Republic of Korea: Seismic activity data collection on the ocean floor

By Katy Barbier-Greenland

Unlike neighbouring Japan, earthquake activity is infrequent in the peninsula area of South Korea, and those that do occur tend to be low in intensity. Typically, earthquakes in this region measure between 2.0 and 5.0 on the Richter scale. The record for the quake with the highest magnitude on the Korean Peninsula stood for some time, recorded at 5.3 in 1980. This was recently overtaken by a 5.8 magnitude tremor in Gyeongju in 2016 and a 5.4 magnitude tremor in Pohang, followed soon after by a 4.8 magnitude aftershock tremor. Data on these events is collected using the existing seismological observation network and tsunami warning system established by the Korea Meteorological Administration (KMA). This network is comprised of broadband seismometers, short-period seismometers and accelerometers – recording velocity and accelerations automatically and transmitting the data to processing stations. Whilst this network has been bolstered in the past couple of decades, there are limits to its reach, as well as gaps in understanding the seriousness of active faults to generate large and tsunamigenic earthquakes.

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There is clearly a need for accurate data on the seismic activity in the region that enables more in-depth observation and prediction. To achieve this objective, the KMA has established and funded an extensive, five-stage investigation that aims to analyse seismic activity and the underlying fault structure in the waters around the South Korean peninsula. This government authority has a broad purview that includes meteorological topics, natural disasters like tsunamis and earthquakes, as well as climate change. The first part of the KMA programme, led by Professor [Kwang-Hee] Kim [a seismologist at Pusan National University], will collect data on fault structures, fault shape and prediction of seismic activity, including earthquake and tsunami magnitudes, enabling prediction and preparation for the impact of such seismic events. The research team will develop a model for the underground structure, integrating this with data collected on seismic velocity. The earth’s internal structures that transform or amplify seismic waves will be captured and analysed alongside data from the existing networks, and the data used to more accurately identify and predict earthquakes.

K.U.M. Ocean-bottom Seismometers

The project team, which includes experts in tsunamis, earthquakes and coastal research, selected the Ocean-Bottom-System (OBS) manufactured by German company Umwelt- und Meerestechnik Kiel (K.U.M.), whose name translates to Environmental and Marine Engineering Keel. K.U.M. has developed products and delivered services to the marine research and maritime science sector since 1997 and the OBS seismometers are a core focus. The OBSs were purchased via Korean supplier GeoTech Systems Corp, a company that provides measurement equipment and marine-related products and services, from tunnel and water surveying to ocean-related exploration equipment. The OBS has been used successfully in other studies investigating the Korean seafloor.

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These OBSs will detect and measure sub-sea seismic movements on the South Korean peninsula to measure the displacement of water level change. The researchers selected this equipment for its ease of use, portability, robust software and continuous data collection capabilities – all of which enable straightforward operation. The team has found the devices are particularly suitable due to their compact size, given the space limitations on board the ship. Ten devices have already been deployed and a further ten will be installed on the ocean floor in identified locations during October 2019. The OBSs will remain in place for up to two years with the team collecting data from them every six months, whereby an acoustic signal will be sent to each OBS, which will subsequently detach from its anchor and rise slowly to the ocean surface.

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