MASW-Based Shear Wave Velocities for Predicting Liquefaction-Induced Sand Boils Caused by the 2017 M5.4 Pohang, South Korea Earthquake

An earthquake of moment magnitude 5.4 occurred in Pohang, South Korea on November 15, 2017. This earthquake of moderate magnitude induced failures of structures and buildings and led to ground displacements. Approximately 600 sand boils were observed in the vicinity of the epicenter. We conduct 26 multichannel analysis of surface waves (MASW) tests and obtain three shear wave velocity (VS) profiles from suspension Compression (P) and Shear (S) wave logging at liquefied and nonliquefied sites near the epicenter. The phase velocities at frequencies lower than 10 Hz at the liquefied sites were smaller than those at the nonliquefied sites. The cyclic stress ratios (CSRs), cyclic resistance ratios (CRRs), and factors of safety were calculated. The CRR curves for these approaches predicted sand boils in Pohang with an accuracy of 79.5%. The liquefaction potential index (LPI) was calculated at 29 sites and is geospatially interpolated to create maps near the epicenter. The LPI maps had a maximum accuracy of 77% when LPI thresholds were 8 and 2 for two approaches from the literature. The high accuracies from both local and regional liquefaction evaluations demonstrate the applicability of the existing approaches for moderate earthquakes in South Korea.