The Tangshan Earthquake: A Lesson in Seismic Preparedness

Abstract

On July 28, 1976, one of the world's most catastrophic natural disasters took place when a magnitude 7.8 earthquake shook the 1.5 million people living in the large Chinese city of Tangshan; of that population, the official death toll was over 240,000 and over 160,000 people were severely injured. The Tangshan region and its surrounding areas have a historical record of seismic activity; therefore, the disaster was not an isolated or unexpected incident. There are dozens of ancient buildings that remained in the Tangshan region, however, a majority of those buildings were subjected to foundation cracking, tilting of structures, and collapsed roofs after the earthquake struck the city. There was virtually no part of public infrastructure in the Tangshan district that was not affected by the earthquake; railways, highways, hydraulic structures, and virtually all aspects of public works utilities were negatively affected by the disaster. Like many other natural disasters throughout history, the circumstances surrounding the great Tangshan earthquake provide insight that could be utilized where seismic hazards pose a risk to public safety: inadequate seismic considerations stemmed from underestimated seismic hazards, virtually all structures in the area of highest seismic intensity collapsed, and failure of public infrastructure was primarily caused by direct damage from the earthquake and collateral damage from fallen seismically inadequate structures.

Introduction

On July 28, 1976, one of the world's most catastrophic natural disasters took place when a magnitude 7.8 earthquake shook the 1.5 million people living in the large Chinese city of Tangshan; of that population, the official death toll was over 240,000 and over 160,000 people were severely injured; other sources claim that the death toll may be as high as 655,000 and the number of injured to be as high as 700,000. (Vol 1, Britannica) The disaster of Tangshan will always go down as a lesson on the consequences of having inadequate seismological considerations in the construction and maintenance of structures near areas of high earthquake probabilities. When appropriate seismological considerations are put into high earthquake-risk structures they will have a greater chance at being able to withstand violent tremors; unfortunately, the buildings in Tangshan were largely under-designed to withstand such tremors (Vol 1). This inadequate design was not due to the neglect of the engineers that designed these structures, but rather, it was due to the inaccuracy of the seismic maps that placed the region of Tangshan in a low seismic-risk zone; naturally, geologic and seismology studies were carried out after the disaster which showed that there were in fact several active faults in the Tangshan region (Vol. 1). With that, this paper aims to inform the reader about the general seismological features that pertain to the disaster, provide an overview of the extent of the damage to ancient Chinese buildings, and to provide an overview of the extent of the damage to public infrastructure.

General Seismological Activity

According to Zhu Chuanzhen of the Institute of Geophysics and State Seismological Bureau in Beijing, China, the Tangshan region and its surrounding areas have a historical record of seismic activity; therefore, the disaster was not an isolated incident, nor should it have been a completely unexpected incident (Vol. 1). The country of China has more than 3000 years of earthquake records, the longest historical earthquake record of any country, that show a belt-like distribution of earthquakes throughout country and over different active periods of seismicity (Vol. 1). Tanghshan is located in North China where many strong earthquakes were recorded over numerous centuries. Refer to Figure 1 below for an illustration of earthquake instances between 1000 A.D. and the later part of 20^th century. It is worth noting that one earthquake had a magnitude of 8.5, five earthquakes had a magnitude of 8.0, and twelve earthquakes had a magnitude between 7.0 and 7.9 (Vol. 1).

Figure 1- Seismic Activity Recorded between 1000 A.D. and Late 20^th Century (Vol. 1)

Note that in Figure 1, a majority of the earthquakes that occur are concentrated in the third and fourth periods of seismic activity. If the instances of earthquakes that occurred during the third and fourth time periods are spread over the area in which they occurred, the resultant geospatial distribution of earthquakes are concentrated in Northern China; moreover, instances of both the third and fourth time periods intersect in the Tangshan region. Refer to Figure 2 below for an illustration of this intersection of active periods over Tangshan.

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Clearly, the Tangshan region sits at an active intersection of two tectonic belts, both of which had active seismicity that resulted in mantle uplift (Vol. 1). The overlapping active tectonic belts resulted in a complex state of horizontal compression that gradually accumulated strain over time in the Tangshan region, forming a massive seismic source; suddenly the seismic source ruptures and the accumulated stress exceeds the strength of the geologic material, causing an earthquake to erupt (Vol. 1).

Figure 2 - Geospatial distribution of Earthquake Occurrences

There are dozens of different building styles that had remained in the Tangshan region by virtue of the Damage to Ancient Chinese Buildings during 3rd and 4th Active Seismic Periods (Vol. 1) timeframe that the development of Chinese civilization spans over. A majority of those buildings were subjected to foundation cracking, tilting of structures, and collapsed roofs after the earthquake struck the city (Vol. 2). For example, the No-Beam Pavilion and Hua Pagoda on Chezhou Hill in Fengren County were located atop of Chezhou Hill just south of Fengren County by about 15 k; these structures were built between 1368 and 1644 during the Ming Dynasty (vol. 2, Botanica Ming). During the earthquake, the no-beam pavilion was substantially damaged: the east gable wall fell down and the roof collapsed, the opposite gable wall completely cracked all the way through, and the foundation platform was found to have a crack with width of nearly 5 cm (vol. 2). Damage to the pagoda was equally devastating: the mast of the pagoda collapsed, the higher levels of the structure fell, the east eaves fell to the ground, and foundational cracks were also found after the earthquake (vol. 2).

Damage to Rail and Roadways

There was no part of public infrastructure in the Tangshan district that was not affected by the earthquake that struck the city; railways, highways, hydraulic structures, and virtually all aspects of public works utilities were negatively affected by the disaster. As a result of this all outside communication and support were cut off from Tangshan. Furthermore, regional aftershocks outside of the Tangshan district caused seismic damage to peripheral infrastructure, making it difficult to provide aid and relief to the area (Vol. 3). For instance, railway infrastructure leading into and out of the city were damaged to different extents, rendering the tracks inoperable. During the earthquake, 28 freight trains and 7 passenger trains were in operation; of those trains, 7 freight trains and 2 passenger trains were either derailed or turned over, however, no passengers were injured (Vol. 3).

Figure 3 - Roadway Damage: Longitudinal Cracks

Another facet of public infrastructure that was negatively affected by the earthquake were public roadways. In the Tangshan region, the cumulative length of roadways in this area amount to 4,104 km; of this cumulative length, 228 km was subjected to earthquake damage (vol. 3). The damage observed included, but was not limited to, longitudinal cracks that were generally between 10-30 cm wide and dozens of meters long, transverse cracks with widths up to 50-60 cm, and settlement of pavement and subgrade of up to 40-60 cm (Vol. 3).

Conclusion

Like many other natural disasters throughout history, the circumstances surrounding the great Tangshan earthquake provides insight that could be utilized all throughout the world. The most important insight being that there were inadequate seismic considerations that stemmed from underestimated hazards in the Tangshan area; this is a stark reminder that even with preexisting knowledge, accurate forecasting of seismic activity was impossible. Secondary to that, a critical insight is that virtually all structures in the area of highest seismic intensity collapsed; which indicates that no measures were taken to prevent collapse during unexpectedly high seismic intensity. Finally, the failure of public infrastructure was primarily caused by similar circumstances; inadequate seismic considerations were carried out and direct damage from the earthquake and collateral damage from fallen nearby structures. Fortunately, by now all of the infrastructure damages have been remediated and accurate seismic maps were subsequently developed to appropriately retrofit surviving structures and appropriately design new structures. Furthermore, a triumph of the disaster is the wealth of insights that stemmed from the catastrophe that can now be utilized all over the world where seismic hazards pose a risk to public safety and welfare; no other earthquake in history has yielded such widely impacting insights.

References

Housner, G., Dixin, H. (2002). The Great Tangshan Earthquake of 1976 (Vol.1). Retrieved from California Institute of Technology, Earthquake Engineering Research Laboratory website: https://authors.library.caltech.edu/26539/1/Tangshan/Volume1.pdf

Housner, G., Dixin, H. (2002). The Great Tangshan Earthquake of 1976 (Vol.2). Retrieved from California Institute of Technology, Earthquake Engineering Research Laboratory website: https://authors.library.caltech.edu/26539/1/Tangshan/Volume2.pdf

Housner, G., Dixin, H. (2002). The Great Tangshan Earthquake of 1976 (Vol.3). Retrieved from California Institute of Technology, Earthquake Engineering Research Laboratory website: https://authors.library.caltech.edu/26539/1/Tangshan/Volume3.pdf