No other earthquake in the 20th century was as catastrophic or claimed as many lives as that which struck the city of Tangshan in Northern China on July 28, 1976.
Tangshan, a thriving industrial city in Hebei Province is located about 95 miles east and slightly south of Beijing, and about 280 miles southwest of Haicheng (Liaoning Province) - where in February 1975 another very destructive earthquake had occurred.
Although the region had experienced moderate seismic activity in the past, there were no foreshocks and no advance warning. The following is a brief summary on this quake and the seismotectonics of the region.
HEBEI Province and Epicentral Area mostly affected by the Earhquake
Date and Time of Origin: July 28, 1976 at 19:42:53.8 UTC (local date and time: July 28, 1976, 03:42).
Epicenter: The earthquake occurred near the coast, in Tangshan in the Hebei Province of northeastern China. Its epicenter was at 39.60° N 118.20° E, about 140 kilometers southeast of Beijing.
Magnitude: Originally reported as 7.8, later revised to Mw 7.6
Focal Depth: Shallow, 15 kilometers
Aftershocks: Many strong aftershocks followed the main earthquake, two of which had magnitudes of 6.0 or more. A major aftershock of 6.1 magnitude struck 15 hours after the main earthquake. In the following days, there were many more aftershocks ranging in magnitude from 5 to 5.5. Several months later, on Nov 15, 1976, a magnitude 6.0 earthquake struck again the same region.
Felt Reports and Intensity of Ground Motions
The earthquake was felt in fourteen provinces of China, and as far as Xian, about 470 miles (756 km) away, in Beijing (about 140 km to the west) and in Tientsin (60 miles to the southwest). The intensities of the earthquake’s ground motions were extensively surveyed and reported. In the epicentral area the intensity of the earthquake was estimated at XI, according to the State Seismological Bureau report. According to eyewitness reports the shaking lasted for about 90 seconds. Ground motions were so strong that people reportedly were thrown in the air.
Intensities of Ground Motions of the Tangshan Earthquake (after Wang Fang 1976, State Seismological Bureau of China)
The region with intensity X was reported as being elliptical in shape and to cover a total area of about 370-km2. The region of intensity of IX was reported as being rhombic in shape, also trending in a northeast direction and to cover an area of about 1,800 km2. In this area, most of the homes were damaged and about 40 percent of them collapsed. The region of intensity VIII extended in a southeastward direction and covered an area of about 7,300 km2. The region of intensity VII was reported to cover an area of about 33,000 km2.
Death Toll and Damages
The earthquake struck at 3:42 a.m. in the morning, when most people were at home asleep. The timing contributed to the great death toll. The zone of maximum destruction was estimated to be is about 47 square kilometers. It included the city of Tangshan and the southern suburb along the Beijing-Shanhaiguan railway.
What made matters worse, was the fact that the city of Tangshan is located in the center of an area surrounded with major faults. Most of the structures in the region were extremely vulnerable because they had been built on unstable, alluvial soils. Consequently most these structures were destroyed. Only a few of the city’s structures and building were earthquake-resistant but even the well built structures suffered serious damage.
Over a four-by-five mile area the devastation of the city was nearly total. About ninety-three percent of residential buildings and seventy-eight percent of commercial and industrial buildings in Tangshan were destroyed.
Death Toll - Within seconds after the thousands of people lost their lives or were trapped in debris. The actual death toll from this earthquake may never been known with certainty. According to official government accounts the earthquake killed 242,769 people and severely injured another 169,851. However, based on the density of the population and the extent of destruction, these figures have been disputed. At the time Tangshan had 1.6 million inhabitants. Since the earthquake destroyed ninety three percent of all residential buildings, the death toll was estimated to be three times greater than what was reported - ranging from 655,000 to 779,000 people. The extremely high death toll makes the 1976 Tangshan event the second worse earthquake disaster in recorded history. The most destructive earthquake ever, occurred four centuries earlier in 1556 in Shaanxi, China. It is estimated that the 1556 earthquake killed 830,000 people. Another earthquake in the Gansu region in 1920 had killed about 200,000.
Damages - The earthquake’s destruction was beyond description. Everything was completely leveled. Highway bridges and at least two dams collapsed. All roads, except for one were closed. Rails were bent causing the derailment of seven commercial trains. Homes, and factories were leveled to the ground. There was total destruction of the region’s infrastructure. Electric power, water supply and sewer systems failed. All telephone and radio communications systems stopped functioning. Almost all of the irrigation wells became inoperative. The damage was not restricted to the Tangshan region only. Damage was reported from as far away as Qinhuangdao, Tianjin and Beijing
Observed Unusual Phenomena
The earthquake caused extensive sagging and severe fissuring of the ground surface. Sand and water gushed from the ground and spread over large tracts of farmland. Mud volcanoes of up to 3 meters in diameter sprung up.
The earthquake ruptured a five-mile (8 km) section of a 25-mile long fault that passes through the city Tangshan. This Tangshan Fault is a strike-slip fault with a north-northeast orientation. The fault is part of an extensive strike-slip fault system, known as Tancheng-Ljiang, or Tan-Lu. This system extends in a north-northeast direction for more than 3,200 miles from the north bank of the Yangtze River in eastern China to the west across the Russian border.
There were substantial ground movements along the segment of the fault that ruptured. Along the west side the ground moved laterally for about five feet, in a north/northeast direction sub parallel to the major axis of the meizoseismic zone. However, in some areas, horizontal ground displacements of up to 7 meters were subsequently measured. On the eastern side of the rupture, the ground block tipped upward near the south end and downward at the northern end.
There were no foreshocks or clear precursory phenomena prior to the Tangshan earthquake - as there had been in other earthquake stricken areas of China. However, more than half a month earlier there had been a series of abnormal signals observed in the regions of Beijing, Tianjin, Tangshan, Bohai and Zhangjiakou. Based on such signals the State Seismological Bureau had correctly concluded that a significant earthquake could be expected between July 22, 1976 and August 5. However, the precursory phenomena differed from those of other earthquakes. Because of the scattered distribution of the signals, there was no determination of the location where this earthquake would strike.
Just prior to the earthquake, many unusual phenomena were observed in the immediate Tangshan region. There were observations of large amplitude variations of groundwater level and of strange animal behavior. As early as July 12, it was reported that gas began to discharge from a well in a village. On July 25 and 26, this discharge increased. The day before the earthquake, well water at another village, reportedly rose and fell three times and other wells showed signs of cracking of their lining. The night before the earthquake, many people in Tangshan reported seeing strange lights in the sky and hearing loud sounds. Some people reported seeing lights of multiple hues and fireballs traversing the skies. Unfortunately the anomalous precursory phenomena were widely scattered and inconclusive. They occurred too late to be of usefulness for short-term prediction and warning purposes. The only community that paid attention to the precursory phenomena was that of Qinglong County. Special emergency meetings for preparedness were held in the three days just prior to the earthquake – and this may have contributed to greater survival rate in this County.
Strange Animal Behavior Prior to the Earthquake
There were numerous reports of unusual animal behavior prior to the Tangshan earthquake – indicating that something was going to happen. Unfortunately, these were isolated incidents that were spread over a large area in a heavily populated region of China, thus no special significance was given at the time. It was reported for example that chickens in Baiguantuan refused to eat and ran around excitedly, twittering. Mice and yellow weasels were seen running around looking for hiding places. In one household in the city of Tangshan, a goldfish began jumping wildly in its bowl. At 2 a.m. on July 28, shortly before the earthquake struck, the goldfish jumped out of its bowl. After its owner had returned it to the bowl, the goldfish continued to jump out until the earthquake struck.
Seismotectonics of the Region
The seismotectonics of China have been summarized elsewhere at this web site. As reported, the high seismicity of central and eastern Asia results from the northward collisional convergence (at about 50 mm/y) of the India tectonic plate against the Eurasian plate. This active collision - which begun about 55 million years ago - is the cause of frequent large earthquakes between India and Tibet and throughout Tibet and the surrounding areas. The convergence has uplifted the Asian highlands and the Tibetan Plateau to an average elevation of over 16,000 feet - the highest and largest plateau on Earth - with hundreds of kilometers of displacement of crustal blocks to the east and southeast in the direction of China. Thus the high seismicity of China is dominated by the northward collisional convergence.
This active collision has resulted in three distinct deformational episodes in China that occurred 200- 240 million years ago and resulted in initial thrusting and subsequent vertical extrusion, while later episodes resulted in folding (Li et al. 2007). This convergence has formed the most active and extensive seismic belts in China and in the formation of major faults. Crustal displacements along China’s seismic zones are responsible for the large destructive earthquakes which occur with high frequency.
The Yan Shan Seismic Zone
The Tangshan earthquake occurred at the junction of the Tangshan fold-fault zone and the Cangdong fault zone. The YanShan fold-fault zone runs in an east-west direction and lies north of the Tangshan region. To the south, there are several sub parallel northeast-trending fault zones known as the Shanxi fault depression structural belt, the Taihang piedmont fault zone, the Cangdong fault zone, and the Tangcheng-Lijiang fault zone. According to the scientific literature, each of these zones has produced several earthquakes. Several episodes of uplift and other anomalous variations along different segments of the fault zones that comprise the Yan Shan Seismic Belt have been reported. The significance of these anomalies remains to be further investigated as to the potential for future destructive earthquakes in the Beijing-Tianjin area, in the Hebei Province and in the Liaoning Province, north of Bohai Bay.
Past Earthquakes in the Northeastern Margin of China
According to the State Seismological Bureau report (Wang Fang, 1976), the high period of seismic activity began in this region in 1815 and intensified in subsequent years.
Historical earthquakes in the region
Since the 1966 Xingtai earthquake - which occurred 425 km southwest of Tangshan - there have been several earthquakes with magnitudes equal or greater than 6. These include the 1967 Hejian earthquake (M=6.3), (about 225 km southwest of Tangshan), the 1969 Bohai earthquake (M=7.4), the 1975 Haicheng earthquake (M=7.3) (400 km east of Tangshan), and the 1976 Horinger earthquake (M=6.3) (550 km west of Tangshan).
After the Haicheng earthquake of February 1975, earthquake swarms and moderate earthquakes occurred frequently in northern China at Miaodao in Shandong Province, Horinger in Inner Mongolia, Daichen (170 km southwest of Tangshan) in the Hebei Province, and in Taiyuan in Shanxi Province.
REFERENCES and ADDITIONAL READING
Wang Fang, The 1976 Tangshan Earthquake. State Seismological Bureau of China Beijing, People's Republic of China.
Yong, Chen, et al. The Great Tangshan Earthquake of 1976: An Anatomy of Disaster. New York: Pergamon Press, 1988H.
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