Recently Aoi et al.[1] reported unusual results for the surface acceleration of the KiK-net IWTH25 station in response to the 14 June 2008 Iwate-Miyagi mainshock, moment mag- nitude 6.9. Accelerations as large as 4g were observed with the acceleration waveform showing distinctive asymmetry, i.e., large, positive accelerations of short duration were bal- anced by small, negative accelerations of long duration. These observations prompted the sobriquet trampoline effect for it seemed that the earth surface was moving much like a jumper on a trampoline. To understand what is going on we undertake examination of the IWTH25 station data sets and introduce a model for what is seen. Examination of the data at the IWTH25 station show the asymmetric vertical acceleration (initially noted by Aoi et al.[1]) and most importantly strong correlation between vertical mo- tion and horizontal motion. A lumped element model[2] for this system, involving nonlinear surface-subsurface coupling, is introduced and shown to have vertical acceleration waveform, asymmetry and Fourier structure in accord with observations. Prompted by the vertical/horizontal correlation seen in data, a nonlinear coupling between vertical motion and horizontal motion is introduced into the lumped element model. Satisfactory description of the data is demonstrated.

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