1964年アラスカ地震: 大アラスカ地震

The Setting: Alaska's Subduction Zone

Alaska sits atop one of the world's most seismically active 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。 systems. The Pacific Plate subducts beneath the North American Plate along the Aleutian Trench at roughly 6 centimeters per year, making this one of the most productive convergent margins on Earth. Historical records and geological evidence document a long history of great earthquakes along the Aleutian arc, and the rupture zone of the 1964 earthquake — the Alaska-Aleutian 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。 — had been identified as a region of high seismic potential. In 1964, Alaska had been a US state for only five years, and much of its infrastructure was relatively new and designed without specific seismic provisions. Anchorage, the largest city with approximately 100,000 people, was built primarily on glacial outwash deposits — a mix of sand, gravel, and silt that was prone to ground failure during strong shaking. The city's newer residential neighborhoods had been developed on Turnagain Heights, an area underlain by particularly sensitive clay deposits.

The Earthquake: March 27, 1964

At 5:36 PM on Good Friday, March 27, 1964, the largest earthquake in US history struck approximately 125 kilometers east of Anchorage, beneath Prince William Sound. The マグニチュード地震が放出した総エネルギー量を表す単一の数値。整数値が1増えるごとに、放出エネルギーはおよそ31.6倍になる。 was M9.2, making it the second most powerful earthquake ever recorded by modern instruments, exceeded only by the 1960 Chile earthquake. The 断層破壊地震時に断層に沿って岩盤が破断し、蓄積された弾性エネルギーが地震波として放出される現象。破壊の長さは、小規模地震では数メートル、大地震では1,000km以上に及ぶこともある。 extended approximately 800 kilometers from near Kodiak Island to southeast Alaska, with the seafloor rising by up to 9 meters in some areas and subsiding by up to 2 meters in others. Strong shaking lasting 4 to 5 minutes was experienced across an area of over 800,000 square kilometers. 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 was so powerful that 地震計地震波によって生じる地面の揺れを検知し記録する装置。現代のデジタル地震計は、ナノメートル未満の変位も検出できる。 needles went off-scale at stations across North America. In Anchorage, the devastating 液状化水を含んだ緩い土壌が強い揺れによって一時的に強度を失い、液体のように振る舞う現象。建物が地面に沈下・傾斜・崩壊することがある。 of the Bootlegger Cove Clay — a sensitive marine clay deposited after the last ice age — caused catastrophic landslides in the Turnagain Heights neighborhood and downtown Anchorage. Thirty blocks of the central business district dropped 3 to 6 meters as the ground failed around them.

The Science: Documenting Ground Failure

The 1964 Alaska earthquake became one of the most thoroughly studied events in earthquake science history, in part because it occurred in a wealthy country with strong scientific institutions and in part because the scale and variety of ground failures were exceptional. The 液状化水を含んだ緩い土壌が強い揺れによって一時的に強度を失い、液体のように振る舞う現象。建物が地面に沈下・傾斜・崩壊することがある。 of the Bootlegger Cove Clay represented one of the most dramatic and well-documented examples of 側方流動液状化の際に、自由面（崖や川岸）に向かって土壌ブロックが水平方向に移動する現象。インフラ・橋梁・パイプラインに広範な被害をもたらすことがある。 ever recorded: entire neighborhoods slid toward Cook Inlet as the clay layer beneath them lost its bearing capacity under cyclic loading from 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。 train. 断層破壊地震時に断層に沿って岩盤が破断し、蓄積された弾性エネルギーが地震波として放出される現象。破壊の長さは、小規模地震では数メートル、大地震では1,000km以上に及ぶこともある。 at the surface was observed along portions of the rupture zone, and geodetic surveys documented the pattern of uplift and subsidence with remarkable precision — valuable data for understanding megathrust earthquake mechanics. 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。 generated by the 1964 earthquake was the most destructive in North American history. Local waves struck the communities of Valdez, Chenega, Whittier, and Seward within minutes, killing dozens and destroying waterfront infrastructure. The regional 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。 propagated throughout the Pacific Ocean: waves 4 to 6 meters high struck Crescent City, California, killing 11 people and causing $7.5 million in damage. Waves were measured in Japan, Antarctica, and throughout the Pacific basin. Use the Earthquake Energy Calculator to explore how the energy of this M9.2 event compares to other great earthquakes in American history.

The Impact: Transformation of Alaska

The 1964 Alaska earthquake killed 139 people — a remarkably low toll given its magnitude, reflecting Alaska's sparse population and the good fortune of the 5:36 PM timing (many residents were home rather than in commercial buildings that subsequently collapsed). The economic damage was approximately $311 million in 1964 dollars. The town of Valdez was so severely damaged by 液状化水を含んだ緩い土壌が強い揺れによって一時的に強度を失い、液体のように振る舞う現象。建物が地面に沈下・傾斜・崩壊することがある。 and 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 it had to be relocated to a more stable site. Seward's waterfront was destroyed, and the railroad terminus burned when oil tanks ignited. The port of Kodiak was devastated. The communities of Chenega on Prince William Sound lost 23 of their 75 residents to the tsunami — 31 percent of the population, one of the highest proportional losses of any community. Across Prince William Sound, the permanent coastal deformation caused by the earthquake dramatically altered the geography: some islands rose more than 10 meters, exposing former seafloor, while other areas dropped below the tidal zone, flooding former forests with seawater.

The Response and Rebuilding

The US federal government, working through the Army Corps of Engineers and the Small Business Administration, provided extensive reconstruction assistance. The rebuilding of Valdez on a new site became a model for how communities can use post-earthquake reconstruction as an opportunity to reduce future risk rather than simply restoring what existed before. Anchorage undertook a systematic program of soil stabilization and foundation retrofitting. The earthquake also prompted a major expansion of the US 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。 in Alaska and a reassessment of 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。 across the Pacific Northwest, where analogous 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。 conditions existed along the Cascadia zone.

The Legacy: Warning Systems and Building Codes

The 1964 Alaska earthquake had lasting impacts on both tsunami science and 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。 development in the United States. It demonstrated that 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。 earthquakes produce not just local tsunamis but ocean-crossing waves capable of killing people thousands of kilometers away, directly motivating improvements to the Pacific Tsunami Warning System. It revealed that 液状化水を含んだ緩い土壌が強い揺れによって一時的に強度を失い、液体のように振る舞う現象。建物が地面に沈下・傾斜・崩壊することがある。 of sensitive marine clays could be a dominant cause of structural damage even at distances far 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。, leading to new provisions in foundation engineering practice. The M9.2 event also recalibrated American scientific understanding of what was possible along the Cascadia 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。 in the Pacific Northwest, where a comparable fault system had been identified. The recognition that Seattle, Portland, and Vancouver might face a Cascadia earthquake of similar magnitude eventually drove major investments in 耐震補強既存の建物の耐震性を向上させるための強化工事。鋼製ブレースの追加、基礎の補強、構造物と基礎のボルト固定などが一般的な手法である。 programs, school safety assessments, and public 地震への備え家具の固定、連絡計画の作成、非常用物資の維持、訓練の実施など、地震の被害を最小限に抑えるための継続的な計画・準備の過程。 campaigns across the Pacific Northwest.