We conducted dynamic viscoelastic measurements of three clay minerals in a solid–liquid two-phase state: kaolinite, illite, and smectite with water. These constituents of concentrated (dense) suspensions were investigated using a high-temperature and high-pressure rheometer, to understand tectonic and non-tectonic phenomena in the shallow part of a fault system, such as shallow slow slip events in subduction zones, and landslides on fault or bed planes. We observed shear strain rate dependencies of phase angles of both dynamic stress and strain waveforms on the rheometer at varying temperature and pressure. The pressure and temperature dependence of the viscoelastic properties of the system can be qualitatively understood by applying the Zwanzig–Mountain theory. The local packing fraction change owing to dynamic oscillations affects the changing viscoelastic properties in systems such as shallow fault systems.