USING ELECTRICAL RESISTIVITY TOMOGRAPHY AND SEDIMENT ANALYSES TO UNDERSTAND THE INFLUENCE OF SEDIMENT PROPERTIES AND ENVIRONMENT OF DEPOSITION ON THE SURFACE EXPRESSION OF EARTHQUAKE-INDUCED LIQUEFACTION

Electrical resistivity tomography (ERT) data collected at a paleoliquefaction site along several profiles reveal how the distribution of subsurface sediments influences the location of liquefaction features. The ERT data show low resistivity values interpreted to reflect fine-grained deposits that contribute to the build-up of fluid pressure and subsequent flow direction of water and entrained sediment resulting from earthquake-induced liquefaction. Soil samples collected from the surface and from auger holes, along with published interpretations of depositional units found in the site vicinity, are used to relate features in the ERT profile to deposition units. Electrical resistivity values are highly influenced by porosity of the depositional unit and composition of fluid in the pore space, in accordance with Archie’s Law. In turn, sediment properties influence the build of pore-water pressure during strong ground shaking and the direction of fluid flow following liquefaction. Braided steam, point bar, abandoned channel, natural levee, and possibly backswamp depositional units are interpreted from the ERT data acquired. Sand blows are represented with high resistivity values and corroborated by sandy textures of samples collected from these domains. Abandoned channel fill deposits exhibit medium to low resistivity values, and silty clay to clay texture. Natural levee depositional units have lower resistivity values and silty clay texture compared to abandoned channel deposits. ERT data combined with soil analyses are consistent with published data of grain-size characteristics of depositional units. We propose that the abandoned channel, natural levee, and backswamp deposits acted as impermeable layers and allowed for the increase in fluid pressure during ground shaking. Once braided stream deposits liquefied, escaping fluids followed the inclined contacts of the abandoned channel deposits leading to the formation of sand blows along its margins.