An ancient city shaken
On July 8, 1975, at 6:34 p.m. local time, a powerful earthquake struck central Myanmar. The event, with a magnitude of M 6.8 to 7.0, had its epicenter about 100 km southwest of Mandalay, near the ancient city of Bagan. This intermediate-depth earthquake occurred within the subducting Indian Plate, between 84 and 157 km below the surface, a result of the ongoing collision between the Indian and Burma tectonic plates. For several seconds, the ground shook violently, and witnesses reported that the shaking continued for up to a minute. The sound of collapsing masonry followed, filling the air with dust.
The earthquake caused severe damage across the Bagan archaeological zone, containing thousands of Buddhist temples, stupas, and monasteries dating from the 9th to 13th centuries. Ninety-four major temples were extensively damaged. The tops of many structures, including the famous Bupaya Pagoda, collapsed entirely, with some tumbling into the nearby Ayeyarwady River. Other famous temples, such as Sulamani, Ananda, and Thatbyinnyu, also sustained significant damage. The event was considered the worst earthquake to affect the region in 900 years of recorded history.
A natural laboratory for seismic engineering
In the aftermath, the Government of Myanmar launched a major survey and restoration campaign, assisted by international experts from UNESCO. The widespread destruction turned Bagan into an unplanned, full-scale laboratory for studying the seismic performance of medieval masonry. A detailed damage survey, led by conservation specialist Pierre Pichard, systematically documented the failure points of hundreds of structures. This survey showed which ancient construction techniques were most resilient to seismic forces.
The findings showed an important structural feature: the presence of internal ring beams. Temples built with horizontal wooden or brick beams tying the walls together at various levels fared significantly better than those without. These ring beams helped the structures maintain their integrity during lateral shaking. The analysis also showed that the overall geometry and quality of construction mattered. Structures that had undergone poor-quality reconstructions in the decades prior to 1975 often failed at the connection points between the original masonry and the newer materials. Conversely, monuments that had been properly strengthened after previous, smaller earthquakes showed greater resistance. The data gathered from the 1975 survey directly informed the subsequent restoration efforts, and the engineering lessons are used in the conservation of heritage sites in seismically active zones worldwide.