確率論的地震ハザード解析(PSHA)ツール

What Is Probabilistic Seismic Hazard Analysis?

確率論的地震ハザード解析（PSHA）起こりうるすべての地震の発生源・マグニチュード・地震動レベルを考慮し、特定の揺れの水準を超える確率として結果を表現する、地震ハザードの定量評価手法。 — Probabilistic Seismic Hazard Analysis — is the methodological framework that transforms geological knowledge about earthquake sources into quantitative estimates of ground shaking probability at specific sites. Developed by C. Allin Cornell in 1968, PSHA integrates three inputs — earthquake source characterization, ground motion prediction, and site response — into a single hazard curve that expresses the annual probability of exceeding any given level of ground shaking. PSHA output drives building codes, insurance pricing, infrastructure design standards, and land-use planning across seismically active regions worldwide.

The Four Steps of PSHA

The first step identifies and characterizes all seismic sources capable of affecting the site of interest. Sources include fault lines地表に現れた断層の痕跡で、線状または破砕された岩石の帯として視認できる。地質学者は活断層線を地図化し、周辺地域の地震リスクを評価する。 with known geometry and すべり速度断層に沿った変位の平均速度で、通常は年間ミリメートル単位で測定される。すべり速度が高いほど、一般に地震発生頻度とリスクが高いことを示す。, area sources representing diffuse seismicity in regions without mapped faults, and background seismicity derived from historical earthquake catalogs. Each source is assigned a magnitude-frequency distribution, typically following the グーテンベルク・リヒター則地震の頻度とマグニチュードの関係を示す統計法則で、マグニチュードが1単位増えるごとに、地震の発生頻度はおよそ10分の1になる。 relation, or the characteristic earthquake model for well-characterized faults.

The second step uses ground motion prediction equations (GMPEs, also called attenuation relations) to predict the probability distribution of ground shaking at the site as a function of earthquake magnitude, source-to-site distance, fault mechanism, and site conditions. Modern GMPEs are derived from large empirical databases of strong motion recordings and express not just the median expected shaking but also the uncertainty (sigma) in that prediction. The large variability in observed ground motion means that sigma terms dominate hazard at very long return periods.

The third step integrates over all possible earthquakes from all sources, weighted by their probability of occurrence, to compute the annual exceedance probability of each ground shaking level. This integration is the PSHA calculation itself and is typically performed using dedicated software packages.

The fourth step expresses results as a hazard curve — annual probability of exceedance versus 最大地動加速度（PGA）地震時における地面の最大加速度で、重力加速度（g）の単位で表される。耐震工学における構造物設計の重要なパラメータ。 or spectral acceleration — or as a hazard map showing ground shaking expected at a specific exceedance probability across a geographic area.

PSHA Software Tools

Several software packages implement PSHA calculations. OpenQuake Engine, developed by the Global Earthquake Model (GEM) Foundation, is an open-source Python-based platform used by researchers and national agencies worldwide. The USGS uses Fortran-based legacy code plus newer OpenQuake-compatible tools to produce the National Seismic Hazard Maps published approximately every six years. EZ-FRISK and R-CRISIS are commercially available tools with graphical interfaces used widely in engineering practice. All implement the same fundamental PSHA mathematics with varying capabilities for complex source geometries and large-scale probabilistic calculations.

The 地震ハザードマップ特定の期間内に地震の揺れが指定の水準を超える確率を示した地図。技術者・都市計画者・保険会社が地震リスクを評価するために用いる。 and Design Ground Motion

The principal product of national PSHA programs is the 地震ハザードマップ特定の期間内に地震の揺れが指定の水準を超える確率を示した地図。技術者・都市計画者・保険会社が地震リスクを評価するために用いる。: a gridded map showing the peak ground acceleration (or spectral acceleration) with a specified probability of exceedance in 50 years. The standard US building code design level uses 2% in 50 years (approximately a 2,475-year return period), while some codes use 10% in 50 years (475-year return period) for different performance objectives. The 地震ハザードマップ特定の期間内に地震の揺れが指定の水準を超える確率を示した地図。技術者・都市計画者・保険会社が地震リスクを評価するために用いる。 values feed directly into building code seismic zone designations — higher hazard zones require stronger structural systems, more detailed analysis, and more conservative design factors.

地震リスク評価特定の地域や構造物について、地震ハザード・建物の脆弱性・想定される損失を評価する過程。ハザードマップ、建物台帳、被害モデルを組み合わせて行われる。 vs. PSHA

確率論的地震ハザード解析（PSHA）起こりうるすべての地震の発生源・マグニチュード・地震動レベルを考慮し、特定の揺れの水準を超える確率として結果を表現する、地震ハザードの定量評価手法。 produces hazard estimates (ground shaking probability); 地震リスク評価特定の地域や構造物について、地震ハザード・建物の脆弱性・想定される損失を評価する過程。ハザードマップ、建物台帳、被害モデルを組み合わせて行われる。 combines hazard with exposure and vulnerability to estimate probable losses. The distinction is important: a remote area may have high seismic hazard but low risk because few people or structures are exposed. A moderately hazardous urban area may have high risk because enormous concentrations of people and infrastructure are exposed to even modest shaking. Risk assessment requires 確率論的地震ハザード解析（PSHA）起こりうるすべての地震の発生源・マグニチュード・地震動レベルを考慮し、特定の揺れの水準を超える確率として結果を表現する、地震ハザードの定量評価手法。 output as input but additionally requires building inventory databases, vulnerability functions特定の建物タイプについて、揺れの水準に応じたさまざまな被害段階の発生確率を示す数学的関数。損失推定モデルに不可欠な要素。, and population data.

The Seismic Risk Checker Tool

The Seismic Risk Checker tool implements a simplified single-site PSHA calculation that allows users to estimate ground shaking probability at any location based on the USGS National Seismic Hazard Model. Entering a US zip code returns the 最大地動加速度（PGA）地震時における地面の最大加速度で、重力加速度（g）の単位で表される。耐震工学における構造物設計の重要なパラメータ。 at the 2%-in-50-year and 10%-in-50-year exceedance levels, along with the corresponding building code seismic design category. This helps property owners, architects, and engineers quickly assess the seismic design requirements applicable to a specific site without performing a full PSHA calculation.

Uncertainties in PSHA

PSHA quantifies both aleatory uncertainty (inherent randomness in earthquake occurrence and ground motion) and epistemic uncertainty (incomplete knowledge of source parameters). Epistemic uncertainty is handled through logic trees — weighted combinations of alternative source models, magnitude-frequency distributions, and GMPEs. A typical national PSHA logic tree may have thousands of branches representing different defensible scientific interpretations. The mean hazard curve averages over this epistemic uncertainty, while fractile curves (16th, 50th, 84th percentile) quantify the spread.

Site-Specific PSHA in Engineering Practice

Building codes provide general 地震ハザードマップ特定の期間内に地震の揺れが指定の水準を超える確率を示した地図。技術者・都市計画者・保険会社が地震リスクを評価するために用いる。 values that conservatively account for typical site conditions. For critical facilities — hospitals, nuclear power plants, dams, LNG terminals — site-specific PSHA is required. This analysis uses a detailed characterization of local seismic sources, site-specific velocity profiles for site amplification modeling, and GMPEs selected for regional appropriateness. Site-specific PSHA may produce hazard estimates substantially different (higher or lower) than the code map values, justifying custom structural designs and potentially significant cost differences.

The Seismic Hazard Model Update Cycle

National seismic hazard models are updated periodically as new data improves source characterization. The 2023 USGS National Seismic Hazard Model update incorporated GPS測地学全地球測位システムの受信機を用いて、プレートの動きや地殻変動をミリメートル単位の精度で測定する手法。地震と地震の間に断層に歪みがどのように蓄積するかを明らかにする。 constraints on fault slip rates, expanded fault databases including many previously unmapped blind thrust faults地表に達しないスラスト断層で、地表からは見えず検出が困難である。1994年のノースリッジ地震はブラインドスラスト断層で発生した。, updated GMPEs incorporating lessons from the 2010 Canterbury and 2011 Tohoku earthquakes, and improved 誘発地震活動水圧破砕（フラッキング）、排水注入、採掘、貯水池の湛水など、人間活動によって引き起こされる地震。ほとんどは小規模（M4未満）だが、M5.5を超えたケースもある。 characterization in the central and eastern United States. Each update can significantly revise hazard estimates in specific regions, driving building code revisions and insurance repricing.

Summary

確率論的地震ハザード解析（PSHA）起こりうるすべての地震の発生源・マグニチュード・地震動レベルを考慮し、特定の揺れの水準を超える確率として結果を表現する、地震ハザードの定量評価手法。 tools transform geological and seismological data into the probabilistic ground shaking estimates that underpin all quantitative earthquake risk management. Understanding the 地震ハザードマップ特定の期間内に地震の揺れが指定の水準を超える確率を示した地図。技術者・都市計画者・保険会社が地震リスクを評価するために用いる。 as a probabilistic product — not a prediction — and recognizing the role of uncertainty in 地震リスク評価特定の地域や構造物について、地震ハザード・建物の脆弱性・想定される損失を評価する過程。ハザードマップ、建物台帳、被害モデルを組み合わせて行われる。 allows engineers, planners, and policymakers to use these powerful tools appropriately. The Seismic Risk Checker tool provides immediate access to national PSHA results for any US location.