メキシコの地震帯: 三重交差点リスク

Tectonic Setting: The Triple Junction

Mexico's seismic hazard is shaped by its position near one of the world's most complex tectonic triple junctions, where the Pacific, North American, Cocos, and Rivera Plates interact along a broad zone of active boundaries. Along Mexico's Pacific coast, the Cocos Plate subducts beneath the North American Plate along the Middle America Trench — a 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。 that has produced some of the hemisphere's most destructive earthquakes. Off northwestern Mexico, the Rivera Plate's subduction beneath the North American Plate creates additional earthquake sources. Inland, the East Pacific Rise — a 発散型境界マントルから上昇したマグマによって新しい地殻がつくられながら、2枚のプレートが互いに離れていくプレート境界。中央海嶺が最も代表的な例。 — is being overridden by the North American Plate, complicating the regional tectonic picture further.

The Middle America 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 notable for its unusually flat subduction geometry in southern Mexico, where the Cocos Plate descends at a shallow angle before steepening to vertical. This geometry has important consequences for earthquake hazard: shallow-angle subduction means the seismogenic zone extends far inland, placing large areas of the Mexican interior within potential earthquake damage zones rather than just the immediate Pacific coast. The Guerrero Gap — a section of the Middle America Trench off the Guerrero coast that has not produced a major thrust earthquake since the 1911 event — is considered one of the highest-risk 地震空白域隣接する区間と比較して、長期間にわたり地震が発生していない活断層の区間。地震空白域は、将来の地震発生確率が高いことを示唆する場合がある。 segments in the Americas.

The 1985 Mexico City Disaster: A Lesson in Soil Amplification

The September 19, 1985 Michoacán Earthquake (magnitude 8.1) demonstrated perhaps the most famous and consequential case 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。 in seismological history. The earthquake's epicenter was approximately 350 kilometers from Mexico City on the Pacific coast, and by the time the seismic waves traveled this distance, their amplitude had naturally decreased. Yet Mexico City suffered catastrophic damage, with hundreds of buildings collapsing and 5,000 to 20,000 people killed — while towns much closer to the epicenter suffered relatively less.

The explanation lay in Mexico City's unusual geology: large portions of the city are built on the former bed of Lake Texcoco, whose deep, soft clay sediments respond to seismic waves like a bowl of jelly responding to a shaking table. The 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。 at these lake bed sites can amplify ground motion by a factor of 50 or more compared to the surrounding firm ground, and the natural resonance period of the soft sediments happens to match the period of the surface waves arriving from distant Cocos Plate earthquakes. Buildings of 6 to 15 stories have structural periods that also match this resonance — 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。 between the ground and building led to enormous forces in exactly the size class of buildings most common in the city.

SASMEX: Pioneer of Public Earthquake Warning

Mexico City operates SASMEX (Sistema de Alerta Símica Mexicana), arguably the world's first operational public earthquake 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。, established in 1991. The system detects earthquakes off the Pacific coast of Oaxaca and Guerrero at seismograph stations near the coast and transmits radio warnings to loudspeakers distributed throughout Mexico City, providing tens of seconds of warning before strong shaking arrives — time enough for people to evacuate buildings, stop trains, and initiate emergency procedures.

The 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。 has performed well during numerous earthquakes, providing warnings for the 1995 Manzanillo earthquake, the 1999 Oaxaca earthquake, and many other events. Its performance during the 2017 Puebla earthquake, however, revealed important limitations: this earthquake occurred inland rather than off the Pacific coast, in a region not well covered by the coastal sensor network, and the warning time was minimal. The system has since been expanded, but the 2017 event demonstrated that a warning system optimized for one earthquake source type can be less effective for others.

The 2017 Puebla Earthquake: September 19 Again

In a remarkable coincidence — or possibly a result of annual earthquake drills held on September 19 — the 2017 Puebla Earthquake (magnitude 7.1) struck Mexico City exactly 32 years to the day after the 1985 disaster, killing 369 people and collapsing dozens of buildings. Unlike the 1985 event, the 2017 earthquake was a shallow intraslab event within the subducting Cocos Plate rather than an interface thrust earthquake, occurring beneath the Mexican interior rather than off the coast. This different source mechanism generated different wave characteristics, and the damage pattern differed from 1985, affecting different building types and different neighborhoods.

Use Seismic Risk Checker to understand how Mexico City's unique soft-soil geology creates dramatically higher hazard levels than the underlying bedrock hazard would suggest.

What Makes Mexico Unique

Mexico's earthquake hazard is defined by the interaction between tectonic complexity, urban geology, and urban density. The Mexico City 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。 phenomenon is among the best-documented examples in the world of how local site conditions can dominate earthquake hazard, and the city's experience has informed hazard assessment worldwide. Mexico has invested substantially in public warning systems, 耐震基準建物の最低限の耐震安全性を確保するための、設計・建設に関する法的要件の体系。大地震で新たな脆弱性が明らかになるたびに更新される。 development, and earthquake risk education since 1985. The combination of an operational 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。 that warns millions of people before shaking arrives, and a culture of earthquake preparedness reinforced by annual drills, makes Mexico City's preparedness infrastructure genuinely impressive. The ongoing threat from the Guerrero Gap — which could produce a magnitude 8+ earthquake at any time — means that preparedness investment must be sustained indefinitely.