2015 네팔 지진: 고르카 지진

The Setting: The Himalayan Collision Zone

Nepal occupies a central position in the Himalayan mountain system, which is formed by the ongoing Plate CollisionThe process of two continental plates converging, creating massive mountain ranges like the Himalayas. Continental collision zones produce shallow but powerful earthquakes. 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 Convergent BoundaryA plate boundary where two plates move toward each other. Can produce subduction zones (ocean-continent), mountain building (continent-continent), or deep trenches (ocean-ocean). Fault (Geology)A fracture in rock along which movement has occurred. Faults range from millimeters to thousands of kilometers long. Major faults that produce earthquakes are called active faults. 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 Unreinforced Masonry (URM)Brick or block construction without steel reinforcement, which is extremely vulnerable to earthquake shaking. URM buildings account for the majority of earthquake fatalities worldwide. brick construction that was known to be highly susceptible to collapse. The Building Code (Seismic)A set of legal requirements governing the design and construction of buildings to ensure minimum levels of earthquake safety. Updated after major earthquakes reveal new vulnerabilities. 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 MagnitudeA single number that quantifies the total energy released by an earthquake. Each whole number increase represents roughly 31.6 times more energy released. was M7.8. The Hypocenter (Focus)The actual point within the Earth where an earthquake rupture initiates. Also called the focus. Depth of the hypocenter significantly affects how an earthquake is felt at the surface. was approximately 15 kilometers deep, relatively shallow for a continental collision zone. The Fault RuptureThe breakage of rock along a fault during an earthquake, releasing stored elastic energy as seismic waves. Rupture length can range from meters (small quakes) to 1,000+ km (great earthquakes). propagated predominantly eastward from the EpicenterThe point on the Earth's surface directly above the hypocenter (focus) where an earthquake originates underground. Often reported as the earthquake's location in news reports., 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 Soil Amplification (Site Effect)The increase in shaking intensity caused by soft soil or sediment layers amplifying seismic waves. Structures built on soft soil can experience 2-10 times stronger shaking than those on bedrock. that increased ground motion by factors of two to four compared to the nearby bedrock sites. Temples, monuments, and Unreinforced Masonry (URM)Brick or block construction without steel reinforcement, which is extremely vulnerable to earthquake shaking. URM buildings account for the majority of earthquake fatalities worldwide. 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. InSAR (Interferometric SAR)A satellite radar technique that measures ground deformation with centimeter accuracy by comparing radar images taken before and after an earthquake. Reveals fault slip patterns. satellite data and GPS GeodesyThe use of Global Positioning System receivers to measure tectonic plate motion and crustal deformation with millimeter precision. Reveals how strain accumulates on faults between earthquakes. 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 AftershockA smaller earthquake that follows the mainshock in the same fault region. Aftershock sequences can last weeks to years, with the largest aftershock typically 1.0-1.2 magnitudes below the mainshock. assessment: not only was the existing ruptured zone subject to stress re-adjustment AftershockA smaller earthquake that follows the mainshock in the same fault region. Aftershock sequences can last weeks to years, with the largest aftershock typically 1.0-1.2 magnitudes below the mainshock. sequences, but the adjacent unruptured segments may have had their failure probability increased by Coulomb Stress TransferThe process by which an earthquake changes stress on nearby faults, potentially triggering or delaying future earthquakes. Used to forecast which faults are brought closer to failure. transfer from the mainshock. The largest AftershockA smaller earthquake that follows the mainshock in the same fault region. Aftershock sequences can last weeks to years, with the largest aftershock typically 1.0-1.2 magnitudes below the mainshock., 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 Unreinforced Masonry (URM)Brick or block construction without steel reinforcement, which is extremely vulnerable to earthquake shaking. URM buildings account for the majority of earthquake fatalities worldwide. 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 Search and Rescue (SAR)Organized efforts to locate and extract survivors trapped in collapsed structures after an earthquake. The first 72 hours are the critical window for finding survivors alive. 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 Search and Rescue (SAR)Organized efforts to locate and extract survivors trapped in collapsed structures after an earthquake. The first 72 hours are the critical window for finding survivors alive. 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 Building Code (Seismic)A set of legal requirements governing the design and construction of buildings to ensure minimum levels of earthquake safety. Updated after major earthquakes reveal new vulnerabilities. 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 AftershockA smaller earthquake that follows the mainshock in the same fault region. Aftershock sequences can last weeks to years, with the largest aftershock typically 1.0-1.2 magnitudes below the mainshock. 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 AftershockA smaller earthquake that follows the mainshock in the same fault region. Aftershock sequences can last weeks to years, with the largest aftershock typically 1.0-1.2 magnitudes below the mainshock. sequence — including the M7.3 May 12 event — demonstrated that Building Code (Seismic)A set of legal requirements governing the design and construction of buildings to ensure minimum levels of earthquake safety. Updated after major earthquakes reveal new vulnerabilities. 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.