1960年チリ大地震: 記録上最大の地震

The Setting: Chile's Subduction Zone

Chile occupies one of the most seismically active strips of land on Earth. The Nazca Plate subducts beneath the South American Plate along the Peru-Chile Trench at a rate of about 7 centimeters per year, making the 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。 one of the most productive sources of large earthquakes on the planet. Historical records document dozens of major earthquakes along the Chilean coast, and the country had experienced multiple catastrophic events in the 20th century alone, including a M8.2 earthquake in 1939 that killed approximately 30,000 people. Chilean culture and architecture had some familiarity with seismic hazard, but the remote and rural areas of southern Chile — Los Lagos and Araucanía regions — had relatively simple timber and adobe construction.

The Earthquake: May 22, 1960

At 3:11 PM on May 22, 1960, the Nazca Plate lurched beneath the South American Plate along a rupture zone stretching approximately 1,000 kilometers from the city of Temuco in the north to Chiloé Island in the south. The earthquake lasted approximately 10 minutes — extraordinarily long even by megathrust standards — and released more seismic energy than any other event in the instrumental era. The モーメントマグニチュード断層面積・平均すべり量・岩石の剛性の積である地震モーメントに基づく、地震規模を測定する現代の標準的な尺度（Mw）。あらゆる規模の地震に対して精度が高い。 was determined to be M9.5, a value that has never been exceeded in recorded history. To put this in context: the 地震エネルギー地震によって放射される総地震エネルギーで、ジュールで測定される。マグニチュード9の地震は、核爆弾約25,000発分に相当するエネルギーを放出する。 released by the 1960 Chile earthquake was approximately 25 percent of all the seismic energy released by earthquakes worldwide between 1906 and 2005. The rupture area was so large that 地震波地震や爆発によって発生し、地球内部を伝播する弾性波。地震波は、震源で放出されたエネルギーを遠方の地点まで運ぶ。 oscillations could be detected for weeks afterward as the Earth rang like a bell — the phenomenon known as free oscillations of the Earth, which was first measured clearly after this earthquake and became a major tool of deep Earth 地震波トモグラフィー医療用CTスキャンに似た手法で、地震波の伝播時間を用いて地球内部構造の3次元イメージを作成する技術。マントルプルームや沈み込むスラブなどの深部構造を明らかにする。.

The Science: The World's Largest Earthquake

The 1960 Chile earthquake was pivotal to the development of モーメントマグニチュード断層面積・平均すべり量・岩石の剛性の積である地震モーメントに基づく、地震規模を測定する現代の標準的な尺度（Mw）。あらゆる規模の地震に対して精度が高い。 as the standard measure of earthquake size. The リヒタースケール1935年にチャールズ・リヒターが考案した、局地地震のマグニチュードを測定するための最初の対数マグニチュード尺度。現在はモーメントマグニチュードに大きく置き換えられているが、報道では今も広く使われている。 and 表面波マグニチュード（Ms）周期約20秒のレイリー波の振幅に基づくマグニチュード尺度。浅発地震にはよく機能するが、マグニチュード8.0以上では飽和する。 scales saturate — they cannot accurately represent events above roughly M8.5 because the seismic waves used to compute them reach maximum amplitudes that don't scale linearly with actual energy release. The moment magnitude formula, developed by Kanamori and Hanks in the 1970s, uses the 地震モーメント断層面積・平均変位量・岩石のせん断弾性率の積として算出される、地震が放出した総エネルギーの尺度。モーメントマグニチュードの算出基盤となる。 — the product of fault area, average slip, and rock rigidity — to compute a magnitude that does not saturate. Applied retroactively to the 1960 Chile earthquake using geodetic data, tide gauge records, and field observations of coastal deformation, the M9.5 value emerged from calculations of the enormous fault area (roughly 1,000 km by 150 km) multiplied by the estimated average slip of approximately 20 meters. The earthquake also generated a 津波海底地震時の海底の急激な変位によって発生する一連の海の波。津波はジェット機並みの速度（時速700km以上）で海洋全域を伝わることがある。 that became one of the most studied in history. The initial waves struck the Chilean coast within minutes, killing an estimated 1,000 to 6,000 people — figures are uncertain because the coastal devastation was so complete. The 津波海底地震時の海底の急激な変位によって発生する一連の海の波。津波はジェット機並みの速度（時速700km以上）で海洋全域を伝わることがある。 then propagated across the Pacific Ocean. Fifteen hours after the earthquake, waves 10 to 11 meters high struck Hilo, Hawaii, killing 61 people. Twenty-two hours after the earthquake, waves 6 meters high struck the coast of Japan, killing 142 people. The 1960 津波海底地震時の海底の急激な変位によって発生する一連の海の波。津波はジェット機並みの速度（時速700km以上）で海洋全域を伝わることがある。 was the first to clearly demonstrate the global reach of Pacific megathrust tsunamis and directly motivated the expansion of the Pacific Tsunami Warning System.

The Impact: Continental-Scale Destruction

The immediate casualties from the earthquake and tsunami in Chile are difficult to establish precisely. Estimates range from 1,655 to over 6,000 deaths, with the wide range reflecting the difficulty of counting casualties in remote areas with destroyed infrastructure. Approximately 2 million people were left homeless. The coastal landscape was permanently altered: sections of coastline subsided by as much as 2 meters, while other areas were uplifted. Lakes formed where none had existed. The Valdivia River was temporarily blocked, creating flooding upstream. The 地震波地震や爆発によって発生し、地球内部を伝播する弾性波。地震波は、震源で放出されたエネルギーを遠方の地点まで運ぶ。 energy triggered volcanic eruptions at the Cordón Caulle volcanic complex, beginning just 47 hours after the mainshock — an example of how mega-earthquakes can modify the stress field in ways that influence volcanic activity. The 津波海底地震時の海底の急激な変位によって発生する一連の海の波。津波はジェット機並みの速度（時速700km以上）で海洋全域を伝わることがある。 caused damage as far away as Japan and the Philippines, demonstrating that this event's 地震エネルギー地震によって放射される総地震エネルギーで、ジュールで測定される。マグニチュード9の地震は、核爆弾約25,000発分に相当するエネルギーを放出する。 transcended its local geography and became a truly global disaster. Use the Earthquake Energy Calculator to compare the energy of this M9.5 event to the next-largest recorded earthquakes — the difference is staggering.

The Response and Legacy

Chile's response to the 1960 earthquake was shaped by the simultaneous political and economic crises facing the country under President Jorge Alessandri. International aid from the United States, Soviet Union, and European nations helped fund reconstruction. The earthquake directly motivated the modernization of Chile's 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。, which became one of the more rigorous in Latin America over subsequent decades — a factor that would prove crucial when Chile faced another M8.8 earthquake in 2010. The 1960 event's global 津波海底地震時の海底の急激な変位によって発生する一連の海の波。津波はジェット機並みの速度（時速700km以上）で海洋全域を伝わることがある。 impact forced the Pacific Tsunami Warning System to expand its scope and improve its modeling capabilities. In Japan, the disaster motivated the construction of seawalls along the Sanriku coast — the same seawalls that were later overtopped by the 2011 Tohoku tsunami. The 1960 Chile earthquake also entered the scientific consciousness as proof that the Earth's tectonic system is capable of releasing energy at a scale that defies intuition: 1,000 kilometers of fault rupture, 20 meters of average slip, and a tsunami that crossed the world's largest ocean as a coherent, deadly wave. This single event reshaped seismology, ocean science, and emergency management simultaneously.