In this paper, the nonlinearity of the gravelly and clayey soils in Japan is investigated based on the Kiban Kyoshin network (KiKnet) downhole array seismic observations. Shear modulus reduction ratios at 18 sites under 165 strong motions are calculated by comparing the response amplification spectra (surface response normalized by bedrock response) subjected to strong motions with those subjected to weak motions and are plotted versus the corresponding seismically induced shear strains. The back-calculated shear modulus reduction ratios are categorized into several groups under different effective confining pressures, and a best-fit line is proposed for each group as the corresponding empirical shear modulus degradation curve. The obtained results are compared with the existing studies based on both in situ and laboratory tests, highlighting the influence of different testing techniques on nonlinear soil behavior. Furthermore, a modified cyclic nonlinear model is used to formulate the proposed empirical curves to account for the shear modulus degradation under a wide range of effective confining pressure. This modified model can then be used to specify material constitutive relationships for the future numerical analysis of geotechnical structures under seismic loads.