地震リスクチェッカー

Check the seismic risk level for any location based on historical earthquake data and geological features.

Assessment

地震リスク評価の仕組み

地震リスク評価は、特定の場所での地震動の発生可能性と潜在的な深刻さを評価します。リスクスコアは、定められた半径内の過去の地震データを分析し、地震の頻度、最大観測マグニチュード、活断層への近さ、地域のテクトニック環境などの要因を考慮して計算されます。プレート境界付近、特に沈み込み帯やトランスフォーム断層付近の地域は、地震リスクが著しく高い傾向があります。

このツールが算出するリスクスコアは、指定された座標を中心とした半径250 kmの検索範囲に基づいています。記録された地震の数とマグニチュードの両方を考慮し、M7.0の地震1回がM6.0の約31.6倍のエネルギーを放出するため、大きな地震により重い重み付けをしています。グーテンベルグ・リヒターのマグニチュード・エネルギー公式によるこの指数関数的関係は、少数の大きな歴史的地震がある地域でも不均衡に高いリスクスコアを持つ可能性があることを意味します。

地震リスクの主要概念

地震ハザードと地震リスクの違い：ハザードは揺れの確率を記述し、リスクは人々と構造物の暴露度と脆弱性も考慮します。
グーテンベルグ・リヒターの法則は、地震のマグニチュードと頻度の統計的関係を記述します。小規模な地震は大規模な地震よりもはるかに頻繁に発生します。
確率論的地震ハザード解析（PSHA）は、所定の期間における地震動の超過確率を推定するために使用される正式な工学的手法です。
地域の地盤条件は地震波を増幅する可能性があり、断層から同じ距離にある2つの場所でも大きく異なる揺れの強度を経験する場合があります。

主な用途

不動産の購入や新しい地域への転居前の地震リスク評価。
教育・研究目的での地域の地震学的背景の理解。
異なる都市や国間の相対的な地震リスクの比較。
高リスクゾーンの特定による防災計画の支援。

How to Use

1
Enter Your Location
Type your city name or coordinates into the location field. The tool uses USGS seismic hazard data to identify your tectonic setting.
2
Review Your Risk Level
See your Peak Ground Acceleration (PGA) percentile and the historic seismicity of your region. Risk levels follow the USGS National Seismic Hazard Model classification.
3
Explore Mitigation Advice
Read the site-specific recommendations for your risk tier, covering building codes, retrofitting priorities, and preparedness actions.

About

Seismic risk assessment quantifies the likelihood and potential consequences of earthquake shaking at a specific location. Unlike a simple map of past earthquakes, a seismic risk calculation integrates probabilistic hazard models, local soil conditions, and the characteristics of structures in the area to produce actionable risk tiers. The foundational concept is the return period: a 475-year return period corresponds to a 10% probability of exceedance in 50 years, the standard design basis for most building codes worldwide. Higher return periods (e.g., 2,475 years, or 2% in 50 years) are used for critical facilities such as hospitals and nuclear plants.

The tectonic setting of a location drives its baseline hazard. Subduction zones—where one oceanic plate descends beneath another—produce the largest earthquakes on Earth, including the 2011 Tohoku M9.1 and 1964 Alaska M9.2 events. Transform faults like the San Andreas slip horizontally and generate frequent moderate-to-large earthquakes. Rift zones such as the East African Rift and the Basin and Range Province in the western US produce extensional faulting. Intraplate regions far from active plate boundaries can still experience significant earthquakes driven by ancient fault systems reactivated by residual tectonic stresses or fluid injection.

Soil amplification profoundly modifies ground shaking intensity at the surface. Soft sediments—lake beds, river deltas, reclaimed land—amplify shaking and extend its duration compared to bedrock sites. The 1985 Mexico City earthquake dramatically illustrated this: distant soft lacustrine sediments beneath the city resonated at the dominant period of the seismic waves, causing collapse of mid-rise buildings 350 km from the epicenter. Site class characterization using shear-wave velocity (Vs30) is now standard in hazard assessments and underlies the site amplification factors in modern building codes.

FAQ

What does seismic risk level mean?

Seismic risk level expresses the probability that ground shaking of a given intensity will be exceeded at a location within a specific time period, typically 50 years. It combines seismic hazard (the physical shaking) with exposure and vulnerability. A 'High' risk level generally corresponds to regions within 50 km of active fault systems or areas that experienced Modified Mercalli Intensity VII or greater in the historical record. Risk is distinct from hazard: two areas can share the same ground motion but differ greatly in risk if one has older, unreinforced masonry buildings.

How is seismic risk calculated?

Seismic risk calculations integrate three components: hazard models derived from fault geometries and historical seismicity catalogs, exposure inventories of buildings and population, and vulnerability functions describing how structures respond to shaking. National hazard maps published by agencies such as the USGS and GEM (Global Earthquake Model) provide probabilistic ground motion values at defined return periods (e.g., 475-year, 2,475-year). These values, combined with local site amplification from soil type, produce site-specific hazard estimates.

Which regions have the highest seismic risk?

The highest seismic risk zones lie along the Pacific Ring of Fire, a 40,000-km arc encircling the Pacific Ocean where roughly 90% of the world's earthquakes occur. Key high-risk regions include the Cascadia subduction zone (Pacific Northwest USA), Japan, the Himalayan collision zone (India, Nepal, Pakistan), the Anatolian fault system (Turkey), and the San Andreas fault system (California). The Mediterranean-Himalayan seismic belt accounts for most of the remaining global seismicity. Stable continental interiors (cratons) generally carry low risk, though intraplate earthquakes such as the 1811–1812 New Madrid sequence demonstrate that zero-risk regions do not exist.

Does building type affect my seismic risk?

Yes, building type is one of the most significant modifiers of seismic risk at the site level. Unreinforced masonry (URM) buildings are highly vulnerable because masonry has low tensile strength and crumbles under lateral shaking forces. Wood-frame light construction performs relatively well due to inherent flexibility. Modern reinforced concrete or steel moment-frame buildings designed to current seismic codes (IBC, Eurocode 8, Japanese BCP) incorporate ductile detailing that dissipates energy without catastrophic collapse. Post-1994 (Northridge) and post-1995 (Kobe) building codes introduced significantly stricter requirements in the US and Japan respectively.

How often should I reassess my seismic risk?

Seismic hazard maps are updated periodically as new fault data, paleoseismic studies, and seismicity catalogs become available. The USGS updates its National Seismic Hazard Maps approximately every 6 years; the most recent edition incorporates data from the 2019 Ridgecrest earthquake sequence. For individual property owners, reassessment is warranted after any nearby major earthquake, after structural modifications to your building, or when purchasing a new property. Local governments update seismic microzonation maps (accounting for soil amplification) on similar cycles, and checking your municipality's current zoning maps provides the most site-specific guidance available.