Historic Earthquakes

The 1976 Tangshan Earthquake

by Wang Fang,
State Seismological Bureau of China
Beijing, People's Republic of China

1976 Tangshan earthquake
The Tangshan People's Bank. This four-story concrete and brick building collapsed completely during the earthquake.

1976 Tangshan earthquake
The Chengli Bridge in Tangshan crumpled during the earthquake.

The Tangshan earthquake of 1976 was one of the largest earthquakes in recent years. It occurred on July 28 at 3:42 a.m., Beijing (Peking) local time, and had magnitude 7.8 (revised to 7.5), focal depth of 15 kilometers, and an epicentral intensity of XI on the New Chinese Seismic Intensity Scale; it caused serious damage and loss of life in this densely populated industrial city. Now, with the help of people from all over China, the city of Tangshan is being rebuilt.

Tangshan is located in northern China, 150 km east of the capital, Beijing. Chinese seismologists had for a long time noticed that this region showed characteristics likely to produce major earthquakes. Indeed, this kind of tectonic activity in northern China is comparatively common.

The Yan Shan fold-fault zone runs in an east-west direction and lies north of the Tangshan region; to the south, there are several subparallel northeast-trending fault zones; the Shanxi fault depression structural belt, the Taihang piedmont fault zone, the Cangdong fault zone, and the Tangcheng-Lijiang fault zone. The Tangshan region is at the junction of the Yan Shan fold-fault zone and the Cangdong fault zone.

Seismic activity in northern China is relatively high. Many strong earthquakes have been recorded there over the centuries. The fourth period of high seismic activity began in 1815 and is still continuing.

Since the 1966 Xingtai (425 km southwest of Tangshan) earthquake, there have been frequent events with magnitudes greater than 6. They include the 1967 Hejian (225 km southwest of Tangshan) earthquake (M=6.3), the 1969 Bohai earthquake (M=7.4), the 1975 Haicheng (400 km east of Tangshan) earthquake (M=7.3), and the 1976 Horinger (550 km west of Tangshan) earthquake (M=6.3). Thus, the Tangshan earthquake came in the wake of a continuous and gradual intensification of seismic activity in northern China.

Since 1972, possible precursory anomalies have been observed regularly in northern China. The seismic activity of the Yan Shan belt presented obvious anomalies, such as: diminution of frequency of small earthquakes, regular distribution of the first motion of small earthquakes, formation of a seismic gap, decrease in the ratio of P- and S-wave velocities (Vp/Vs) and in b-value (a measure of the frequency of earthquakes of a given magnitude).

1976 Tangshan earthquake
Sand and water gushed from the ground and spread over large tracts of farmland.

A large area of the southern piedmont of the Yan Shan belt uplifted from 1970 to 1975. About 1973, anomalous variations in surface deformation, radon content in water, gravity, telluric currents, geomagnetism, and so forth were observed at stations and observatories located between Beijing and the Bohai coastal region, and at stations in southern Liaoning Province.

After the 1975 Haicheng earthquake, these variations appeared again in southern Liaoning Province, and they continued to develop after some pause in the Beijing-Tianjin-Tangshan area.

Based upon a joint analysis of tectonic conditions, seismicity, and precursory observations in June 1974, it was forecast that earthquakes ranging in magnitude from 5 to 6 might occur within 1 or 2 years in the Beijing-Tianjin area and in the northern districts above Bohai Bay.

Based on these considerations and the fact that most of the stations showing anomalies were in eastern Hebei Province and southwestern Liaoning Province, the following new propositions were made in January 1976 by the States Seismological Bureau: "earthquakes of magnitude 5-6 may still occur this year in the Beijing-Tianjin-Tangshan area and the Bohai-Zhangjiakou region" (Zhangjiakou is 310 km northwest of Tangshan) and "special attention should be paid to the regions between Tangshan and Chaoyang (225 km northwest of Tangshan) and between Beijing and Tianjin."

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 Hebei Province, and Taiyuan in Shanxi Province.

This created problems in the analysis and evaluation of the seismic regime. Were the above-mentioned anomalies a result of the Haicheng earthquake? were they related to the earthquake swarms and moderate earthquakes? or were they the precursory phenomena of an impending great earthquake?

In the light of China's experience, no definite conclusion could be drawn. It was true that some abrupt anomalies, such as large amplitude variations of groundwater level and anomalous animal behavior in some places, had been observed a few days before the Tangshan earthquake, but these anomalous phenomena occurred so late and were so inconclusive that a short-term prediction was impossible.

The zone of maximum destruction (meizoseismic) of the Tangshan earthquake is elliptical in shape, is about 47 square kilometers in area, and includes the city of Tangshan and extends to its southern suburb along the Beijing-Shanhaiguan railway. According to the New Chinese Seismic Intensity Scale, which corresponds approximately to other 12-degree scales, its intensity was XI.

In this region, structures were generally destroyed; all ordinary buildings lay in ruins, and some strongly built structures suffered serious damage. Rails were bent, and highway bridges were broken or collapsed. The belt of surface faulting related to seismogenic structures ran through this region to a length of more than 8 km, trending northeast, subparallel to the major axis of the meizoseismic zone. Walls, buildings, roads, and canals where the fissure zone passed were sheared.

The 370-km2 region of intensity X is also approximately elliptical in shape and trends northeast. About 80 percent of all dwellings and almost half the industrial buildings were destroyed. Fountains and mud volcanoes were formed. Almost all irrigation wells ceased to function.

The region of intensity of IX is rhombic in shape, has an area of about 1,800 km2, and also trends northeast. Most of the dwelling houses suffered damage, and 40 percent of them collapsed. Half of the industrial structures were destroyed.

Because the southern part of this region is close to the seashore, the ground condition there is rather poor, having water-saturated silt interlayers. Ejection of water and sand was comparatively common; the observed maximum diameter of mud volcanoes was 3 meters. The ejected sand covered large tracts of farmland; sagging and fissuring of the ground surface were also severe.

The region of intensity VIII extends southeastward and has an area of about 7,300 km2. Because most of the buildings in this region were very old and nonearthquake-resistant, destruction was also comparatively serious.

The region of intensity VII has an area of about 33,000 km2. Destruction here was on a lesser scale; a small number of old houses collapsed, and most buildings suffered some degree of damage.

The damage to buildings and structures caused by this earthquake was obviously influenced by the ground conditions. In the northern part of the region, the depth to bedrock is comparatively small; the ground condition, which is good, generally belongs to the I or II category in the Chinese building code; and, therefore, damage was relatively light.

However, the overburden in the southern region is comparatively thick, mostly consisting of sandy or puddly soil, and the water table is high. Thus, damage was quite serious, especially for constructions with longer natural periods of vibrations.

Although Yutian Province is situated in the zone (isoseismal) of intensity VII, structures suffered practically no damage because of good ground conditions and shallow overburden. However, in Tianjin city, which is also located in the zone of intensity VII, destruction was similar in that in regions of intensity VIII because of thick alluvial deposits and the high water table. These are obvious examples of the influence of ground conditions on seismic destructiveness.

REMARKS

For different types of earthquakes, the precursory phenomena are different. The precursory phenomena of the Tangshan earthquake differed from those other earthquakes. For the Tangshan earthquake, there were no obvious foreshocks, and other precursory phenomena were either obscure or occurred too late and were of short duration and scattered distribution. Therefore, in earthquake prediction, the types of precursory phenomena of different earthquakes, their morphology and differences in the process of development, must be taken into consideration. Simple imitation will lead to erroneous judgement.

Many elements may contribute to the precursory phenomena of earthquakes. For example, when earthquakes occur successively in the same region, anomalous phenomena connected with them influence each other and are intermingled. This made it difficult to predict the Tangshan earthquake.

Under such conditions, it is important to distinguish the precursors for one earthquake from the aftereffects of preceding earthquakes and to discriminate among the precursory effects belonging to different earthquakes.

Insufficient research on seismogenic structures of this region was the main cause of the difficulties in the assessment of seismic risk of the Tangshan region before the great earthquake of 1976. Tectonic conditions play an important role in the assessment of seismic risk. Surface faults only reflect tectonic movements of the corresponding geologic age and this cannot be used to infer deep structures. Therefore, Chinese seismologists are putting emphasis on studying the conditions of the deeper parts of the Earth's crust in their current research into earthquake genesis.

The basic seismic intensity to be expected at a site is an important basis for the design of earthquake resistant buildings. Because the basic seismic intensity of Tangshan had been underestimated, the city suffered serious damage during the great earthquake.

Buildings designed to cope with the expected basic seismic intensity of a given area should not suffer structural damage during the earthquake. They should be serviceable, having few or no repairs needed.

Because it is still difficult to asses basic seismic intensity accurately, the possibility of accidental strong earthquakes should be taken into account in earthquake-resistant design. Nevertheless, the probability of an accidental occurrence of a strong earthquake is very small, and, generally, it is uneconomic to design against it.

A reasonable compromise is to lower the standard of earthquake-resistant measures, allowing for a certain degree of damage, but safeguarding personnel and important industrial equipment. That is to say, the design principle should be to avoid destruction during earthquakes by using a strength corresponding to design intensity: buildings should be able to withstand the impact of earthquakes with intensities one or two grades higher than expected.

Under present economic conditions, it is a difficult problem to design structures that can sustain a heavy impact such as the Tangshan earthquake. But it may help to increase structural unity, especially by strengthening the joints of structural elements and by selecting malleable structural types and construction materials with high strength.

(Editor's note: This article uses the Pinyin system of spelling Chinese names and places, which is now the Chinese Government's official transliteration system. The word Pinyin means "phonetic spelling.")

Abridged from Earthquake Information Bulletin, May-June 1979, Volume 11, Number 3.