2015年ネパール地震: ゴルカ地震

The Setting: The Himalayan Collision Zone

Nepal occupies a central position in the Himalayan mountain system, which is formed by the ongoing プレート衝突2枚の大陸プレートが収束し、ヒマラヤ山脈のような巨大な山脈を形成する過程。大陸衝突帯では、浅いながらも強力な地震が発生する。 of the Indian subcontinent with the Eurasian Plate. The Indian Plate moves northward at approximately 5 centimeters per year and subducts beneath the Tibetan Plateau along the Main Himalayan Thrust — a 収束型境界2枚のプレートが互いに近づき合うプレート境界。海洋プレートと大陸プレートでは沈み込み帯を、大陸プレート同士では造山運動を、海洋プレート同士では深い海溝を生じる。 断層岩盤に沿って動きが生じた破断面。断層は数ミリメートルから数千キロメートルまで様々な長さがある。地震を引き起こす主要な断層は活断層と呼ばれる。 system extending over 2,400 kilometers. This collision has been ongoing for approximately 50 million years and has produced the tallest mountain range on Earth. It also generates regular large earthquakes. The historical record documents devastating earthquakes in Nepal in 1255, 1408, 1681, 1833, and 1934. The 1934 Bihar-Nepal earthquake caused widespread damage in the Kathmandu Valley. Seismologists had identified the section of the Main Himalayan Thrust beneath central Nepal as a region of accumulated seismic strain, and a 2013 study by geophysicist Laurent Bollinger and colleagues estimated a high probability of a major earthquake in the coming decades. Nepal's building stock in 2015 was highly vulnerable: the Kathmandu Valley had experienced explosive informal urban growth, and the vast majority of residential buildings were 無補強組積造（URM）鉄筋補強のないレンガまたはブロック造の建物で、地震の揺れに極めて脆弱である。URM建物は、世界の地震死者数の大部分を占める。 brick construction that was known to be highly susceptible to collapse. The 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。 existed on paper but enforcement was nearly nonexistent.

The Earthquake: April 25, 2015

At 11:56 AM local time on April 25, 2015, the Main Himalayan Thrust ruptured approximately 77 kilometers northwest of Kathmandu, near the town of Gorkha. 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 M7.8. The 震源地震の破壊が実際に始まる地球内部の地点。フォーカスとも呼ばれる。震源の深さは、地表での揺れ方に大きく影響する。 was approximately 15 kilometers deep, relatively shallow for a continental collision zone. 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。 propagated predominantly eastward 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。, extending roughly 150 kilometers and releasing accumulated strain over a patch of the Main Himalayan Thrust. Kathmandu, a city of approximately 1.5 million people, experienced intense shaking lasting 50 to 60 seconds. The open plazas and central areas of the city — many built on the soft lacustrine sediments of the former Kathmandu lake bed — experienced severe 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。 that increased ground motion by factors of two to four compared to the nearby bedrock sites. Temples, monuments, and 無補強組積造（URM）鉄筋補強のないレンガまたはブロック造の建物で、地震の揺れに極めて脆弱である。URM建物は、世界の地震死者数の大部分を占める。 buildings throughout the historic city center collapsed. At the same time, the earthquake triggered avalanches on the slopes of Mount Everest and surrounding peaks, killing 22 mountaineers at Base Camp in what became the deadliest day in Everest's history.

The Science: A Partially Filled Gap

The 2015 Gorkha earthquake partially filled the seismic gap that geophysicists had identified beneath central Nepal, but it did not fill the entire locked patch. 干渉SAR（InSAR）地震前後に撮影されたレーダー画像を比較することで、センチメートル単位の精度で地表変動を測定する衛星レーダー技術。断層のすべりパターンを明らかにする。 satellite data and 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。 measurements showed that the earthquake ruptured the deeper, updip portion of the locked fault zone but did not propagate all the way to the surface — a pattern described as a blind thrust rupture. The remaining locked portions of the Main Himalayan Thrust still carry significant accumulated strain, meaning the 2015 earthquake did not fully relieve the seismic potential of the region. This finding had important implications for 余震本震と同じ断層領域で本震の後に続く、より規模の小さい地震。余震活動は数週間から数年続くことがあり、最大の余震は通常本震よりマグニチュードが1.0〜1.2小さい。 assessment: not only was the existing ruptured zone subject to stress re-adjustment 余震本震と同じ断層領域で本震の後に続く、より規模の小さい地震。余震活動は数週間から数年続くことがあり、最大の余震は通常本震よりマグニチュードが1.0〜1.2小さい。 sequences, but the adjacent unruptured segments may have had their failure probability increased by クーロン応力伝達地震が周辺の断層の応力状態を変化させ、将来の地震を誘発または遅延させうる過程。どの断層が破壊に近づいたかを予測するために用いられる。 transfer from the mainshock. The largest 余震本震と同じ断層領域で本震の後に続く、より規模の小さい地震。余震活動は数週間から数年続くことがあり、最大の余震は通常本震よりマグニチュードが1.0〜1.2小さい。, a M7.3 on May 12, 2015, struck east of the main rupture zone, in exactly the area where stress transfer models predicted elevated failure probability. Use the Earthquake Energy Calculator to understand how the M7.3 aftershock relates energetically to the M7.8 mainshock.

The Impact: Kathmandu and Mountain Communities

The 2015 Nepal earthquake killed 8,964 people and injured over 22,000. Approximately 604,000 houses were destroyed and another 288,000 were damaged. The Kathmandu Valley, despite being the center of population and government, was not the hardest-hit area relative to its size. The most severe devastation occurred in the hill districts northwest of Kathmandu — Gorkha, Sindhupalchowk, and Rasuwa — where remote mountain villages built of traditional stone 無補強組積造（URM）鉄筋補強のないレンガまたはブロック造の建物で、地震の揺れに極めて脆弱である。URM建物は、世界の地震死者数の大部分を占める。 were almost entirely obliterated. Road access to these districts was poor even before the earthquake, and landslides triggered by the shaking blocked mountain roads for days. The mountainous terrain made helicopter evacuation the only option for many communities. The historic structures of Kathmandu — temples, stupas, and palace complexes in the Durbar Squares of Kathmandu, Patan, and Bhaktapur — suffered severe damage, with UNESCO World Heritage sites destroyed or partially collapsed. The 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。 operations were complicated by the scale of affected area: roughly 500,000 square kilometers of territory across Nepal, India, China, and Bangladesh experienced moderate to strong shaking.

The Response: International Aid and Local Challenges

The international 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。 response to the 2015 Nepal earthquake was the largest ever deployed in the Himalayan region. Over 60 countries offered assistance. Urban search and rescue teams from China, India, Israel, Japan, the UK, and the United States arrived within 48 hours. The Nepali Army and Armed Police Force were mobilized in full. However, several structural challenges hampered the response. Nepal's single international airport in Kathmandu became severely congested as aid flights arrived simultaneously. The road network in the hill districts was inadequate for the scale of aid movement required. Government coordination structures were strained. The combination of 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。 enforcement failure, geographic remoteness, and infrastructure inadequacy created a response environment in which the gap between arriving international aid and reaching the most severely affected communities could be measured in days to weeks rather than hours.

The Legacy: Building Back Better

The 2015 Nepal earthquake generated enormous international attention and billions of dollars in reconstruction pledges. The National Reconstruction Authority was established to oversee rebuilding. Nepal's building codes were reviewed and strengthened. A major debate emerged about whether to rebuild traditional stone architecture — culturally irreplaceable but seismically vulnerable — or to shift entirely to reinforced concrete construction more resistant to 余震本震と同じ断層領域で本震の後に続く、より規模の小さい地震。余震活動は数週間から数年続くことがあり、最大の余震は通常本震よりマグニチュードが1.0〜1.2小さい。 sequences. The resolution was a hybrid approach: culturally significant structures rebuilt using traditional techniques with hidden reinforcement, while ordinary residential buildings encouraged to adopt confined masonry or reinforced concrete frames. Critically, the 余震本震と同じ断層領域で本震の後に続く、より規模の小さい地震。余震活動は数週間から数年続くことがあり、最大の余震は通常本震よりマグニチュードが1.0〜1.2小さい。 sequence — including the M7.3 May 12 event — demonstrated that 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。 compliance during reconstruction was not just aspirational but immediately life-saving, as the few new buildings that met seismic standards performed dramatically better than the legacy stock in the aftershock-damaged hillside communities.