Earthquakes and the Ancient Capital of Gyeongju!
Ensuring safety so that structures remain standing even when they shake…
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The Hidden Seismic Performance of Cheomseongdae and the Seokgatap Pagoda
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On September 12, 2016, Gyeongju experienced the strongest earthquake recorded by the Korea Meteorological Administration since instrumental monitoring began in 1978.
The powerful quake—unlike anything people had experienced before—was followed by months of aftershocks, leaving many residents unsettled. Gyeongju sits near the Yangsan Fault Zone and is a historic city that served as the capital of the Silla Kingdom for a thousand years, from the late 1st century BCE during King Hyeokgeose’s reign until Silla’s fall in 935 CE.
Historical records such as the Samguk Sagi and Samguk Yusa document numerous earthquakes in the Gyeongju region. Despite being an earthquake-prone area, Gyeongju remains home to many enduring cultural treasures—including Seokguram, Bulguksa, and Cheomseongdae—that continue to showcase the legacy of this ancient capital.
However, the 2016 Gyeongju earthquake prompted scientific stability assessments and practical preservation measures. The very fact that these cultural assets have withstood multiple earthquakes makes them noteworthy from architectural and structural engineering perspectives.
Cheomseongdae, constructed during Queen Seondeok’s reign (632–646), is an ancient astronomical observatory with a rare and distinctive bottle-shaped form. Its elegant silhouette adds a mysterious, historic charm, attracting great public interest. It has also inspired numerous scholarly interpretations and theories regarding its construction principles.
During the 2016 earthquake, Cheomseongdae sustained no significant structural damage aside from ground subsidence that caused it to lean slightly more. Its central axis tilted an additional 2 cm, building upon a longstanding 20 cm lean toward the north. The top stone—shaped like the Chinese character 井—shifted about 5 cm due to its joint edges slipping from their original placement.
In 2010, a seismic hazard assessment was conducted along with a geotechnical survey. Results showed that Cheomseongdae meets Korea’s highest seismic design category—equivalent to a “1,000-year return period” seismic structure capable of withstanding earthquakes expected once every millennium.
During seismic vibration tests simulating a magnitude corresponding to a 1,000-year return period (approximately M6.4), the top stone’s joint edges showed patterns of dislocation similar to those observed after the 2016 earthquake.
Taken together, these findings suggest that Cheomseongdae likely endured major earthquakes exceeding a 1,000-year return cycle in the distant past.
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Seokgatap Pagoda of Bulguksa: The Strongest Seismic Resistance
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The Seokgatap Pagoda at Bulguksa is one of the most iconic stone pagodas from the Silla period. Historical records note that earthquakes struck Gyeongju shortly after Bulguksa’s construction in 742, with documented events in 755, 779, and 1036. Notably, the Samguk Sagi mentions that “Seokgatap leaned, so supports were installed” after the 1036 earthquake.
A 2011 seismic risk assessment determined that Seokgatap falls into Korea’s “special grade” seismic category—equivalent to a 2,400-year return period structure. Based on seismic vibration tests, the leaning reported in 1036 is believed to have resulted from a magnitude 7 or larger earthquake—the strongest in the region’s historical record—and the pagoda likely had already suffered structural fatigue and deformation before the quake’s vibrations further affected it.
In 2010, damage to the pagoda’s platform was discovered, leading to a full dismantling and restoration. Based on prior seismic assessments, restoring the pagoda to its original structural state became the project’s highest priority. Restoration began in 2012. The central structural axis—critical to Seokgatap’s seismic resilience—was reconstructed using precise modern measuring technology. The inner fill was strengthened using a durable mix of titanium components and a newly developed inorganic binder (a mineral adhesive), enhancing long-term stability.
Restoration finished in July 2016, just two months before the magnitude 5.8 Gyeongju earthquake struck. Post-earthquake analysis revealed no deformation in the central structural axis, and construction deviations in the second-story section had actually decreased—confirming the stability of the restoration.
Cultural heritage sites express their seismic resilience through their long survival. Although Cheomseongdae and Seokgatap were not built with modern “seismic design,” it is accurate to say that traditional construction methods inherently incorporated seismic-resistant characteristics.
The case of Seokgatap clearly indicates how Korea should approach earthquake protection for cultural heritage. Ensuring the structural principles used at the time of construction remain intact is critical for effectively maintaining seismic performance. These structural principles must be identified through scientific research and preserved as a significant part of Korea’s intangible academic heritage.
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1) The pagoda body suppresses amplification of upper-level vibrations through inertia generated by vertical load.
2) The internal fill structure disperses seismic waves and acts as a “damper,” reducing vibration and impact as seismic energy passes through. A damper is a device that absorbs seismic energy, helping prevent structural damage.
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What Is a Seismic Return Period?
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When defining the seismic load a structure must withstand, engineers use the term “return period.”
For example, a 100-year return period refers to an earthquake likely to occur once every 100 years. A 2,400-year return period means the structure must be able to withstand a quake expected roughly once every 2,400 years. The longer the return period, the stronger the associated seismic event.
Korea’s seismic design standards traditionally used the 2,400-year return period as the upper limit for structural calculations. More recently, a 4,800-year return period category has been under consideration.
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What Is Seismic Performance?
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Seismic performance represents a building’s overall ability to resist earthquake forces. It is commonly categorized into four levels:
1) Operational: The building remains fully functional after an earthquake.
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2) Immediate Occupancy: The building remains usable, though minor repairs may be needed.
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3) Life Safety: Significant damage occurs, and substantial repairs are required before reuse.
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4) Collapse Prevention: The structure avoids collapse, but aftershocks may make failure likely, and reuse is nearly impossible.
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