Geological and Structural Control on Localized Ground Effects within the Heunghae Basin during the Pohang Earthquake (MW 5.4, 15th November 2017), South Korea

Abstract

On 15th November 2017, the Pohang earthquake (Mw 5.4) had strong ground shaking that caused severe liquefaction and lateral spreading across the Heunghae Basin, around Pohang city, South Korea. Such liquefaction is a rare phenomenon during small or moderate earthquakes (MW < 5.5). There are only a few examples around the globe, but more so in the Korean Peninsula. In this paper, we present the results of a systematic survey of the secondary ground effects—i.e., soil liquefaction and ground cracks—developed during the earthquake. Most of the liquefaction sites are clustered near the epicenter and close to the Heunghae fault. Based on the geology, tectonic setting, distribution, and clustering of the sand boils along the southern part of the Heunghae Basin, we propose a geological model, suggesting that the Heunghae fault may have acted as a barrier to the propagation of seismic waves. Other factors like the mountain basin effect and/or amplification of seismic waves by a blind thrust fault could play an important role. Liquefaction phenomenon associated with the 2017 Pohang earthquake emphasizes that there is an urgent need of liquefaction potential mapping for the Pohang city and other areas with a similar geological setting. In areas underlain by extensive unconsolidated basin fill sediments—where the records of past earthquakes are exiguous or indistinct and there is poor implementation of building codes—future earthquakes of similar or larger magnitude as the Pohang earthquake are likely to occur again. Therefore, this represents a hazard that may cause significant societal and economic threats in the future.

Land Damage Mapping and Liquefaction Potential Analysis of Soils from the Epicentral Region of 2017 Pohang Mw 5.4 Earthquake, South Korea

“…The distribution of Quaternary faults and mechanisms involved are consistent with the maximum principal stress direction (E-W; ENE-WSW) during the Quaternary period [54][55][56]. The recorded seismic data along this area suggest that this area has experienced four moderate magnitude earthquakes (Mw 5-6) during the last 14 years, including the 2016 Gyeongju earthquake (Mw 5.5) and the 2017 Pohang earthquake (Mw 5.4) [47][48][49]. Figure 1.…”

“…The southern part of the Korean Peninsula is located on the Eurasian Plate and its tectonic activities are controlled by the ongoing subduction of the adjoining Pacific and Philippine Sea Plates. Moreover, the collision between the Indian Plate with Eurasian Plate controls the tectonism of the Korean Peninsula [47][48][49][50][51]. The Cenozoic deformation along the southeastern part of the Korean Peninsula is accommodated by the Yangsan-Ulsan Fault system.…”

“…Geological map of the study area showing the location of the Heunghae basin, the epicenter of the 15 November 2017 Pohang earthquake and its focal mechanism (modified from [47]). In some places, the liquefaction features were even more than 2 m in diameter ( Figure 2).…”

“…In some places, the liquefaction features were even more than 2 m in diameter ( Figure 2). The shaking was so intense that there was a report of liquefaction of gravelly soil along the dried river beds and rice farms [47]. Due to the presence of soft ground and shallow focal depth of the earthquake, there were several cases of ground settlements of up to 0.39 m. Some residential buildings were tilted by an angle of 1.6 • with differential settlement of 0.15 m [47,50].…”

“…The shaking was so intense that there was a report of liquefaction of gravelly soil along the dried river beds and rice farms [47]. Due to the presence of soft ground and shallow focal depth of the earthquake, there were several cases of ground settlements of up to 0.39 m. Some residential buildings were tilted by an angle of 1.6 • with differential settlement of 0.15 m [47,50]. The grain size analysis was done as per the American Standard for Testing Materials (ASTM) D6913 [58] for the ejected soil from the sand boil, which is classified as mostly silty sand, silt, and sandy ground ( Figure 2) and falls under liquefiable soil category [47,59,60].…”