The 1976 Tangshan earthquake (Chinese: 唐山大地震; pinyin: Tángshān dà dìzhèn; lit. 'Great Tangshan earthquake') occurred at 19:42:55 UTC on 27 July (03:42:55, 28 July local time). The maximum intensity of the 7.6 earthquake around Tangshan, Hebei, China, was XI (Extreme) on the Mercalli scale. In minutes, 85 percent of the buildings in Tangshan collapsed or were rendered unusable, all services failed, and most of the highway and railway bridges collapsed or were seriously damaged. The official count stated 242,469 deaths, while historians accepted at least 300,000 died, making it the deadliest earthquake in recorded history (excluding the famine deaths from the 1556 Shaanxi earthquake) and one of the worst disasters in China by death toll.
Tectonic setting
Tangshan lies at the northern edge of the Beijing-Tianjin-Tangshan Plain, an alluvial plain that stretches from Beijing to the Sea of Bohai. This plain – the northeastern corner of the great North China Plain – is where sediments eroded from the Yanshan mountains to the north have filled in the ancient Sea of Bohai, with Tangshan near where the shore was about 4,000 years ago. To the south these sediments have formed a layer of weak soils as much as 3 kilometres (1.9 mi) thick. At Tangshan and northward these sediments are thinner where the underlying strata crops out to form isolated hills. This underlying strata is a thick —typically 10 kilometres (6.2 mi) — layer of mainly sedimentary strata such as limestone and sandstone, with large deposits of coal. Tangshan is located particularly over a northeast oriented syncline, a fold in the sedimentary strata that has brought massive deposits of coal close enough to the surface to be mined. In this area the overlying alluvium varies in thickness from several meters to around 600 metres (2,000 ft).
Underlying all this is the ancient bedrock of different kinds of metamorphic rock (such as schist, gneiss, quartzite, granulite, etc.) that form the Eastern Block of the North China Craton. This craton (see figure) was formed approximately two billion years ago by the collision of two major crustal blocks that left a belt of uplifted mountains – the Central (China) Orogenic Belt (COB) – that crosses China approximately southwest to northeast, passing just west and north of Beijing. Just north of Zunhua another orogenic belt, the east–west trending Yanshan mountain fault-fold belt (also known as the Yanshan seismic belt) marks the northern edge of the North China Craton (and of the alluvial plain). It is also the location of over half of the destructive earthquakes in Hebei province, as under the plain several fault zones (oriented parallel to the Central Orogenic Belt) terminate against the Yanshan mountains.
Many of these faults are ancient, but have been reactivated by the force transmitted from the collision of the Indian plate against the Eurasian plate, making the Eastern Block unusually active seismically, accounting for six of the ten deadliest earthquakes in recorded history.
The Tangshan fault that ruptured 28 July runs right under the center of Tangshan City. One of three faults in the Changdong fault zone, it runs approximately east-northeast about 36 kilometres (22 mi) to where it terminates against the north–south trending fault where, just to the south, the secondary M 7.1 quake occurred ("B" on the 'shaking' map). The southern end of the Tangshan fault (it bends slightly at Tangshan) is near Ninghe, which was also the site of a M 6.2 earthquake several hours after the main shock, and an M 6.9 quake ("C") the following November. The Tangshan fault is considered shallow, but corresponds with a deeper and younger fault with somewhat differing characteristics.
Earthquakes
The earthquake sequence comprised two shocks with magnitudes of at least 7.0. The first struck at 3:42:55 in the morning (local time), approximately 12 kilometres (7.5 mi) under the southern part of Tangshan. The magnitude was initially estimated at 8.1, subsequently recalculated to be 7.6 on the standard scale. However, that scale measures only the total energy released by an earthquake, and earthquakes vary in how much of that energy is converted to seismic shaking. The Tangshan quake, being relatively shallow, converted much of its energy to surface shaking, and on the (surface magnitude) scale it also measured 7.6, and 7.8 on the Chinese surface magnitude scale. This shock ruptured along a vertically dipping, northeast trending right-lateral strike-slip fault. Based on the location of the aftershocks, the rupture initiated from the epicenter and extended outwards along the fault for 140 km (87 mi). The crust on the southeast side of the fault was displaced by about 3 metres (9.8 ft) to the southwest. This resulted from tectonic compression on a nearly west–east axis. Surface rupturing occurred in five en echelon segments extending 8 kilometres (5 mi) through the center of Tangshan.
The second main shock, with a magnitude 7.0 Mw, or 7.4 Ms, struck that afternoon at 18:45:36 near Luanxian, about 70 kilometres (43 mi) to the east-northeast ("B" on the intensity map in the next section), just south of the northeastern end of the Tangshan fault. This occurred in a zone of north-northwest striking conjugate faults that cut across the north end of the Tangshan fault. The left-lateral motion here, along with the right-lateral motion on the Tangshan fault, suggests that as the crustal blocks to the west and east are compressed together the block between these two earthquakes is being squeezed out to the south.
Aftershocks
A long sequence of aftershocks followed, with twelve of magnitude 6 or greater. The first of these struck just three and a half hours after the initial shock, at 7:17, at the southern end of Tangshan fault, near Ninghe ("C" on the map in the section below), with a magnitude of 6.2 Ms. Another significant aftershock (Ms 6.9) occurred in November near Ninghe. Most aftershocks occurred between these end points, in a zone 140 kilometres (87 mi) long and about 50 kilometres (31 mi) wide. Many buildings were further damaged by the aftershocks.
The aftershock zone remains seismically active in the twenty-first century. Earthquakes measuring 4.5 Msto 4.7 Msoccurred in 2012, 2016, and 2019. On 12 July 2020, a 5.1 Msstrike-slip earthquake struck the northern part of the Tangshan Fault. It produced its own sequence of aftershocks that ended on 17 July 2020. The earthquake caused only minor damage to buildings in Tangshan. Its occurrence stirred a debate among scientists into whether the recent earthquakes are aftershocks of the 1976 event or are background seismicity.
Damage
The damage done by an earthquake depends primarily on two factors. First, the intensity of shaking, which depends mainly on the magnitude of the earthquake rupture, the distance from the epicenter, and the nature of the local soil and topography, with soft soils (e.g., sediments and fill) more likely to amplify the intensity and duration of the shaking. Second, the design and construction of the structures being shaken, with houses built of adobe or stone, wooden houses without a well-built frame, and unreinforced masonry construction being especially vulnerable. The seismic risk had been greatly underestimated and almost all buildings and structures were designed and built without seismic considerations. As a result, Tangshan was "mainly a city of unreinforced brick buildings", sitting right on top of a major fault line.
The power (magnitude) of the Tangshan earthquake is indicated by the extent of where it was felt: up to 1,100 km (680 miles) away, across most of northeastern China, and even in Mongolia and Korea. In and around Beijing, 140 km (87 miles) from the epicenter, the shaking reached an intensity of VI on the Chinese intensity scale (similar to the Modified Mercalli Intensity Scale), with nearly 10% of all buildings damaged, and at least 50 fatalities.
The economic loss totaled to 10 billion yuan.
Intensity XI and X zone
The rupture occurred under the southern part of the city, and propagated northeastward on a fault that runs through the middle of the city. The maximum intensity was "XI" (eleven) on the 12-degree Chinese scale. Nearly every building and structure in the city collapsed, wholly or partially, infrastructure was severely damaged, and essential services, such as electric power, water supply, and communications, were entirely knocked out.
This area of maximum damage – the meizoseismal area – was approximately 10.5 kilometres (6.5 mi) long and from 3.5 to 5.5 kilometres (2.2 to 3.4 mi) wide, centered roughly along the railway.
The area of intensity X shaking – where only new, one-story brick buildings were merely "damaged or slightly damaged", the rest being severely damaged or worse – was 36 kilometres (22 mi) long and 15 kilometres (9.3 mi) across. In this "high intensity" zone (intensity X and XI, within the red contour on the map) 20 highway bridges and five railway bridges cross the Douhe River in Tangshan; only six survived with only minor damage.
Intensity IX and VIII zone
Shaking of intensity IX (or greater) occurred in a zone roughly 78 km (48 mi) long and 42 km (26 mi) — about 1,800 square kilometres (695 sq mi) wide, inner orange contour on the map – and also around the aftershocks at Luanxian and Ninghe. In this zone most buildings classified as Class III (well-built buildings of wood, masonry, or reinforced concrete) survived, but many Class II buildings (typically old wood-frame buildings lacking a well-built frame, and quite common outside of the cities) were destroyed, while a majority of Class I buildings (built of adobe or stone) were destroyed.
Further out (to the outer orange contour), and around the city of Tianjin and a few isolated patches, intensity VIII shaking mostly affected Class I buildings (more than half destroyed), bridges, and tall brick chimneys. Railway track was also subject to bending or displacement, depending on soil conditions.
Intensity VII zone
The zone of intensity VII shaking – inside the dark brown contour – marks the extent of moderate damage, where many Class I structures (of weak design or construction) were damaged but only "few"—between 10% and 30%—were destroyed, and only a "few" Class two buildings damaged. This ellipsoid zone extended about 75 kilometres (47 mi) north and south of Tangshan and 120 kilometres (75 mi) east and west, from about 25 kilometres (16 mi) short of Beijing to short of Qinhuangdao City (which had anomalously higher shaking), and from the Sea of Bohai in the south and southwest to just north of Zunhua. The north–south shortening of this zone is attributed to buttressing by the bedrock of the Yanshin mountains.
Significant damage occurred beyond this in the VI zone, but (like in Beijing) affected less than 10% of the buildings, or occurred in small localized areas.
Coal mines
Mining coal is Tangshan's principal industry; when the quake struck, around 10,000 miners were underground. For the most part, the mine roadways (tunnels) were not seriously damaged, but with the loss of electrical power there were no illumination (aside from headlamps), no ventilation, and no working lifts. It is reported that most miners escaped within hours, but that some others took two weeks to reach the surface.
Most of the damage to the vertical shafts occurred within the first 50 metres (164 ft) where they pass through the water-bearing alluvium. In many cases the concrete liner built to keep out the water cracked (particularly where not built properly), allowing a much greater inflow of water. Coupled with damage to the underground drainage system and lack of power to drive the pumps, many of the various mines flooded.
Some electrical power to the mines was restored in three days, and some coal production resumed within ten days. However, de-watering, overhauling of flooded electrical equipment, and rebuilding of surface buildings and structures continued for a year and a half; the pre-earthquake level of production was not reached until the end of 1977.