The peak ground acceleration at K-NET in Ojiya during the Mid Niigata Prefecture Earthquake in 2004 is significantly larger than that at JMA in Ojiya, while the trend reverses in other earthquakes with less intensity. Strain-dependent shear moduli of surface soils at both stations are estimated based on the accelerograms obtained only at the ground surface together with in-situ tests including boring and PS logging. Despite its low shear wave velocity, the near-surface peat at K-NET show weaker nonlinearity than the near-surface silt at JMA, which might have had strong effects on the observed ground motions at the two sites during different earthquakes.