津波リスク推定器

Estimate tsunami risk based on earthquake parameters and your coastal proximity.

Assessment

地震がどのように津波を発生させるか

津波は、主に沈み込み帯に沿った海底地震によって引き起こされる、水の突然の大規模な変位によって生成される海洋波です。巨大地震の際に海洋プレートの一部が上方に突き上げられたり下方に沈んだりすると、膨大な量の水が変位し、長周期波の連続として外側に伝播します。深海では津波の波は500〜800 km/h（ジェット機に匹敵）の速度で移動し、波高はわずか30〜60 cmであるため、ほとんど検出できません。波が浅い沿岸水域に近づくと減速・圧縮され、波高が劇的に増幅されます。この過程は浅水効果と呼ばれ、海岸線で10〜30メートル以上に達する可能性があります。

すべての地震が津波を発生させるわけではありません。主要な要因は：地震が海底（海中）であること、浅い（通常70 km未満）こと、大きい（局地津波の場合は通常M7.0以上、広域津波はM7.5以上）こと、そして海底の顕著な垂直変位を伴うことです。主に水平方向の断層運動を伴う横ずれ断層地震は、重大な津波を発生させることはまれです。最も危険な津波発生メカニズムは、沈み込み帯のメガスラストにおける逆断層運動であり、2004年のインド洋地震（M9.1）、2011年の東北地方太平洋沖地震（M9.1）、1960年のチリ地震（M9.5）に見られます。

津波科学の主要概念

津波の警報時間は距離に依存します：近地津波は10〜30分以内に到達する可能性がありますが、遠洋津波は数時間かかることがあります。2011年の日本の津波は22時間後にチリに到達しました。
津波地震は、そのマグニチュードに対して不釣り合いに大きな津波を発生させる緩速破壊の特殊な地震であり、近隣の海岸にとって特に危険です。
遡上高（陸上で水が到達する最大垂直高度）は、海岸地形や湾の共鳴効果により沖合の波高をはるかに超えることがあります。
太平洋津波警報センター（PTWC）や地域の警報センターは、DART（深海圧力センサー）ブイを使用して津波をリアルタイムで検出・確認しています。

主な用途

報告された地震のパラメータに基づいて津波を発生させる可能性があるかどうかの判断。
地震の特性と津波発生の関係についての教育的な探究。
旅行や移住計画のための沿岸の津波ハザードへの暴露評価。
津波警報システムと近地津波に対する即時避難の重要性について学ぶこと。

How to Use

1
Enter Earthquake Parameters
Input the earthquake magnitude, focal depth, and location. Tsunamis are most efficiently generated by shallow (< 50 km depth) thrust earthquakes with vertical fault displacement; the tool checks these criteria automatically.
2
Specify Your Coastal Location
Enter your coastal city or coordinates. The tool calculates your approximate distance from the source and identifies whether you are in a mapped tsunami inundation zone based on NOAA and national tsunami center data.
3
Read Your Risk Summary
Review the estimated wave arrival time, indicative wave height range, and evacuation tier. Treat all outputs as supplementary to official warnings from PTWC, NTHMP, or JMA, which must always take precedence.

About

Tsunami science sits at the intersection of seismology, physical oceanography, and coastal engineering. The word tsunami derives from the Japanese 津波 (tsu, harbor; nami, wave), reflecting Japan's millennia of devastating experience with these events. Despite their colloquial name 'tidal waves,' tsunamis have no connection to tidal forces; they are long-period gravity waves with wavelengths of 100–500 km in deep water and wave periods of 10–60 minutes, compared to wind-driven ocean waves with periods of seconds.

The physics of tsunami generation requires a mechanism that displaces a large volume of water vertically over a large area. Megathrust earthquakes accomplish this by the sudden elastic rebound of the overriding plate—the 2011 Tohoku earthquake caused the seafloor to rise by 5–8 meters over a 300 × 200 km area instantaneously, displacing an estimated 5 cubic kilometers of water. The resulting wave system propagates radially, with energy concentrated perpendicular to the fault strike. Directivity effects mean that the coast directly opposite the rupture typically receives the highest waves.

The Deep-ocean Assessment and Reporting of Tsunamis (DART) buoy network, operated primarily by NOAA, provides real-time sea-level measurements from the deep ocean floor. These buoys detect tsunami wave amplitudes of centimeters in the open ocean, enabling confirmation or cancellation of warnings within 15–30 minutes of a potentially tsunamigenic earthquake. Combined with coastal tide gauge networks and numerical propagation models, DART data allows warning centers to issue probabilistic wave height forecasts for coastal communities hours before wave arrival in distant-field scenarios. Local and regional tsunamis remain the most challenging problem in warning science because the lead times are measured in minutes.

FAQ

What earthquakes generate tsunamis?

Tsunamis are most commonly generated by shallow (< 50 km depth) submarine thrust earthquakes that produce significant vertical seafloor displacement, typically exceeding 1 meter over large areas. The key factors are: magnitude ≥ M7.5 (though some M7.0 events with favorable geometry have generated destructive tsunamis), a thrust (reverse) or oblique-thrust focal mechanism with a large vertical displacement component, shallow focal depth in the crust or uppermost mantle, and occurrence beneath ocean floor rather than continental crust. Strike-slip earthquakes—where motion is primarily horizontal, as on transform faults—rarely generate significant tsunamis. The 2018 Sulawesi earthquake was exceptional: a predominantly strike-slip event triggered localized submarine landslides that generated the devastating Palu Bay tsunami.

How fast do tsunamis travel and when do they arrive?

Tsunami propagation speed in open ocean is governed by the shallow-water wave formula: v = √(g × d), where g is gravitational acceleration and d is ocean depth. In the deep Pacific (average depth ~4,000 m), tsunamis travel at approximately 700–800 km/h—comparable to a commercial jet aircraft. As waves enter shallower coastal water, they slow dramatically (to 50–100 km/h near shore) while their amplitude increases through shoaling. This means a tsunami generated 4,000 km away arrives in about 5–6 hours in deep water, but can take 30–60 minutes to inundate a coast after entering shallow shelf waters. The Pacific Tsunami Warning Center (PTWC) issues initial bulletins within 3 minutes of detecting a significant seismic event.

How far inland can tsunami waves travel?

Tsunami inundation distance depends on wave height, coastal topography, and land elevation. The 2011 Tohoku tsunami waves reached heights of 40.5 m (at Miyako, Japan) and inundated up to 10 km inland across flat coastal plains, destroying the town of Rikuzentakata. In contrast, mountainous coastlines confine inundation to narrow strips. Maximum inundation run-up is generally defined as the highest elevation reached by wave water, measured as meters above mean sea level. NOAA and national emergency management agencies have mapped tsunami inundation zones for high-risk coastlines using numerical models calibrated against historical events. These maps define evacuation zones (A through E or Zone 1 through 3 depending on jurisdiction) for emergency planning.

Is a tsunami warning always issued after a large earthquake?

Warning centers issue tsunami warnings, advisories, and watches based on rapid seismic analysis within minutes of a significant event. PTWC monitors earthquakes globally and issues initial bulletins for any event M7.0+ in ocean basin settings within 3 minutes. However, the first message is often a 'tsunami information statement' that may not indicate confirmed wave generation—definitive warnings come after sea-level gauges and DART buoys confirm or deny wave propagation. Local tsunamis—generated by earthquakes within 50–100 km of shore—can arrive within 5–20 minutes, before official warnings are disseminated. In these cases, the natural warning is the shaking itself: the international standard guidance is that prolonged strong shaking near the coast (> 20 seconds) is itself a tsunami warning signal, and coastal residents should evacuate immediately without waiting for official messages.

Are there tsunamis caused by non-earthquake sources?

Tsunamis can be generated by submarine landslides, volcanic activity, meteorite impacts, and atmospheric pressure disturbances (meteotsunamis). Submarine landslides are the second most common cause: the 1958 Lituya Bay event in Alaska, triggered by an earthquake-induced rockslide, produced a 524-meter run-up—the tallest wave in recorded history. The 2022 Hunga Tonga-Hunga Ha'apai volcanic eruption generated an unusual meteotsunami-like pressure wave that propagated globally at the speed of sound in the atmosphere (~340 m/s), reaching Peru and Japan within hours. Volcanic island collapses—hypothesized for the Canary Islands—could theoretically generate Atlantic basin tsunamis, though the probability and magnitude of such events are debated in the scientific community.