Assessment of Liquefaction Potentials and Building Damage Based on Shear Wave Velocities in Pohang, South Korea

Abstract

An earthquake with a moment magnitude of 5.4 occurred on November 15, 2017 in Pohang, South Korea. This earthquake was the second largest earthquake ever recorded in South Korea and caused significant damage to the ground and structures. In terms of structural damage, approximately 30,000 buildings were damaged near the epicenter. Approximately 1,900 experts investigated the damaged buildings and classified them into three levels (“Usable,” “Restricted,” and “Unusable”). There are certain factors that influence the damage to buildings, such as the construction year, number of floors, and average shear wave velocity from the 30 m soil deposit. In terms of geotechnical damage, 601 liquefaction-induced sand boils were observed in the vicinity of the epicenter. Forty-four shear wave velocity profiles were collected through a multichannel analysis of surface waves, a spectral analysis of surface waves, a downhole, and a suspension PS-logging test to evaluate the liquefaction potentials near the epicenter. The profiles from the tests and dispersion curves from the multichannel analysis of surface waves tests between the liquefied and non-liquefied sites were compared. The factor of safety against liquefaction was calculated based on the work of Andrus et al. and Kayen et al. by using the shear wave velocity profiles at 44 sites. Both methods could predict the liquefaction triggered by the Pohang earthquake with an accuracy of 74% and 75% (Andrus et al. and Kayen et al., respectively). The liquefaction potential index (LPI) suggested by Iwasaki et al. was calculated for the 44 sites and geospatially interpolated with the LPI values for local evaluation. The LPI map based on the work of Andrus et al. exhibits a maximum accuracy of 75.5% when the LPI thresholds are 4–6, whereas the LPI map based on the work of Kayen et al exhibits a maximum accuracy of 76.5% at an LPI threshold of 1.5.