放出エネルギー
711.7 atomic bombs
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14:28 CST: The Longmenshan Fault Ruptures for 300 Kilometres
The afternoon of May 12, 2008, was ordinary in Sichuan Province. In Chengdu, China's fourth-largest city, workers were at their desks or returning from lunch. In the mountain towns of Beichuan, Yingxiu, and Wenchuan — smaller communities strung along the narrow valleys at the foot of the Longmenshan range — students were in afternoon classes, farmers worked terraced fields, and the spring tourist season was generating modest activity at the heritage sites of the region. At 2:28 PM China Standard Time, this ordinary afternoon became catastrophic.
The Longmenshan Fault — a thrust fault system marking the abrupt eastern edge of the Tibetan Plateau — ruptured. The earthquake was 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。 7.9, later refined by the China Earthquake Administration to the same value through additional analysis. 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 northeast from the hypocenter near Wenchuan County along approximately 300 kilometres of the fault system, producing one of the longest surface ruptures documented from any earthquake since the 1906 San Francisco event. The shaking lasted approximately 80 seconds in the near-field areas — long enough to complete multiple collapse cycles in poorly constructed buildings.
The death toll when finally determined: 87,587 confirmed dead, including 5,335 students who died in collapsed school buildings. Another 374,643 were injured. Approximately 4.8 million people were left homeless. The economic losses exceeded $85 billion, making it the most costly natural disaster in Chinese history to that point and one of the most expensive earthquake disasters in global history. The disaster struck a region that had not experienced a comparable earthquake in recorded history, striking a population and infrastructure that had evolved without the benefit of seismic code enforcement or earthquake culture.
The physical scale of the disaster was defined by the fault's orientation and rupture direction. The 300-kilometre rupture propagated northeast along the Longmenshan system, and the towns embedded in the narrow valleys at the fault's surface trace — Yingxiu, Beichuan, Qingchuan, Wenchuan — suffered near-total destruction. Yingxiu, located approximately 9 kilometres from the rupture initiation point, lost approximately 80 percent of its population of 10,000. Beichuan county town was so thoroughly destroyed by the combination of intense shaking and subsequent 地震誘発地すべり地震の揺れによって引き起こされる、土砂や岩石の斜面下方への移動。地すべりは地域全体を埋没させることがあり、揺れそのものより多くの犠牲者を出すこともある。s from the surrounding mountains that the central government ultimately decided it could not be rebuilt at its original location, and a new town was constructed several kilometres away.
Tibetan Plateau Collision Zone: Geology of a Tectonic Boundary
The tectonic setting of the 2008 Sichuan earthquake is one of the grandest geological phenomena on Earth. The Tibetan Plateau — average elevation exceeding 4,500 metres, the highest and most extensive high plateau on the planet — exists because of 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。 between the Indian and Eurasian プレート（テクトニックプレート）移動し、浮遊し、時に破断する地球のリソスフェアの巨大な区画。7つの主要プレートと約8つの小規模プレートがあり、それらの相互作用がほとんどの地震の原因となる。s. India is moving northward at approximately 40 millimetres per year, and the collision that began roughly 50 million years ago has produced the Himalayas by crustal shortening and thickening, the Tibetan Plateau by wholesale uplift of a crustal block two to three times the normal thickness, and a complex system of active faults throughout a vast region of Asia that accommodates the ongoing collision through a combination of crustal thickening and lateral extrusion.
The Longmenshan Fault system marks the eastern margin of this crustal thickening zone — the sharp boundary where the thick, buoyant crust of the eastern Tibetan Plateau meets the thinner, denser crust of the Sichuan Basin. The topographic relief across this boundary is extreme: within 50 kilometres of horizontal distance, the land surface rises from the nearly flat Sichuan Basin at approximately 500 metres elevation to peaks exceeding 6,500 metres in the Longmenshan range. This extreme topographic gradient is a direct expression of the active 逆断層（スラスト断層）圧縮力によって、上盤が下盤に対して上方にずれる断層。傾斜の緩い逆断層（スラスト断層）は、最大級の地震の原因となる。 motion along the Longmenshan system, which has been progressively thrusting the plateau crust over the basin crust for millions of years.
The Longmenshan system consists of three parallel northwest-dipping fault zones: the Maowen-Wenchuan Fault to the west, the Beichuan-Yingxiu Fault in the middle (which produced the primary 2008 rupture), and the Guanxian-Anxian Fault to the east (which produced a secondary rupture). GPS measurements in the years preceding the 2008 earthquake had shown convergence rates across the Longmenshan of only about 2-4 millimetres per year — relatively low compared to classic subduction zones. This low rate contributed to assessments that the Longmenshan system might not be capable of very large earthquakes, assessments that were dramatically revised after 2008.
The explanation for the apparent paradox — a fault with low geodetic convergence rate producing a magnitude-7.9 earthquake — lies in the concept of strain storage over long periods. A fault that slips at 3 mm/year needs approximately 667 years to accumulate 2 metres of elastic strain, enough for a large earthquake. If the fault only ruptures in large, infrequent events rather than releasing strain continuously through smaller earthquakes, the seismogenic record will show relative quiescence between major events. Paleoseismic trenching across the Longmenshan faults after 2008 found evidence of previous large surface-rupturing earthquakes, with estimated recurrence intervals of 2,000 to 10,000 years — consistent with the hypothesis that the 2008 earthquake was the most recent in a very long-recurrence sequence that had produced no comparable events in the recorded historical period.
300 km of Surface Rupture: Field Observations
The 300 kilometres of surface rupture produced by the 2008 Sichuan earthquake was documented in extraordinary detail by Chinese and international geological field teams in the weeks following the event. The primary rupture followed the Beichuan-Yingxiu Fault for approximately 240 kilometres, from near Yingxiu in the southwest to near Qingchuan County in the northeast. A secondary rupture on the Guanxian-Anxian Fault appeared as a shorter, parallel trace approximately 20-30 kilometres southeast of the main rupture, extending roughly 70 kilometres.
The character of the surface rupture reflected the 逆断層（スラスト断層）圧縮力によって、上盤が下盤に対して上方にずれる断層。傾斜の緩い逆断層（スラスト断層）は、最大級の地震の原因となる。 mechanism of the earthquake: the southwest block (the Tibetan Plateau side) moved upward and northeastward relative to the northeast block (the Sichuan Basin side). Vertical offsets along the rupture trace were striking — reaching 6-8 metres in maximum, with the hanging wall standing as a stark, fresh scarp above the footwall. Horizontal shortening components of similar magnitude were documented by offset road features, stream channels, and field boundaries. The combined thrust and lateral components of displacement confirmed the oblique-thrust character of the Longmenshan fault system.
The surface rupture trace crossed numerous roads, streams, and settlements, providing an inadvertent field experiment in the effects of fault proximity on building damage. Structures sitting directly on or within a few metres of the rupture trace were invariably destroyed, split by the differential motion of the ground on either side of the fault. Buildings within a few hundred metres experienced intense shaking amplified by the proximity to the rupture source and by directivity effects — the concentration of seismic energy in the forward direction of rupture propagation. Detailed mapping of damage patterns relative to fault distance allowed researchers to quantify fault zone setback distances appropriate for land use planning: the distance from an active fault surface rupture trace within which construction of occupied buildings should be prohibited.
Post-earthquake paleoseismic investigation of the Longmenshan faults — examining exposures created by the rupture and trenching across the fault trace — revealed the archaeological and geological record of previous surface ruptures. Multiple prior events were identified at various locations along the fault system, providing data on recurrence intervals that will inform future 地震ハザードマップ特定の期間内に地震の揺れが指定の水準を超える確率を示した地図。技術者・都市計画者・保険会社が地震リスクを評価するために用いる。s of the region and allow quantitative assessment of the expected recurrence time for the next comparable event.
The School Collapse Scandal: 7,000 Buildings, 5,335 Children
Among all the human dimensions of the 2008 Sichuan earthquake, the deaths of 5,335 students in collapsed school buildings generated the most sustained public grief, political controversy, and long-term policy consequence. The pattern was unmistakable and visually undeniable: school buildings had collapsed while adjacent structures — government office buildings, commercial facilities, private houses — stood relatively intact. The buildings that killed children appeared systematically weaker than those that protected adults.
The earthquake struck at 2:28 PM, approximately 2.5 hours into the afternoon school session. Virtually all students were in their classrooms. The collapse of school buildings throughout the affected region was essentially simultaneous across hundreds of communities, killing children across the full age range of primary and secondary schooling in a single 80-second event.
Engineering investigations of collapsed school buildings found recurring structural deficiencies: columns with inadequate reinforcement and insufficient confinement ties, beam-column connections that lacked the continuity reinforcement needed to prevent progressive failure, concrete of below-specified compressive strength (indicating substitution of cheaper materials or inadequate quality control during construction), and design layouts that created structurally irregular buildings prone to torsional response. Many of the buildings showed evidence of construction that deviated significantly from their structural drawings, suggesting that the drawings — even where they had incorporated 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。 requirements — had not been faithfully executed.
The term that Chinese parents and journalists applied to these buildings — 'tofu-dreg schoolhouses' — captured the public perception of shoddy construction using substandard materials. The political sensitivity was acute because the failures implicated local government officials responsible for school construction procurement, and because the pattern of better performance by government office buildings — constructed under more direct official scrutiny — suggested that the quality failures were not random but systematic.
Parents whose children had died in collapsed schools organised independently of official channels, compiled lists of dead children's names, and sought legal accountability and public acknowledgment of what had happened. These efforts were met with a combination of financial settlements offered to silence grieving families, censorship of online accounts, and official pressure not to speak to foreign journalists. The artist Ai Weiwei compiled a comprehensive list of the names and birthdays of all identified student victims — a memorial that simultaneously documented official neglect and served as testimony about construction corruption. The school collapse controversy thus became one of the most politically complex and emotionally charged aspects of the entire earthquake aftermath.
Landslide Dams: When Mountains Block Rivers
The steep terrain of the Longmenshan mountain range responded to the 80 seconds of intense shaking with one of the largest sequences of earthquake-triggered 地震誘発地すべり地震の揺れによって引き起こされる、土砂や岩石の斜面下方への移動。地すべりは地域全体を埋没させることがあり、揺れそのものより多くの犠牲者を出すこともある。s ever documented from a single event. Researchers ultimately mapped approximately 15,000 landslides triggered directly by the earthquake and its aftershocks, ranging from small rockfalls to massive deep-seated failures involving millions of cubic metres of rock and debris. The total area covered by earthquake-triggered landslide material was approximately 800 square kilometres.
The Hattian Bala-scale landslide dams created in Sichuan were technically distinct from the Kashmir analog — larger rivers, higher volumes, and greater risk to downstream populations. Thirty-four individual landslide masses were large enough and appropriately positioned to dam significant river channels, creating 'quake lakes' that accumulated rapidly and threatened catastrophic dam-break floods downstream. The largest and most hazardous of these was the Tangjiashan landslide dam on the Jianjiang River, approximately 60 kilometres northeast of Beichuan county town.
The Tangjiashan dam was roughly 800 metres long, 600 metres wide, and between 80 and 120 metres high, created by a massive bedrock landslide that blocked the river valley on May 12. The lake behind it grew rapidly as the river continued to flow — reaching a maximum volume of approximately 316 million cubic metres before engineering intervention. If the dam had failed suddenly in an uncontrolled breach, calculations suggested a flood wave reaching the city of Mianyang — population approximately 1.2 million — within hours, potentially adding thousands of deaths to the earthquake toll.
Chinese Army engineers and civilians began work on the Tangjiashan dam two weeks after the earthquake, despite continued aftershock risk and the physical challenges of accessing the remote mountain site. Their approach was to excavate a controlled spillway channel through the upper portion of the dam, creating a controlled path through which the impounded water could drain gradually rather than catastrophically. The work required drilling and blasting in unstable terrain while monitoring the dam and lake level continuously. The controlled drainage began on June 10, 2008, and the lake was successfully drawn down without catastrophic failure over the subsequent weeks. The engineering achievement was significant and was executed under conditions of physical risk and time pressure that few infrastructure projects have matched.
China's Largest Peacetime Military Mobilization
The Chinese government's response to the 2008 Sichuan earthquake was characterised by the fastest and largest military mobilisation in Chinese peacetime history, and by an openness to international assistance and media access that was notably greater than China's typical approach to domestic disasters. Within hours of the earthquake, Premier Wen Jiabao had boarded a plane for Sichuan, arriving at the disaster site that evening. Over the subsequent weeks, approximately 130,000 military personnel — Army, Air Force, and People's Liberation Army Reserve — were deployed to the affected region.
The logistical scale of the response was extraordinary. The road network linking the mountain communities to the Chengdu plain had been severely disrupted by the combination of direct earthquake damage to bridges and road surfaces and the hundreds of landslides that blocked mountain passes. Many affected communities were initially accessible only by helicopter. Army aviation units flew thousands of sorties in the first days, inserting paratroopers and rescue teams into otherwise inaccessible valleys and evacuating severely injured survivors. Engineers with heavy equipment followed to clear landslide debris from key road routes, re-establishing surface access to affected communities within days rather than the weeks it might otherwise have taken.
International rescue teams were welcomed from more than 60 countries — a significant departure from China's historically restrictive approach to foreign assistance in domestic emergencies. Teams from Japan, Russia, France, Singapore, Taiwan, and elsewhere operated alongside Chinese military and civilian rescue units. The decision to accept international assistance reflected both the genuine need for specialised search-and-rescue expertise and a political calculation that openness to international help would serve China's reputation in the context of the upcoming Beijing Olympics, three months away.
The earthquake disaster and the successful military-civilian response temporarily elevated public confidence in the Chinese government's disaster response capacity. The school collapse controversy — unfolding simultaneously — complicated this narrative, demonstrating the tensions between the government's genuine organisational capacity in responding to physical disasters and its political defensiveness in acknowledging the institutional failures that had made the school buildings so vulnerable.
Aftermath: Building Codes, Activism, and Accountability
The 2008 Sichuan earthquake produced lasting changes in Chinese building standards, construction oversight, and the geography of civil society. The most rapid and formally traceable changes were in regulatory frameworks. A national programme to assess and strengthen existing school buildings in seismically active regions was launched in 2009, backed by central government funding and explicitly motivated by the school collapse tragedy. The programme aimed to retrofit or replace unsafe school buildings throughout earthquake-prone regions, and represented the largest single programme of school building seismic improvement ever undertaken in China.
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。 revisions accelerated after 2008 strengthened requirements for column confinement reinforcement, increased design ground motions for western China in recognition of the Longmenshan fault's demonstrated capability, and extended the special requirements for life-safety structures (schools, hospitals, emergency response facilities) that had existed in principle but were inconsistently applied. New provisions required that school and hospital buildings be designed to a higher performance level than ordinary buildings — the principle that the structures most likely to be needed after a disaster must be the structures most likely to survive it.
The civic activism that emerged from the school collapse controversy — parents demanding accountability, independent researchers documenting construction failures, journalists attempting to report despite official restrictions — represented an important moment in the development of Chinese civil society, even though it was substantially suppressed in its immediate form. The grief and anger of earthquake-bereaved parents found expression in art, in underground documentation, and in the persistent advocacy of groups that refused to accept official silence as a substitute for accountability. These movements contributed to broader conversations about government accountability and citizens' rights that have continued in Chinese public life since 2008. Seismic Risk Checker tools that display both earthquake hazard and building vulnerability alongside each other embody the central lesson of 2008 Sichuan: the deadliness of an earthquake is determined by the intersection of ground shaking intensity and building fragility.