Changes in soil conditions before and after earthquakes at a repetitive soil liquefaction site in Taiwan

Abstract

Beishih Village of Hsinhua Township in southern Taiwan is a unique location for studying soil liquefaction. Soil liquefaction has been observed at the same site after earthquakes in 1946, 2010, and 2016, each of which had a Richter magnitude greater than six. This recurrence provides an opportunity for analyzing soil condition variations resulting from soil liquefaction. Seismic data sets have been collected in 2011, 2014, 2016, and 2017. We have used seismic refraction tomography and the multichannel analysis of surface waves to estimate P- and S-wave velocities. In S-wave velocity profiles, low shear velocity zones have been located beneath sand volcanoes shortly after two earthquakes and have disappeared four years after a 2010 earthquake. However, the P-wave velocity has been less sensitive to soil condition changes, possibly because groundwater obscures the effect of soil liquefaction on velocity profiles. In addition, we use seismic wave velocities to determine the importance of soil properties such as Poisson’s ratio, shear modulus, and porosity to identify the cause of the low shear velocity zone. Notably, although porosity decreases after soil grain rearrangement, sand and clay mixing increases the Poisson’s ratio, reducing the shear modulus of the soil. In addition, a soil layer between 2 and 7 m and a deeper layer below 10 m that resulted in sand volcanoes have been liquefied. We also consider how the evaluation of soil liquefaction potential can be affected by long-term variations in soil conditions and changes resulting from liquefaction. The factor of safety is used to evaluate the liquefaction potential of the site. The results reveal that the assessment conducted long after the earthquake underestimated risk because the soil developed shear strength after the earthquake.