Fault Rock Bearing Traces of Earthquakes to Become Natural Monument

Designation proposed for fault rock on Gyeongju's Bonggil-ri coast
First case reported in South Korean academia... up to 40 cm thick

“Gyeongju Bonggil-ri Pseudotachylite,” breccia-type pseudotachylite / National Heritage Administration.

A fault rock bearing traces of earthquakes has been nominated for national protection. The Korea Heritage Service announced on October 15 that it will designate the “Gyeongju Bonggil-ri Pseudotachylyte,” located along the coast of Bonggil-ri, Munmudaewang-myeon, Gyeongju, Gyeongsangbuk-do, as a Natural Monument. The agency will gather public opinion for one month and make a final decision on the designation after deliberation by the National Heritage Committee.

Pseudotachylyte is a fault rock formed when frictional heat generated as a fault rapidly slips during an earthquake melts rock, which then cools and solidifies. The Bonggil-ri pseudotachylyte is the first case in South Korea to be reported in academic circles after its occurrence and its geological and chemical characteristics were identified. Three forms can be observed: a single-dike type, which appears as a relatively large dike; a fault-vein type, which appears as thin veins; and a breccia type, in which the rock is fractured into angular pieces. The site offers a comprehensive view of the process in which material produced as rock fractures and melts moves, is injected, and solidifies.

Overview of the Gyeongju Bonggil-ri pseudotachylite / National Heritage Administration.

The single-dike type averages about 21 cm in thickness and reaches a maximum of 40 cm, making it exceptionally thick by global standards. It is believed to have formed from frictional heat and energy generated as a fault slipped, and is regarded as an important resource for studying large-scale seismic fault movements in the southeastern Korean Peninsula in the past.

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