One component of horizontal ground acceleration recorded on flexible soil during the 1968 November 1 Cook strait earthquake is shown to be well modelled as a single degree of freedom oscillator excited by the recorded component of transverse acceleration in nearby bed rock. This is shown to be consistent with the cellular resonant mode hypothesis of seismic response of flexible soil. The mechanical oscillator is identified as a layer of fine-grained, post-glacial alluvium of approximately 20 m thickness.