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Soil liquefaction in Pohang

Posted November. 21, 2017 07:31,   

Updated November. 21, 2017 08:21


Concerns are rising over the soil losing stiffness like a swamp following the Pohang earthquake. A research team led by Sohn Mun, a geology professor of Pusan National University and a member of the Active Faults Research Team under the Ministry of the Interior and Safety, concluded Sunday that the mud volcano, which is created by the eruption of water or mud, found near the epicenter is the result of soil liquefaction caused by the earthquake. Aftershocks of 3.5 and 3.6 magnitude hit the city five days after the earthquake and soil liquefaction has been observed in more than 200 places in the city, unnerving many Pohang residents.

Once soil loses strength due to liquefaction, buildings and structures could suffer structural damage or even collapse in the process of hardening of the soil. This suggests that an aftershock of a low magnitude can tear down buildings or infrastructures such as roads and drain pipes. Soil liquefaction is the most common secondary damage to earthquakes, which is not uncommon in the United States and Japan. Since the 1964 Niigata earthquake, the Japanese government has distributed national seismic hazard and liquefaction maps to local governments and made them available to the public. But the Japanese government does not regulate constructing buildings on the affected areas.

The Korean government has decided to investigate the scope of soil liquefaction through excavation and drilling with the lead of the Active Faults Research Team under the Ministry of the Interior and Safety and the Korea Meteorological Administration. Many experts say that it is too early to confirm‎ that there is soil liquefaction with the sand or mud volcanoes currently found in the affected areas. In order to conclude that an earthquake caused soil liquefaction, there must be a strong earthquake, and the ground must be loose and sandy soils. But the Pohang earthquake was not a strong one. There is no need to panic by jumping to a conclusion. Even if the soil has actually been liquefied, mostly the building built on the shoreline will be affected. It is not like all buildings will suddenly collapse all at once.

Natural disasters such as earthquakes serve as a test-bed for measuring the level of competency of the government. A few years ago, the Ministry of Public Safety and Security made a liquefaction hazard map using the government’s drilling research database but did not made it public on two reasons. First of all, the map was not based on actual drilling but on research database only. Secondly, and more importantly, the ministry was concerned that the people living in the liquefied areas could protest against making the information public. Now that we know Korea is not safe from earthquakes anymore, investments should be made in investigating the characteristics of our soil. We knew so little about our geological features.