resonant period — five times higher. The 1985 earthquake exposed this disparity for the world to see: Zone III districts looked as if they had been close to the epicenter of a very large earthquake; Zone I districts showed almost no damage.

Structural Resonance: The Selective Destruction of Mid-Rise Buildings

Among all the observations from the 1985 Mexico City earthquake, none was more striking to structural engineers — or more immediately actionable for future code development — than the extreme selectivity of building collapses. Not all buildings in the lakebed zone fell. Not even all mid-rise buildings fell. But the pattern of collapse, when mapped across the city, showed a pronounced concentration in buildings of specific heights, with a remarkable correlation between building height and collapse rate.

Buildings of roughly 6 to 15 stories collapsed at dramatically higher rates than buildings outside that range. Shorter buildings — 1 to 5 stories — survived the 1985 ground motion with generally lower damage rates. Taller buildings — 20 stories and above — also survived more frequently, though some suffered significant structural damage. It was the mid-rise class — the dominant form of apartment housing in much of central Mexico City — that was devastated.

The explanation, confirmed by subsequent analysis of both building natural periods and ground motion spectra, was 構造物の共振地震波の周波数が建物の固有振動数と一致したときに生じる、建物の揺れの増幅現象。低層建物は高周波数の波と、高層建物は低周波数の波と共振しやすい。. A reinforced concrete frame building's natural period — the frequency at which it tends to oscillate most readily when disturbed — is approximately 0.1 seconds per storey for stiff, shear-wall-dominated construction, and somewhat longer for flexible moment-frame systems. For the flexible concrete frame buildings typical of Mexico City construction from the 1960s through the 1980s, natural periods for 8- to 12-story buildings clustered around 1.5 to 2.5 seconds. The lakebed resonant period was approximately 2 seconds. The overlap between ground motion frequency content and building natural period was nearly perfect.

When an earthquake drives a building at its natural frequency, each cycle of ground motion pushes the structure a little further than the previous cycle — a phenomenon called resonant amplification. The building sways further and further with each wave, demanding ductility from its structural system that in many cases was not available. Buildings constructed with inadequate reinforcement, poor detailing of column-beam connections, or soft-story irregularities were driven to failure by ground motion that, on a different soil type or against a building of different height, might have caused only minor damage.

The lesson was absorbed rapidly by the Mexican structural engineering community and by engineers worldwide. Post-1985 Mexican seismic design codes were fundamentally revised to account explicitly for the site-dependent resonant characteristics of different zones within the city, requiring that new buildings be designed to avoid resonance with the local soil rather than merely to resist a specified peak ground acceleration. The concept of site-specific design spectra — ground motion design spectra tailored to the resonant characteristics of the soil at a specific site — became standard in Mexican practice and was subsequently adopted, in various forms, in building codes throughout the world.

Civil Society Rising: When Citizens Organised Before Government

The Mexican government's response to the 1985 earthquake was slow, disorganised, and ultimately inadequate to the scale of the disaster. President Miguel de la Madrid, in an early decision that was later widely criticised, initially declined offers of international assistance — a position rooted in a combination of political pride and a genuine underestimation of the disaster's scale in the first hours. The government's communication to the public was poor, its coordination of rescue efforts was weak, and its capacity to organise the distribution of supplies to hundreds of thousands of displaced residents was quickly overwhelmed.

Into this governmental vacuum stepped ordinary citizens. In a development that historians of Mexican politics regard as a turning point in the country's democratic evolution, people organised spontaneously and effectively before and often without government direction. University students formed rescue brigades, arriving at collapsed buildings with whatever tools they could find — shovels, picks, their bare hands. Neighbourhood associations organised the distribution of food and water to people sleeping in parks and on the streets. Seamstresses and factory workers whose workplaces had collapsed organised to demand accountability from their employers and from the government. Professional engineers volunteered their expertise to guide rescue operations in collapsed buildings.

The civil society organisations that emerged from the 1985 earthquake became the foundation of Mexican civil society more broadly. Mutual aid networks that had formed for disaster relief converted to permanent community organisations. Advocacy groups that had demanded government accountability over earthquake response transformed into human rights organisations and eventually into the political opposition movements that would ultimately end the PRI's seven-decade dominance of Mexican political life. The 2000 presidential election of Vicente Fox — the first opposition candidate to win the Mexican presidency in over seven decades — has roots in the political awakening that began in the ruins of September 19, 1985.

The contrast between the organised effectiveness of civil society and the disorganised inadequacy of the government response in 1985 resonated through subsequent decades of Mexican disaster management. Subsequent governments invested in improving emergency response capabilities, public alert systems, and institutional frameworks for disaster coordination — recognising that the 1985 earthquake had permanently changed public expectations about what the government owed its citizens in a disaster.

SASMEX: Mexico Builds the World's First Public Early Warning

One of the most consequential technical legacies of the 1985 earthquake was the development and eventual implementation of the Mexican Seismic Alert System — SASMEX (Sistema de Alerta Sismica Mexicano) — which became the world's first fully operational public 地震警報システム1991年から運用されている、世界初の公共向け緊急地震速報システムの一つであるメキシコのSASMEX。沿岸部の地震から、メキシコシティに最大60秒の警報時間を提供する。 when it began operation in 1993.

The concept behind SASMEX is elegant in its exploitation of physics. The source of the most dangerous earthquakes for Mexico City — the Cocos 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。 — is located on the Pacific coast, 350 kilometres from the capital. 地震波地震や爆発によって発生し、地球内部を伝播する弾性波。地震波は、震源で放出されたエネルギーを遠方の地点まで運ぶ。s travel at 3-8 kilometres per second depending on frequency. At those speeds, it takes approximately 60-120 seconds for the most damaging surface waves to travel from the subduction zone to Mexico City. Electronic signals travel at the speed of light — essentially instantaneous over that distance.

SASMEX deploys an array of strong-motion accelerograph sensors along the Pacific coast of Mexico, in the Guerrero, Oaxaca, and Michoacan regions where Cocos subduction zone earthquakes most frequently occur. When these sensors detect ground motion above a threshold amplitude consistent with a damaging earthquake, the system automatically calculates an estimated 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。 and broadcasts a radio alert signal to Mexico City. The alert reaches the city seconds to more than a minute before the damaging surface waves arrive, depending on the epicentral distance.

The public alert system consists of loudspeakers installed throughout the city that broadcast a distinctive two-tone alarm signal when the alert is received. Mexico City residents have been trained — through decades of annual September 19 earthquake drills — to respond to this alarm by dropping, covering, and if possible moving to open areas. The 緊急地震速報（EEW）地震を検知し、強い揺れが到達する前に人々やシステムに警報を送るシステム。数秒から数十秒の猶予を提供し、身を守る行動をとるのに十分な時間となる。 provided by SASMEX has been used effectively in several earthquakes since 1993 to initiate protective actions that reduced injuries: people getting out from under heavy shelves, workers moving away from dangerous machinery, elevator systems being automatically stopped at the nearest floor.

The SASMEX model demonstrated the technical feasibility and social value of public earthquake early warning systems, and has been adopted in modified forms by Japan (which launched its nationwide EEW system in 2007), the United States (which launched ShakeAlert for California, Oregon, and Washington in stages from 2019 to 2021), and numerous other countries. Mexico's earthquake science community thus made a globally consequential contribution to earthquake risk reduction through the post-1985 development of SASMEX — a contribution that has inspired early warning systems protecting hundreds of millions of people worldwide.

September 19 Curse: The 2017 Earthquake on the Same Date

On September 19, 2017 — exactly 32 years to the day after the 1985 earthquake — another destructive earthquake struck central Mexico. The 2017 event (M7.1) had its epicenter in Puebla state, approximately 120 kilometres south-southeast of Mexico City, at a depth of about 51 kilometres. Unlike the 1985 event — a classic Cocos plate-interface earthquake — the 2017 earthquake was an intraslab event, occurring within the downgoing Cocos Plate itself rather than on the interface between the Cocos and North American plates.

The date coincidence was so extraordinary that many Mexico City residents, when the SASMEX alarm sounded that morning, initially assumed it was the annual September 19 earthquake drill that commemorates the 1985 event. The drill had ended just two hours before the actual earthquake struck. The alarm was real; the earthquake was real; and the result was 369 deaths, primarily from building collapses in Mexico City's lakebed zone.

The 2017 earthquake caused damage that in some respects differed systematically from the 1985 event — reflecting the different source characteristics of an intraslab earthquake, which generates relatively stronger short-period ground motion than a shallow plate-interface event — and it damaged some buildings constructed after 1985 to the improved codes, a sobering demonstration that buildings designed to resist one ground motion spectrum may still be vulnerable when the ground motion has a different frequency content. It also validated SASMEX's operational performance: the system issued a warning for the 2017 earthquake, though the relatively close epicenter (120 km rather than 350 km) provided only 10-20 seconds of warning rather than the full 90+ seconds that a Guerrero or Michoacan coast event would provide.

The two September 19 earthquakes form bookends around three decades of intensive Mexican earthquake engineering research — a period during which Mexico City has become arguably the most intensively studied site of urban soil-structure interaction in the world, with 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。 networks, geotechnical instrument arrays, and structural monitoring systems providing data of unmatched quality on how the city's soil and buildings respond to earthquake loading. The ongoing work to understand and reduce Mexico City's earthquake vulnerability draws on both the catastrophic lessons of 1985 and the refined scientific understanding that has accumulated since.