Dynamic properties of compacted earth core materials are difficult to measure in the laboratory because of sampling difficulties and the inclusion of larger-size particles. Significant uncertainties exist in selecting the modulus reduction curves of earth core materials in seismic design practice. This study reviews the acceleration time series recorded at multiple earthfill dams in Japan. The study describes the data processing methodology to extract the apparent shear wave velocities between downhole sensors during strong shaking by calculating the wave travel time. The study also describes the calculation of transient shear strains from the difference in displacement time series between sensors by double-integrating filtered acceleration time series. By combing these apparent shear wave velocities and shear strains between sensors, the insitu modulus reduction curves are extracted for earthfill central core materials for selected dams. The results are compared to the empirical models of modulus reduction curves for dam materials.