Acceleration and wave-induced pore pressure were recorded in a saturated sand during the 1980 Mammoth Lakes, California, earthquake sequence. For the largest event recorded, the pore pressure was observed to be proportional to vertical surface acceleration during the P-wave arrivals and proportional to horizontal surface velocity during the S-wave arrivals. The results can be quantitatively explained with a linear elastic model of a porous saturated medium, such that pore pressure depends on dilatation and is independent of shear strain. A slight frequency dependence in the ratio of pore pressure to dilatation indicates local fluid flow on the scale of individual pores. The good agreement between observations and theory indicates that the deformation was primarily linear, even though maximum shear strains were close to the typical thresholds for liquefaction.