イランの地震ハザード: 衝突帯地震

Tectonic Setting: Arabia Meets Eurasia

Iran sits in the zone of collision between the Arabian Plate, moving northward at approximately 2 to 3 centimeters per year, and the Eurasian Plate — a プレート衝突2枚の大陸プレートが収束し、ヒマラヤ山脈のような巨大な山脈を形成する過程。大陸衝突帯では、浅いながらも強力な地震が発生する。 that has built the Zagros and Alborz mountain ranges and sustains one of the world's highest concentrations of active fault systems. Unlike the relatively clean 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。 boundaries of the Pacific, the Arabia-Eurasia 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。 involves complex crustal shortening distributed across dozens of active 逆断層（スラスト断層）圧縮力によって、上盤が下盤に対して上方にずれる断層。傾斜の緩い逆断層（スラスト断層）は、最大級の地震の原因となる。 and fold systems across the Iranian Plateau. The absence of a single dominant fault means that destructive earthquakes can occur almost anywhere across the country's vast territory.

The Zagros Fold and Thrust Belt, running from northwestern Iran through Kurdistan and Khuzestan to the Persian Gulf, accommodates a significant fraction of the Arabia-Eurasia convergence through a series of northwest-trending thrust and 逆断層（スラスト断層）圧縮力によって、上盤が下盤に対して上方にずれる断層。傾斜の緩い逆断層（スラスト断層）は、最大級の地震の原因となる。 systems. The Zagros is one of the world's most seismically active continental collision zones, generating frequent moderate earthquakes that are broadly distributed across the belt rather than concentrated on individual fault lines. The Alborz Mountains in northern Iran, including the fault systems flanking Tehran, represent a separate zone of active deformation linked to the collision of the South Caspian Basin microplate with the Iranian Plateau. The North Tehran Fault, running directly beneath Iran's capital of over 15 million people, is considered one of the most dangerous urban seismic hazards in the world.

Historical Seismicity: A Relentless Record

Iran's earthquake history is among the most consistently destructive of any country, with major events recurring across the entire territory at intervals of years to decades throughout recorded history. The 856 Damghan Earthquake (estimated magnitude 8.0) killed approximately 200,000 people in northeastern Iran, making it one of the deadliest earthquakes in history if ancient population estimates are accepted. The 893 Ardabil Earthquake killed another 150,000. The 1990 Manjil-Rudbar Earthquake (magnitude 7.4) killed approximately 35,000 to 50,000 people in Gilan Province, leaving 100,000 homeless in the Alborz Mountains region. The 2003 Bam Earthquake (magnitude 6.6) killed at least 26,000 people — approximately 30 percent of the city's population — in a single, early-morning event that destroyed 80 percent of Bam's buildings, including the UNESCO-recognized Arg-e Bam citadel, the world's largest adobe structure.

The 2005 Zarand Earthquake (magnitude 6.4), the 2012 Ahar-Varzaqan Earthquakes (magnitude 6.4 and 6.2), the 2017 Kermanshah Earthquake (magnitude 7.3, killing 630 people), and the 2019 Mashhad earthquake are among dozens of destructive events in recent decades. The geographical distribution of these disasters across different provinces and fault systems reflects the pervasive nature of seismic hazard across the entire Iranian Plateau.

Construction Vulnerability: The Rural Challenge

Iran's most severe earthquake losses consistently occur in rural areas where construction is dominated by traditional 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。 — adobe, sun-dried brick, and stone masonry built without any seismic reinforcement. Rural Iranian construction traditions developed over millennia in a pre-engineering era, producing buildings that are well-adapted to climate and local materials but catastrophically vulnerable to ground shaking. Adobe construction in particular, while durable in dry conditions, fails dramatically in earthquakes as the heavy mud-brick walls crack, lose cohesion, and collapse inward, burying occupants under heavy debris.

Iran has invested substantially in earthquake-resistant construction programs following major disasters, including the significant post-Bam reconstruction that attempted to introduce modern building techniques to rural areas. However, the challenge of changing construction practices across tens of thousands of villages spread across a vast territory is enormous. Official surveys have found that a significant fraction of rural housing stock continues to use highly vulnerable 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。 techniques despite government programs promoting reinforced construction. 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。 enforcement in rural areas remains an ongoing challenge.

Tehran: An Unresolved Urban Catastrophe in Waiting

Tehran's seismic risk is perhaps the most alarming urban earthquake hazard in the world in terms of the combination of extreme hazard, enormous population, and limited mitigation. The city of 15 million people sits in a basin flanked by active 逆断層（スラスト断層）圧縮力によって、上盤が下盤に対して上方にずれる断層。傾斜の緩い逆断層（スラスト断層）は、最大級の地震の原因となる。 systems including the North Tehran Fault, the Rey Fault, and the Mosha Fault, all capable of generating magnitude 7 or greater earthquakes with minimal warning. The North Tehran Fault in particular, located on the mountain front directly overlooking the wealthy northern sections of the city, could generate a scenario earthquake with peak ground accelerations exceeding 1 g in some zones.

Iran has conducted detailed probabilistic seismic hazard analyses for Tehran and has developed emergency management plans, but the scale of potential losses in an extreme scenario — potentially hundreds of thousands of deaths given the density of vulnerable construction and the deep alluvial basin that will amplify shaking — represents a risk management challenge of extraordinary difficulty. The scenario of a direct hit on Tehran from the North Tehran Fault is one of the most severe urban earthquake catastrophes that scientists consider plausible anywhere in the world.

What Makes Iran Unique

Iran's seismic challenge is defined by the combination of extremely high hazard distributed across the entire national territory, large rural populations in highly vulnerable construction, and the specific threat of a catastrophic scenario earthquake beneath one of the world's largest metropolitan areas. Iran has capable earthquake engineering institutions and has produced significant research on Zagros and Alborz seismicity. The fundamental challenge is resources and governance: implementing earthquake risk reduction across a vast, diverse nation where traditional construction remains deeply embedded in rural culture requires sustained investment and commitment that competes with other national priorities.