Studying undersea faults crucial for earthquake management

When the city of Pohang was hit by a 5.4-magnitude earthquake on last Wednesday, it was primarily ascribed to Yangsan Fault, a seismic fault in the southeastern parts of the Korean Peninsula. It is speculated that the fault in question is originated from the Uljin area and stretched across the west of Pohang and Gyeongju, reaching all the way down to the west of Busan. However, there is no knowing exactly how far the fault can reach to. Experts are not excluding the possibility of Yangsan Fault reaching deeper into the bottom of the sea since the seabed also has underground strata and faults.

In July this year, the Ministry of the Interior and Safety, the Korea Meteorological Administration, and the Nuclear Safety and Security Commission announced a plan to make joint investments worth 108 million U.S. dollars by 2041 in a probe into some 450 faults. However, there is no plan to investigate undersea faults. Though the Ministry of Oceans and Fisheries is planning to study the undersea faults in the waters off the southeastern coasts starting in 2019, the budget has not even fixed.

Understanding the importance of undersea faults, U.S. seismic experts are keeping a close tab on them. In 2008, they launched a project for mapping the seafloor of California to identify any correlation between undersea faults and land faults. “The data on the sediments and faults of the seafloor has added accuracy to our seismic data on California,” explains Samuel Johnson, a researcher at the United States Geological Survey (USGS), which is leading the program to create the seafloor map. “The accuracy of our seismic hazard map is so improved that now we can predict that there will be a 6.7 or greater earthquake on the Hayward Fault in San Francisco in the next 30 years, with a probability of 14.4 percent.”

Since the Pohang earthquake, experts are increasingly voicing the need for an investigation into seafloor faults. The fault newly identified in the vicinity of the Pohang areas is also located near the sea. It is possible that the fault may be rooted deep into the seafloor.

“Reaching a conclusion on a seismic fault solely based on the surface can be likened to a blind man trying to figure out the shape of an elephant by touching it,” says Gong Ki-soo, a chief researcher at an undersea geological exploration research center of Korea Institute of Geoscience and Mineral Resources. “Identifying the entirety of a fault is important in order to make a more precise prediction on the cause of an earthquake and the concomitant damage.”

solea@donga.com