Geology is the key: Understanding the liquefaction susceptibility of Niigata City soil

Abstract

The Niigata M7.5 Earthquake of 1964 remains uniquely important among field case histories for understanding liquefaction triggering and manifestations. Much has been written about the Kawagishi-cho strong motion record, the Niigata case histories of seismic-soil liquefaction triggering, and the post-triggering lateral spread displacements. This paper explores a new and different perspective on the disaster - the geologic setting and geomorphic processes reworking Holocene sand units that ultimately create the most severe liquefaction effects during the earthquake. Across the city, liquefaction was most pronounced in fluvially-reworked sands derived from three aeolian and barrier island dune fields upriver and along the coastline. The largest source of beach and aeolian sand material that liquefied in 1964 is a mid-Holocene maximum transgressive barrier island that deposited fifty–sixty meters of sand along the then coastline five-eight-thousand years ago. Tectonic-downwarping and -subsidence of the Echigo Plain has allowed for delta-progradational processes to build out a thick sedimentary prism beneath the current location of Niigata City. Within this prism, the Shinano and Agano Rivers have eroded and fluvially-redeposited these barrier island sands, and those of a closer-in two-three-thousand-year beach-ridge deposit, beneath districts of the city. Most recently, for human-placed fills the materials are sourced almost entirely from modern coastal beach-ridge and sand dune deposits fronting the Sea of Japan. More than any other factors, these geologic conditions and geomorphic depositional histories controlled the locations and severity of soil liquefaction during the 1964 event. Today, these geologic units persist as future a risk to infrastructure of Niigata City.