ShakeMapの理解: リアルタイム震度

What Is a ShakeMap?

シェイクマップ（ShakeMap）地震発生後の揺れの強さの分布を示すUSGSの成果物。地震計データ、地震動モデル、「揺れを感じましたか？」の報告を組み合わせて作成される。 is a product of the アメリカ地質調査所（USGS）地震の監視、National Earthquake Information Centerの運用、世界の地震データのリアルタイム公開を担う、アメリカ政府の主要機関。 and regional 地震観測網地震活動を継続的に監視する、連携した地震観測点の集合。世界地震観測網（GSN）は150以上の観測点から構成され、世界規模の観測範囲を提供する。 partnerships that converts raw seismometer recordings into a spatially continuous map of ground shaking intensity. Within minutes of a significant earthquake, ShakeMap delivers color-coded maps showing where shaking was strongest, providing emergency managers, utilities, insurers, and individuals with rapid situational awareness. Understanding how to read these maps and what their values represent is an essential skill in earthquake-prone regions.

From Seismic Stations to Spatial Maps

When an earthquake occurs, seismometers at dozens to hundreds of stations record ground motion simultaneously. ShakeMap ingests these recordings and extracts peak ground acceleration, peak ground velocity, and spectral acceleration values at each station. Because stations are sparsely distributed relative to the affected area, the software uses ground motion prediction equations to interpolate values across the gaps between stations. The result is a smooth, continuous surface that estimates shaking intensity at every point within the affected region.

The Role of Site Amplification

Raw interpolation would be inadequate because soil conditions dramatically influence local shaking. 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。 can multiply shaking intensity by a factor of five or more compared to bedrock. ShakeMap incorporates Vs30 data — the average shear-wave velocity in the upper 30 meters of soil — to adjust estimates at each interpolated grid point. In urban areas where detailed Vs30 maps exist, this correction significantly improves accuracy. Where site data is sparse, the adjustment relies on topographic proxies developed through regional calibration.

Reading the Color Scale

ShakeMap uses a standard color palette mapped to instrumental intensity values, which correlate with the 修正メルカリ震度階級感知できない揺れ（I）から壊滅的な被害（XII）まで、特定の地点で観測された地震の影響を測定する12段階の階級。マグニチュードと異なり、震央からの距離によって値が変化する。 scale. Green tones represent weak shaking (MMI I–III), yellow represents light to moderate (MMI IV–V), orange represents strong (MMI VI–VII), red represents very strong to violent (MMI VIII–IX), and purple represents extreme to catastrophic (MMI X–XII). The boundaries between colors are not precise lines — shaking transitions gradually, and the color at any location represents the statistical best estimate given available data.

Peak Ground Acceleration and Velocity

ShakeMap reports ground motion in physically meaningful units. 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。 is expressed as a percentage of g (gravitational acceleration). Human perception begins around 0.5%g; structural damage begins around 10%g for poorly built structures; engineered buildings designed to modern codes may sustain damage at 50%g or more depending on duration. Peak ground velocity (cm/s) better captures the damage potential for flexible structures because it relates directly to spectral displacement demands.

The Distance-from-Epicenter Relationship

A key use of ShakeMap is understanding how shaking attenuates with distance. Use the Distance from Epicenter tool alongside a ShakeMap to quantify how far different intensity zones extend from 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。. As a general rule, strong shaking (MMI VII+) is concentrated within tens of kilometers of the rupture for M 6.0 events but can extend hundreds of kilometers for M 8.0+ megathrust earthquakes. Rupture directivity — the direction in which the fault rupture propagates — creates asymmetric patterns where shaking is stronger in the direction of rupture propagation.

Uncertainty and Data Density

ShakeMap includes an uncertainty layer showing where estimates are most and least reliable. Areas with dense station coverage have lower uncertainty, while remote areas rely more on model predictions. Immediately after a large event, the ShakeMap is provisional — it may be updated multiple times as additional station data is processed, 「揺れを感じましたか？」（DYFI）地震後に一般市民から震度に関する報告を収集し、市民参加型の震度マップを作成するUSGSのプログラム。地震を感じた誰もが報告を投稿できる。 reports are integrated, and moment tensor solutions refine the source model. Always note the version timestamp when using ShakeMap for post-event analysis.

Did You Feel It Integration

Citizen intensity reports from the 「揺れを感じましたか？」（DYFI）地震後に一般市民から震度に関する報告を収集し、市民参加型の震度マップを作成するUSGSのプログラム。地震を感じた誰もが報告を投稿できる。 system feed directly into ShakeMap as additional data points. Each report provides an estimated 震度人・構造物・自然環境への観測された影響から判定される、特定地点における揺れの強さの尺度。震央からの距離が離れるほど小さくなる。 at a geocoded location, supplementing instrumental data particularly in areas with few sensors. During the 2011 Virginia earthquake, which occurred far from densely instrumented regions, DYFI reports were essential to mapping the broad felt area. The system has collected tens of millions of responses since its launch, creating a historical archive of felt reports for thousands of events.

Applications in Emergency Response

Emergency managers rely on ShakeMap as a primary situational awareness tool in the first hours after a major earthquake. FEMA's HAZUS loss estimation software ingests ShakeMap data to produce rapid estimates of casualties, displaced households, and infrastructure damage. These estimates guide decisions about where to concentrate 捜索救助（SAR）地震後、倒壊した構造物に閉じ込められた生存者を発見・救出するための組織的な活動。発生から最初の72時間が、生存者を発見できる重要な時間帯とされる。 resources and how large a response effort will be required. Utilities use ShakeMap to prioritize inspection routes for gas and water pipeline systems.

Operational ShakeAlert Integration

In regions covered by シェイクアラート（ShakeAlert）USGSと大学パートナーが運用する、アメリカの緊急地震速報システム。西海岸（カリフォルニア州・オレゴン州・ワシントン州）をカバーし、緊急速報メールを通じて警報を送信する。, ShakeMap output becomes available within one to three minutes of an earthquake, faster than the several minutes historically required. The speed improvement comes from pre-event parameter estimates transmitted by ShakeAlert's real-time source characterization, which gives ShakeMap a head start before station recordings are fully processed. This rapid availability is critical for the first wave of emergency response decisions.

Reading Scenario ShakeMaps

Beyond real-time events, the USGS publishes scenario ShakeMaps for hypothetical future earthquakes on known faults. These scenario products show what shaking would look like if a given fault segment ruptured with a specified magnitude. Urban planners, building departments, and emergency planners use scenario ShakeMaps to evaluate infrastructure vulnerability and develop response plans before a disaster occurs. California's HayWired scenario, for example, modeled a M 7.0 earthquake on the Hayward Fault and generated ShakeMap products that revealed catastrophic expected shaking in the East Bay.

Summary

シェイクマップ（ShakeMap）地震発生後の揺れの強さの分布を示すUSGSの成果物。地震計データ、地震動モデル、「揺れを感じましたか？」の報告を組み合わせて作成される。 translates the abstract numbers of seismology into spatial context that operators, responders, and communities can act upon. By understanding the color scale, recognizing uncertainty zones, appreciating the role 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 knowing how to supplement maps with the Distance from Epicenter tool, you gain a clearer picture of the real geographic footprint of any earthquake.