Strong motion records of 18 earthquakes observed at 12 borehole sites deployed in the 20kmX 20km region in Sendai were analyzed. First, S wave velocities and frequency dependent damping factors were identified for each spectral ratio between surface and borehole records assuming one-dimensional wave propagation. The engineering bedrock wave, that is, the wave supposed to be observed at the bedrock level defined for an engineering purpose, can be calculated from these identified soil constants. Second, we characterized the envelope characteristics of the engineering bedrock waves by using Jennings' type envelope model which consists of three parts, namely, the initial part, the strong part and the attenuating part and we identified the duration of each part by the identification method based on the non-linear least square method. The dependent valuables for each duration were determined based on either the physical meanings of the duration or the statistical test of regression coefficients. Thus we have proposed here a statistical envelope model for engineering bedrock waves only with physically plausible dependent variables.