1976 Tangshan Earthquake

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The Tangshan Earthquake:

The deadliest earthquake to strike in the twentieth century occurred on July 28, 1976 near the east coast of the Republic of China. The epicenter was at the city of Tangshan, in the Hebei province, about 90 miles southeast of Beijing; the hypocenter was directly under the city at a depth of 11 km. The initial tremor was a magnitude 8.2 followed by an aftershock of a magnitude 7.8. Although the 1964 Alaska earthquake was a magnitude 8.4, the damage it caused was far exceed in Tangshan, where great losses of life and property were suffered. However, China's political isolation and strict governmental control of information makes accurate analysis of the earthquake difficult. Few foreigners were allowed into the country, and nonChinese sources of information were primarily anecdotal. Nearly twenty years later, political changes within China have resulted in the surfacing of new information which sheds light on the following factors which contributed to making the Tangshan quake so devastating, 1) the geology of the quake, 2) the timing of the quake, 3) the inadequacy of the building codes, 4) the damaged infrastructure which hindered rescue efforts, 5) the great loss of human life, and 6) perhaps the most important of all, human dereliction.

Geology of the Tangshan Earthquake:

Tangshan's geographic location makes it particularly vulnerable to earthquakes, on both a regional and local scale. Just three weeks after the Tangshan quake, over a two-day period, China's Szechwan Province, the Philippine island of Mindanao, and Japan's Honshu Island were all struck by earthquakes (Rollins and Bryant). The Indian plate and the Pacific plate are squeezing the Asian continent. As the Indian plate moves northeastward, it pushes on the Asian continent. The Pacific plate is squeezing the Asian continent from the east (Figures 1 and 2). The steep angle of this continent-oceanic subduction zone exerts a powerful horizontal force. The area of northern China hit by the Tangshan earthquake is recognized as being particularly prone to the westward movement of the Pacific plate ("The Screams of Tortured Rocks").

On a local scale, the earthquake at Tangshan occurred when a 150-kilometer fault ruptured. The Tancheng-Lujiang, or Tan-Lu, wrench fault system is a large fault system. It extends from the north bank of the Yangtze River all the way into the former USSR. The zone of intertwined faults is 5000 km long and up to 1000 km wide. All of the faults are straight or nearly straight and have steep dips, 70 - 80+ as would be expected in a strike-slip fault ("The Screams of Tortured Rocks"). Faults are an indication of rock deformation that occurs when rocks are under stress. Earthquakes are a response of rocks to the built-up stress. The amount of energy released in the Tangshan quake was great because the fault had not moved in such a long time. The stress of the Tangshan quake was caused by the compression along the plate boundaries of the Indian and Asian plates, as well as the compression along the boundaries of the Pacific and Asian plates.

Damage on the earth's surface could be attributed to two principal types of mechanisms: brittle cracking and soil liquefaction. Most affected area was an area four by five miles long, intersected by an extensive lateral fault. In some places the earth was rent apart by several feet, and in many others the ground caved in to form thousands of craters ("Tangshan Quake: Portrait of a Catastrophe"). Half a mile from the fault line, a field of corn the size of an airport was knocked over, in the same direction, as though by some giant wind. Trees were snapped off or uprooted (Malcolm). Much of the previously flat agricultural land was undulated ("Tangshan Quake: Portrait of a Catastrophe"). Another destructive aspect of the Tangshan quake has to do with the phenomenon of liquefaction -the violent mixing of sandy soil and subterranean water under pressure that turns what...
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