A fault under Tangshan city broke at 3:42 in the morning
A magnitude 7.6 quake struck Tangshan, Hebei, at 03:42 local time on 28 July 1976 (27 July UTC). Intensity reached XI, and the Tangshan fault—one of three in the Changdong zone—ran directly beneath the urban core on an alluvial plain at Beijing–Tianjin’s edge.
Within minutes about 85 percent of Tangshan's buildings collapsed or became unusable, every service failed and most road and rail bridges came down. The official toll was 242,469, but historians accept at least 300,000 deaths, which makes it the deadliest earthquake ever documented if famine deaths after the 1556 Shaanxi quake are set aside. Seismic risk had been badly underestimated, so the city was built largely of unreinforced brick, standing directly on a major fault. Shaking was felt up to 1,100 kilometres away, in Mongolia and Korea, and in Beijing, 140 kilometres off, nearly a tenth of buildings were damaged.
First estimated at magnitude 8.1, the mainshock was later recalculated at 7.6. Being shallow, it turned much of its energy into surface shaking. The fault slipped sideways, right-laterally, the rupture spread 140 kilometres, and the crust southeast of the fault shifted about 3 metres southwest, with surface breaks in five staggered segments through the city centre. Hours later a 6.2 aftershock hit near Ninghe, and a 6.9 followed there in November. The afternoon shock near Luanxian moved the opposite way on crossing faults, suggesting a wedge of crust was being squeezed south between them.
The setting explains the violence. Tangshan sits near where the Bohai shoreline lay about 4,000 years ago, on sediment washed down from the Yanshan mountains, above roughly 10 kilometres of limestone and sandstone rich in coal, folded into a syncline that brings seams within mining reach. Beneath lies metamorphic bedrock of the North China Craton, assembled about two billion years ago. The collision of India with Eurasia has reawakened its old faults, and this Eastern Block has produced six of the ten deadliest earthquakes known.
The zone is still restless. Quakes of 4.5 to 4.7 struck in 2012, 2016 and 2019, and a 5.1 in July 2020 revived debate over whether these are late aftershocks or ordinary background activity.
Source: 1976 Tangshan earthquake
