Holographic photograph is applied to study surface deformation of core-contained rock samples under single-axial pressure and visible image of off-surface displacement field is displayed. When load is 90% of the critical value, strong deformation zone and macroscopic cracks appear around cores. Such zone may be found on one side of the core, if the strength of core is below or equal to that of the peripheral material. If the strength of core is above that outside, deformation inside the core may be small in contrast to outside.
Based on the experimental results, a new explanation of 'gap'formation before earthquakes is proposed.