Broadband seismometers can be deployed at three (or more) separate terrace-top seismic stations along a transect perpendicular to the sea cliff edge.These instruments measure velocity for three separate directions of ground motion (e.g. vertical, north-south, and east-west). These ground motion velocities can be differentiated to obtain ground accelerations, or integrated to obtain absolute displacements.
Microseismic Sea Cliff Motion
Adams, P. N., C. D. Storlazzi, and R. S. Anderson (2005), Nearshore wave-induced cyclical flexing of sea cliffs, J. Geophys. Res., 110, F02002, doi:10.1029/2004JF000217.